10BASE5
7.1.2 This page will discuss the original 1980 Ethernet product, which is 10BASE5. 10BASE5 transmitted 10 Mbps over a single think coaxial cable bus.
10BASE5 is important because it was the first medium used for Ethernet. 10BASE5 was part of the original 802.3 standard. The primary benefit of 10BASE5 was length. 10BASE5 may be found in legacy installations. It is not recommended for new installations. 10BASE5 systems are inexpensive and require no configuration. Two disadvantages are that basic components like NICs are very difficult to find and it is sensitive to signal reflections on the cable. 10BASE5 systems also represent a single point of failure.
10BASE5 uses Manchester encoding. It has a solid central conductor. Each segment of thick coax may be up to 500 m (1640.4 ft) in length. The cable is large, heavy, and difficult to install. However, the distance limitations were favorable and this prolonged its use in certain applications.
When the medium is a single coaxial cable, only one station can transmit at a time or a collision will occur. Therefore, 10BASE5 only runs in half-duplex with a maximum transmission rate of 10 Mbps.
Figure illustrates a configuration for an end-to-end collision domain with the maximum number of segments and repeaters. Remember that only three segments can have stations connected to them. The other two repeated segments are used to extend the network.
The next page will discuss 10BASE2.
Tuesday, January 26, 2010
10-Mbps and 100-Mbps Ethernet
10-Mbps Ethernet
7.1.1 This page will discuss 10-Mbps Ethernet technologies.
10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, the frame format, transmission processes, and a basic design rule.
Figure displays the parameters for 10-Mbps Ethernet operation. 10-Mbps Ethernet and slower versions are asynchronous. Each receiving station uses eight octets of timing information to synchronize its receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters. For example, 1 bit time at 10 Mbps = 100 nanoseconds (ns) = 0.1 microseconds = 1 10-millionth of a second. This means that on a 10-Mbps Ethernet network, 1 bit at the MAC sublayer requires 100 ns to transmit.
For all speeds of Ethernet transmission 1000 Mbps or slower, transmission can be no slower than the slot time. Slot time is just longer than the time it theoretically can take to go from one extreme end of the largest legal Ethernet collision domain to the other extreme end, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station to be detected.
10BASE5, 10BASE2, and 10BASE-T also have a common frame format.
The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. As the frame passes from the MAC sublayer to the physical layer, other processes occur before the bits move from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. The SQE is a transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. The SQE is also called a heartbeat. The SQE signal is designed to fix the problem in earlier versions of Ethernet where a host does not know if a transceiver is connected. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active in the following instances:
• Within 4 to 8 microseconds after a normal transmission to indicate that the outbound frame was successfully transmitted
• Whenever there is a collision on the medium
• Whenever there is an improper signal on the medium, such as jabber, or reflections that result from a cable short
• Whenever a transmission has been interrupted
All 10-Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding. Line encoding describes how the bits are actually signaled on the wire. The simplest encodings have undesirable timing and electrical characteristics. Therefore, line codes have been designed with desirable transmission properties. This form of encoding used in 10-Mbps systems is called Manchester encoding.
Manchester encoding uses the transition in the middle of the timing window to determine the binary value for that bit period. In Figure , the top waveform moves to a lower position so it is interpreted as a binary zero. The second waveform moves to a higher position and is interpreted as a binary one. The third waveform has an alternating binary sequence. When binary data alternates, there is no need to return to the previous voltage level before the next bit period. The wave forms in the graphic show that the binary bit values are determined based on the direction of change in a bit period. The voltage levels at the start or end of any bit period are not used to determine binary values.
Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is most important in mixed-media networks. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on the following types of parameters:
• Cable length and propagation delay
• Delay of repeaters
• Delay of transceivers
• Interframe gap shrinkage
• Delays within the station
10-Mbps Ethernet operates within the timing limits for a series of up to five segments separated by up to four repeaters. This is known as the 5-4-3 rule. No more than four repeaters can be used in series between any two stations. There can also be no more than three populated segments between any two stations.
The next page will describe 10BASE5.
7.1.1 This page will discuss 10-Mbps Ethernet technologies.
10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, the frame format, transmission processes, and a basic design rule.
Figure displays the parameters for 10-Mbps Ethernet operation. 10-Mbps Ethernet and slower versions are asynchronous. Each receiving station uses eight octets of timing information to synchronize its receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters. For example, 1 bit time at 10 Mbps = 100 nanoseconds (ns) = 0.1 microseconds = 1 10-millionth of a second. This means that on a 10-Mbps Ethernet network, 1 bit at the MAC sublayer requires 100 ns to transmit.
For all speeds of Ethernet transmission 1000 Mbps or slower, transmission can be no slower than the slot time. Slot time is just longer than the time it theoretically can take to go from one extreme end of the largest legal Ethernet collision domain to the other extreme end, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station to be detected.
10BASE5, 10BASE2, and 10BASE-T also have a common frame format.
The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. As the frame passes from the MAC sublayer to the physical layer, other processes occur before the bits move from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. The SQE is a transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. The SQE is also called a heartbeat. The SQE signal is designed to fix the problem in earlier versions of Ethernet where a host does not know if a transceiver is connected. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active in the following instances:
• Within 4 to 8 microseconds after a normal transmission to indicate that the outbound frame was successfully transmitted
• Whenever there is a collision on the medium
• Whenever there is an improper signal on the medium, such as jabber, or reflections that result from a cable short
• Whenever a transmission has been interrupted
All 10-Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding. Line encoding describes how the bits are actually signaled on the wire. The simplest encodings have undesirable timing and electrical characteristics. Therefore, line codes have been designed with desirable transmission properties. This form of encoding used in 10-Mbps systems is called Manchester encoding.
Manchester encoding uses the transition in the middle of the timing window to determine the binary value for that bit period. In Figure , the top waveform moves to a lower position so it is interpreted as a binary zero. The second waveform moves to a higher position and is interpreted as a binary one. The third waveform has an alternating binary sequence. When binary data alternates, there is no need to return to the previous voltage level before the next bit period. The wave forms in the graphic show that the binary bit values are determined based on the direction of change in a bit period. The voltage levels at the start or end of any bit period are not used to determine binary values.
Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is most important in mixed-media networks. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on the following types of parameters:
• Cable length and propagation delay
• Delay of repeaters
• Delay of transceivers
• Interframe gap shrinkage
• Delays within the station
10-Mbps Ethernet operates within the timing limits for a series of up to five segments separated by up to four repeaters. This is known as the 5-4-3 rule. No more than four repeaters can be used in series between any two stations. There can also be no more than three populated segments between any two stations.
The next page will describe 10BASE5.
Module 7: Ethernet Technologies
Overview
Ethernet has been the most successful LAN technology mainly because of how easy it is to implement. Ethernet has also been successful because it is a flexible technology that has evolved as needs and media capabilities have changed. This module will provide details about the most important types of Ethernet. The goal is to help students understand what is common to all forms of Ethernet.
Changes in Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the early 1980s. The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100-Mbps Fast Ethernet. In recent years, an even more rapid growth in media speed has moved the transition from Fast Ethernet to Gigabit Ethernet. The standards for Gigabit Ethernet emerged in only three years. A faster Ethernet version called 10-Gigabit Ethernet is now widely available and faster versions will be developed.
MAC addresses, CSMA/CD, and the frame format have not been changed from earlier versions of Ethernet. However, other aspects of the MAC sublayer, physical layer, and medium have changed. Copper-based NICs capable of 10, 100, or 1000 Mbps are now common. Gigabit switch and router ports are becoming the standard for wiring closets. Optical fiber to support Gigabit Ethernet is considered a standard for backbone cables in most new installations.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Describe the differences and similarities among 10BASE5, 10BASE2, and 10BASE-T Ethernet
• Define Manchester encoding
• List the factors that affect Ethernet timing limits
• List 10BASE-T wiring parameters
• Describe the key characteristics and varieties of 100-Mbps Ethernet
• Describe the evolution of Ethernet
• Explain the MAC methods, frame formats, and transmission process of Gigabit Ethernet
• Describe the uses of specific media and encoding with Gigabit Ethernet
• Identify the pinouts and wiring typical to the various implementations of Gigabit Ethernet
• Describe the similarities and differences between Gigabit and 10-Gigabit Ethernet
• Describe the basic architectural considerations of Gigabit and 10-Gigabit Ethernet
Ethernet has been the most successful LAN technology mainly because of how easy it is to implement. Ethernet has also been successful because it is a flexible technology that has evolved as needs and media capabilities have changed. This module will provide details about the most important types of Ethernet. The goal is to help students understand what is common to all forms of Ethernet.
Changes in Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the early 1980s. The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100-Mbps Fast Ethernet. In recent years, an even more rapid growth in media speed has moved the transition from Fast Ethernet to Gigabit Ethernet. The standards for Gigabit Ethernet emerged in only three years. A faster Ethernet version called 10-Gigabit Ethernet is now widely available and faster versions will be developed.
MAC addresses, CSMA/CD, and the frame format have not been changed from earlier versions of Ethernet. However, other aspects of the MAC sublayer, physical layer, and medium have changed. Copper-based NICs capable of 10, 100, or 1000 Mbps are now common. Gigabit switch and router ports are becoming the standard for wiring closets. Optical fiber to support Gigabit Ethernet is considered a standard for backbone cables in most new installations.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Describe the differences and similarities among 10BASE5, 10BASE2, and 10BASE-T Ethernet
• Define Manchester encoding
• List the factors that affect Ethernet timing limits
• List 10BASE-T wiring parameters
• Describe the key characteristics and varieties of 100-Mbps Ethernet
• Describe the evolution of Ethernet
• Explain the MAC methods, frame formats, and transmission process of Gigabit Ethernet
• Describe the uses of specific media and encoding with Gigabit Ethernet
• Identify the pinouts and wiring typical to the various implementations of Gigabit Ethernet
• Describe the similarities and differences between Gigabit and 10-Gigabit Ethernet
• Describe the basic architectural considerations of Gigabit and 10-Gigabit Ethernet
Summary of Module 6
Summary
This page summarizes the topics discussed in this module.
Ethernet is not one networking technology, but a family of LAN technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. Ethernet operates at two layers of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various physical topologies. Layer 1 bits need structure so OSI Layer 2 frames are used. The MAC sublayer of Layer 2 determines the type of frame appropriate for the physical media.
The one thing common to all forms of Ethernet is the frame structure. This is what allows the interoperability of the different types of Ethernet.
Some of the fields permitted or required in an 802.3 Ethernet Frame are:
• Preamble
• Start Frame Delimiter
• Destination Address
• Source Address
• Length/Type
• Data and Pad
• Frame Check Sequence
In 10 Mbps and slower versions of Ethernet, the Preamble provides timing information the receiving node needs in order to interpret the electrical signals it is receiving. The Start Frame Delimiter marks the end of the timing information. 10 Mbps and slower versions of Ethernet are asynchronous. That is, they will use the preamble timing information to synchronize the receive circuit to the incoming data. 100 Mbps and higher speed implementations of Ethernet are synchronous. Synchronous means the timing information is not required, however for compatibility reasons the Preamble and SFD are present.
The address fields of the Ethernet frame contain Layer 2, or MAC, addresses.
All frames are susceptible to errors from a variety of sources. The Frame Check Sequence (FCS) field of an Ethernet frame contains a number that is calculated by the source node based on the data in the frame. At the destination it is recalculated and compared to determine that the data received is complete and error free.
Once the data is framed the Media Access Control (MAC) sublayer is also responsible to determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data. There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served).
Examples of deterministic protocols include Token Ring and FDDI. The carrier sense multiple access with collision detection (CSMA/CD) access method is a simple non-deterministic system. The NIC listens for an absence of a signal on the media and starts transmitting. If two nodes or more nodes transmit at the same time a collision occurs. If a collision is detected the nodes wait a random amount of time and retransmit.
The minimum spacing between two non-colliding frames is also called the interframe spacing. Interframe spacing is required to insure that all stations have time to process the previous frame and prepare for the next frame.
Collisions can occur at various points during transmission. A collision where a signal is detected on the receive and transmit circuits at the same time is referred to as a local collision. A collision that occurs before the minimum number of bytes can be transmitted is called a remote collision. A collision that occurs after the first sixty-four octets of data have been sent is considered a late collision. The NIC will not automatically retransmit for this type of collision.
While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. Ethernet errors result from detection of frames sizes that are longer or shorter than standards allow or excessively long or illegal transmissions called jabber. Runt is a slang term that refers to something less than the legal frame size.
Auto-Negotiation detects the speed and duplex mode, half-duplex or full-duplex, of the device on the other end of the wire and adjusts to match those settings.
This page summarizes the topics discussed in this module.
Ethernet is not one networking technology, but a family of LAN technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. Ethernet operates at two layers of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various physical topologies. Layer 1 bits need structure so OSI Layer 2 frames are used. The MAC sublayer of Layer 2 determines the type of frame appropriate for the physical media.
The one thing common to all forms of Ethernet is the frame structure. This is what allows the interoperability of the different types of Ethernet.
Some of the fields permitted or required in an 802.3 Ethernet Frame are:
• Preamble
• Start Frame Delimiter
• Destination Address
• Source Address
• Length/Type
• Data and Pad
• Frame Check Sequence
In 10 Mbps and slower versions of Ethernet, the Preamble provides timing information the receiving node needs in order to interpret the electrical signals it is receiving. The Start Frame Delimiter marks the end of the timing information. 10 Mbps and slower versions of Ethernet are asynchronous. That is, they will use the preamble timing information to synchronize the receive circuit to the incoming data. 100 Mbps and higher speed implementations of Ethernet are synchronous. Synchronous means the timing information is not required, however for compatibility reasons the Preamble and SFD are present.
The address fields of the Ethernet frame contain Layer 2, or MAC, addresses.
All frames are susceptible to errors from a variety of sources. The Frame Check Sequence (FCS) field of an Ethernet frame contains a number that is calculated by the source node based on the data in the frame. At the destination it is recalculated and compared to determine that the data received is complete and error free.
Once the data is framed the Media Access Control (MAC) sublayer is also responsible to determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data. There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served).
Examples of deterministic protocols include Token Ring and FDDI. The carrier sense multiple access with collision detection (CSMA/CD) access method is a simple non-deterministic system. The NIC listens for an absence of a signal on the media and starts transmitting. If two nodes or more nodes transmit at the same time a collision occurs. If a collision is detected the nodes wait a random amount of time and retransmit.
The minimum spacing between two non-colliding frames is also called the interframe spacing. Interframe spacing is required to insure that all stations have time to process the previous frame and prepare for the next frame.
Collisions can occur at various points during transmission. A collision where a signal is detected on the receive and transmit circuits at the same time is referred to as a local collision. A collision that occurs before the minimum number of bytes can be transmitted is called a remote collision. A collision that occurs after the first sixty-four octets of data have been sent is considered a late collision. The NIC will not automatically retransmit for this type of collision.
While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. Ethernet errors result from detection of frames sizes that are longer or shorter than standards allow or excessively long or illegal transmissions called jabber. Runt is a slang term that refers to something less than the legal frame size.
Auto-Negotiation detects the speed and duplex mode, half-duplex or full-duplex, of the device on the other end of the wire and adjusts to match those settings.
Link establishment and full and half duplex
Link establishment and full and half duplex
6.2.10 This page will explain how links are established through Auto-Negotiation and introduce the two duplex modes.
Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation.
Auto-Negotiation is optional for most Ethernet implementations. Gigabit Ethernet requires its implementation, though the user may disable it. Auto-Negotiation was originally defined for UTP implementations of Ethernet and has been extended to work with other fiber optic implementations.
When an Auto-Negotiating station first attempts to link it is supposed to enable 100BASE-TX to attempt to immediately establish a link. If 100BASE-TX signaling is present, and the station supports 100BASE-TX, it will attempt to establish a link without negotiating. If either signaling produces a link or FLP bursts are received, the station will proceed with that technology. If a link partner does not offer an FLP burst, but instead offers NLPs, then that device is automatically assumed to be a 10BASE-T station. During this initial interval of testing for other technologies, the transmit path is sending FLP bursts. The standard does not permit parallel detection of any other technologies.
If a link is established through parallel detection, it is required to be half duplex. There are only two methods of achieving a full-duplex link. One method is through a completed cycle of Auto-Negotiation, and the other is to administratively force both link partners to full duplex. If one link partner is forced to full duplex, but the other partner attempts to Auto-Negotiate, then there is certain to be a duplex mismatch. This will result in collisions and errors on that link. Additionally if one end is forced to full duplex the other must also be forced. The exception to this is 10-Gigabit Ethernet, which does not support half duplex.
Many vendors implement hardware in such a way that it cycles through the various possible states. It transmits FLP bursts to Auto-Negotiate for a while, then it configures for Fast Ethernet, attempts to link for a while, and then just listens. Some vendors do not offer any transmitted attempt to link until the interface first hears an FLP burst or some other signaling scheme.
There are two duplex modes, half and full. For shared media, the half-duplex mode is mandatory. All coaxial implementations are half duplex in nature and cannot operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only.
In half duplex only one station may transmit at a time. For the coaxial implementations a second station transmitting will cause the signals to overlap and become corrupted. Since UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time. Both stations in a point-to-point full-duplex link are permitted to transmit at any time, regardless of whether the other station is transmitting.
Auto-Negotiation avoids most situations where one station in a point-to-point link is transmitting under half-duplex rules and the other under full-duplex rules.
In the event that link partners are capable of sharing more than one common technology, refer to the list in Figure . This list is used to determine which technology should be chosen from the offered configurations.
Fiber-optic Ethernet implementations are not included in this priority resolution list because the interface electronics and optics do not permit easy reconfiguration between implementations. It is assumed that the interface configuration is fixed. If the two interfaces are able to Auto-Negotiate then they are already using the same Ethernet implementation. However, there remain a number of configuration choices such as the duplex setting, or which station will act as the Master for clocking purposes, that must be determined.
This page concludes this lesson. The next page will summarize the main points from the module.
6.2.10 This page will explain how links are established through Auto-Negotiation and introduce the two duplex modes.
Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation.
Auto-Negotiation is optional for most Ethernet implementations. Gigabit Ethernet requires its implementation, though the user may disable it. Auto-Negotiation was originally defined for UTP implementations of Ethernet and has been extended to work with other fiber optic implementations.
When an Auto-Negotiating station first attempts to link it is supposed to enable 100BASE-TX to attempt to immediately establish a link. If 100BASE-TX signaling is present, and the station supports 100BASE-TX, it will attempt to establish a link without negotiating. If either signaling produces a link or FLP bursts are received, the station will proceed with that technology. If a link partner does not offer an FLP burst, but instead offers NLPs, then that device is automatically assumed to be a 10BASE-T station. During this initial interval of testing for other technologies, the transmit path is sending FLP bursts. The standard does not permit parallel detection of any other technologies.
If a link is established through parallel detection, it is required to be half duplex. There are only two methods of achieving a full-duplex link. One method is through a completed cycle of Auto-Negotiation, and the other is to administratively force both link partners to full duplex. If one link partner is forced to full duplex, but the other partner attempts to Auto-Negotiate, then there is certain to be a duplex mismatch. This will result in collisions and errors on that link. Additionally if one end is forced to full duplex the other must also be forced. The exception to this is 10-Gigabit Ethernet, which does not support half duplex.
Many vendors implement hardware in such a way that it cycles through the various possible states. It transmits FLP bursts to Auto-Negotiate for a while, then it configures for Fast Ethernet, attempts to link for a while, and then just listens. Some vendors do not offer any transmitted attempt to link until the interface first hears an FLP burst or some other signaling scheme.
There are two duplex modes, half and full. For shared media, the half-duplex mode is mandatory. All coaxial implementations are half duplex in nature and cannot operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only.
In half duplex only one station may transmit at a time. For the coaxial implementations a second station transmitting will cause the signals to overlap and become corrupted. Since UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time. Both stations in a point-to-point full-duplex link are permitted to transmit at any time, regardless of whether the other station is transmitting.
Auto-Negotiation avoids most situations where one station in a point-to-point link is transmitting under half-duplex rules and the other under full-duplex rules.
In the event that link partners are capable of sharing more than one common technology, refer to the list in Figure . This list is used to determine which technology should be chosen from the offered configurations.
Fiber-optic Ethernet implementations are not included in this priority resolution list because the interface electronics and optics do not permit easy reconfiguration between implementations. It is assumed that the interface configuration is fixed. If the two interfaces are able to Auto-Negotiate then they are already using the same Ethernet implementation. However, there remain a number of configuration choices such as the duplex setting, or which station will act as the Master for clocking purposes, that must be determined.
This page concludes this lesson. The next page will summarize the main points from the module.
Ethernet auto-negotiation
Ethernet auto-negotiation
6.2.9 This page explains auto-negotiation and how it is accomplished.
As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto-Negotiation of speeds at half or full duplex was developed. Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer.
10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst. Each FLP burst is sent at the same timing interval as an NLP, and is intended to allow older 10BASE-T devices to operate normally in the event they should receive an FLP burst.
Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners. The burst communicates the capabilities of the transmitting station to its link partner. After both stations have interpreted what the other partner is offering, both switch to the highest performance common configuration and establish a link at that speed. If anything interrupts communications and the link is lost, the two link partners first attempt to link again at the last negotiated speed. If that fails, or if it has been too long since the link was lost, the Auto-Negotiation process starts over. The link may be lost due to external influences, such as a cable fault, or due to one of the partners issuing a reset.
The next page will discuss half and full duplex modes.
6.2.9 This page explains auto-negotiation and how it is accomplished.
As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto-Negotiation of speeds at half or full duplex was developed. Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer.
10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst. Each FLP burst is sent at the same timing interval as an NLP, and is intended to allow older 10BASE-T devices to operate normally in the event they should receive an FLP burst.
Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners. The burst communicates the capabilities of the transmitting station to its link partner. After both stations have interpreted what the other partner is offering, both switch to the highest performance common configuration and establish a link at that speed. If anything interrupts communications and the link is lost, the two link partners first attempt to link again at the last negotiated speed. If that fails, or if it has been too long since the link was lost, the Auto-Negotiation process starts over. The link may be lost due to external influences, such as a cable fault, or due to one of the partners issuing a reset.
The next page will discuss half and full duplex modes.
FCS and beyond
FCS and beyond
6.2.8 This page will focus on additional errors that occur on an Ethernet network.
A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded.
High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network. If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system.
A message that does not end on an octet boundary is known as an alignment error. Instead of the correct number of binary bits forming complete octet groupings, there are additional bits left over (less than eight). Such a frame is truncated to the nearest octet boundary, and if the FCS checksum fails, then an alignment error is reported. This is often caused by bad software drivers, or a collision, and is frequently accompanied by a failure of the FCS checksum.
A frame with a valid value in the Length field but did not match the actual number of octets counted in the data field of the received frame is known as a range error. This error also appears when the length field value is less than the minimum legal unpadded size of the data field. A similar error, Out of Range, is reported when the value in the Length field indicates a data size that is too large to be legal.
Fluke Networks has coined the term ghost to mean energy (noise) detected on the cable that appears to be a frame, but is lacking a valid SFD. To qualify as a ghost, the frame must be at least 72 octets long, including the preamble. Otherwise, it is classified as a remote collision. Because of the peculiar nature of ghosts, it is important to note that test results are largely dependent upon where on the segment the measurement is made.
Ground loops and other wiring problems are usually the cause of ghosting. Most network monitoring tools do not recognize the existence of ghosts for the same reason that they do not recognize preamble collisions. The tools rely entirely on what the chipset tells them. Software-only protocol analyzers, many hardware-based protocol analyzers, hand held diagnostic tools, as well as most remote monitoring (RMON) probes do not report these events.
The next page will describe Auto-Negotiation.
6.2.8 This page will focus on additional errors that occur on an Ethernet network.
A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded.
High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network. If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system.
A message that does not end on an octet boundary is known as an alignment error. Instead of the correct number of binary bits forming complete octet groupings, there are additional bits left over (less than eight). Such a frame is truncated to the nearest octet boundary, and if the FCS checksum fails, then an alignment error is reported. This is often caused by bad software drivers, or a collision, and is frequently accompanied by a failure of the FCS checksum.
A frame with a valid value in the Length field but did not match the actual number of octets counted in the data field of the received frame is known as a range error. This error also appears when the length field value is less than the minimum legal unpadded size of the data field. A similar error, Out of Range, is reported when the value in the Length field indicates a data size that is too large to be legal.
Fluke Networks has coined the term ghost to mean energy (noise) detected on the cable that appears to be a frame, but is lacking a valid SFD. To qualify as a ghost, the frame must be at least 72 octets long, including the preamble. Otherwise, it is classified as a remote collision. Because of the peculiar nature of ghosts, it is important to note that test results are largely dependent upon where on the segment the measurement is made.
Ground loops and other wiring problems are usually the cause of ghosting. Most network monitoring tools do not recognize the existence of ghosts for the same reason that they do not recognize preamble collisions. The tools rely entirely on what the chipset tells them. Software-only protocol analyzers, many hardware-based protocol analyzers, hand held diagnostic tools, as well as most remote monitoring (RMON) probes do not report these events.
The next page will describe Auto-Negotiation.
Ethernet errors
Ethernet errors
6.2.7 This page will define common Ethernet errors.
Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks.
The following are the sources of Ethernet error:
• Collision or runt – Simultaneous transmission occurring before slot time has elapsed
• Late collision – Simultaneous transmission occurring after slot time has elapsed
• Jabber, long frame and range errors – Excessively or illegally long transmission
• Short frame, collision fragment or runt – Illegally short transmission
• FCS error – Corrupted transmission
• Alignment error – Insufficient or excessive number of bits transmitted
• Range error – Actual and reported number of octets in frame do not match
• Ghost or jabber – Unusually long Preamble or Jam event
While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. The presence of errors on a network always suggests that further investigation is warranted. The severity of the problem indicates the troubleshooting urgency related to the detected errors. A handful of errors detected over many minutes or over hours would be a low priority. Thousands detected over a few minutes suggest that urgent attention is warranted.
Jabber is defined in several places in the 802.3 standard as being a transmission of at least 20,000 to 50,000 bit times in duration. However, most diagnostic tools report jabber whenever a detected transmission exceeds the maximum legal frame size, which is considerably smaller than 20,000 to 50,000 bit times. Most references to jabber are more properly called long frames.
A long frame is one that is longer than the maximum legal size, and takes into consideration whether or not the frame was tagged. It does not consider whether or not the frame had a valid FCS checksum. This error usually means that jabber was detected on the network.
A short frame is a frame smaller than the minimum legal size of 64 octets, with a good frame check sequence. Some protocol analyzers and network monitors call these frames “runts". In general the presence of short frames is not a guarantee that the network is failing.
The term runt is generally an imprecise slang term that means something less than a legal frame size. It may refer to short frames with a valid FCS checksum although it usually refers to collision fragments.
The next page will continue the discussion of Ethernet frame errors.
6.2.7 This page will define common Ethernet errors.
Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks.
The following are the sources of Ethernet error:
• Collision or runt – Simultaneous transmission occurring before slot time has elapsed
• Late collision – Simultaneous transmission occurring after slot time has elapsed
• Jabber, long frame and range errors – Excessively or illegally long transmission
• Short frame, collision fragment or runt – Illegally short transmission
• FCS error – Corrupted transmission
• Alignment error – Insufficient or excessive number of bits transmitted
• Range error – Actual and reported number of octets in frame do not match
• Ghost or jabber – Unusually long Preamble or Jam event
While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. The presence of errors on a network always suggests that further investigation is warranted. The severity of the problem indicates the troubleshooting urgency related to the detected errors. A handful of errors detected over many minutes or over hours would be a low priority. Thousands detected over a few minutes suggest that urgent attention is warranted.
Jabber is defined in several places in the 802.3 standard as being a transmission of at least 20,000 to 50,000 bit times in duration. However, most diagnostic tools report jabber whenever a detected transmission exceeds the maximum legal frame size, which is considerably smaller than 20,000 to 50,000 bit times. Most references to jabber are more properly called long frames.
A long frame is one that is longer than the maximum legal size, and takes into consideration whether or not the frame was tagged. It does not consider whether or not the frame had a valid FCS checksum. This error usually means that jabber was detected on the network.
A short frame is a frame smaller than the minimum legal size of 64 octets, with a good frame check sequence. Some protocol analyzers and network monitors call these frames “runts". In general the presence of short frames is not a guarantee that the network is failing.
The term runt is generally an imprecise slang term that means something less than a legal frame size. It may refer to short frames with a valid FCS checksum although it usually refers to collision fragments.
The next page will continue the discussion of Ethernet frame errors.
Types of collisions
Types of collisions
6.2.6 This page covers the different types of collisions and their characteristics.
Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are:
• Local
• Remote
• Late
To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters a signal from the other station. The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The doubling of the signal pushes the voltage level of the signal beyond the allowed maximum. This over-voltage condition is then sensed by all of the stations on the local cable segment as a collision.
In the beginning the waveform in Figure represents normal Manchester encoded data. A few cycles into the sample the amplitude of the wave doubles. That is the beginning of the collision, where the two waveforms are overlapping. Just prior to the end of the sample the amplitude returns to normal. This happens when the first station to detect the collision quits transmitting, and the jam signal from the second colliding station is still observed.
On UTP cable, such as 10BASE-T, 100BASE-TX and 1000BASE-T, a collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair. Since the two signals are on different pairs there is no characteristic change in the signal. Collisions are only recognized on UTP when the station is operating in half duplex. The only functional difference between half and full duplex operation in this regard is whether or not the transmit and receive pairs are permitted to be used simultaneously. If the station is not engaged in transmitting it cannot detect a local collision. Conversely, a cable fault such as excessive crosstalk can cause a station to perceive its own transmission as a local collision.
The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum, but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity. This sort of collision usually results from collisions occurring on the far side of a repeated connection. A repeater will not forward an over-voltage state, and cannot cause a station to have both the TX and RX pairs active at the same time. The station would have to be transmitting to have both pairs active, and that would constitute a local collision. On UTP networks this is the most common sort of collision observed.
There is no possibility remaining for a normal or legal collision after the first 64 octets of data has been transmitted by the sending stations. Collisions occurring after the first 64 octets are called “late collisions". The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late. As far as the NIC is concerned everything went out fine, and the upper layers of the protocol stack must determine that the frame was lost. Other than retransmission, a station detecting a late collision handles it in exactly the same way as a normal collision.
The next page will discuss the sources of Ethernet errors.
6.2.6 This page covers the different types of collisions and their characteristics.
Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are:
• Local
• Remote
• Late
To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters a signal from the other station. The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The doubling of the signal pushes the voltage level of the signal beyond the allowed maximum. This over-voltage condition is then sensed by all of the stations on the local cable segment as a collision.
In the beginning the waveform in Figure represents normal Manchester encoded data. A few cycles into the sample the amplitude of the wave doubles. That is the beginning of the collision, where the two waveforms are overlapping. Just prior to the end of the sample the amplitude returns to normal. This happens when the first station to detect the collision quits transmitting, and the jam signal from the second colliding station is still observed.
On UTP cable, such as 10BASE-T, 100BASE-TX and 1000BASE-T, a collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair. Since the two signals are on different pairs there is no characteristic change in the signal. Collisions are only recognized on UTP when the station is operating in half duplex. The only functional difference between half and full duplex operation in this regard is whether or not the transmit and receive pairs are permitted to be used simultaneously. If the station is not engaged in transmitting it cannot detect a local collision. Conversely, a cable fault such as excessive crosstalk can cause a station to perceive its own transmission as a local collision.
The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum, but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity. This sort of collision usually results from collisions occurring on the far side of a repeated connection. A repeater will not forward an over-voltage state, and cannot cause a station to have both the TX and RX pairs active at the same time. The station would have to be transmitting to have both pairs active, and that would constitute a local collision. On UTP networks this is the most common sort of collision observed.
There is no possibility remaining for a normal or legal collision after the first 64 octets of data has been transmitted by the sending stations. Collisions occurring after the first 64 octets are called “late collisions". The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late. As far as the NIC is concerned everything went out fine, and the upper layers of the protocol stack must determine that the frame was lost. Other than retransmission, a station detecting a late collision handles it in exactly the same way as a normal collision.
The next page will discuss the sources of Ethernet errors.
Error handling
Error handling
6.2.5 This page will describe collisions and how they are handled on a network.
The most common error condition on Ethernet networks are collisions. Collisions are the mechanism for resolving contention for network access. A few collisions provide a smooth, simple, low overhead way for network nodes to arbitrate contention for the network resource. When network contention becomes too great, collisions can become a significant impediment to useful network operation.
Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput.
The considerable majority of collisions occur very early in the frame, often before the SFD. Collisions occurring before the SFD are usually not reported to the higher layers, as if the collision did not occur. As soon as a collision is detected, the sending stations transmit a 32-bit “jam” signal that will enforce the collision. This is done so that any data being transmitted is thoroughly corrupted and all stations have a chance to detect the collision.
In Figure two stations listen to ensure that the cable is idle, then transmit. Station 1 was able to transmit a significant percentage of the frame before the signal even reached the last cable segment. Station 2 had not received the first bit of the transmission prior to beginning its own transmission and was only able to send several bits before the NIC sensed the collision. Station 2 immediately truncated the current transmission, substituted the 32-bit jam signal and ceased all transmissions. During the collision and jam event that Station 2 was experiencing, the collision fragments were working their way back through the repeated collision domain toward Station 1. Station 2 completed transmission of the 32-bit jam signal and became silent before the collision propagated back to Station 1 which was still unaware of the collision and continued to transmit. When the collision fragments finally reached Station 1, it also truncated the current transmission and substituted a 32-bit jam signal in place of the remainder of the frame it was transmitting. Upon sending the 32-bit jam signal Station 1 ceased all transmissions.
A jam signal may be composed of any binary data so long as it does not form a proper checksum for the portion of the frame already transmitted. The most commonly observed data pattern for a jam signal is simply a repeating one, zero, one, zero pattern, the same as Preamble. When viewed by a protocol analyzer this pattern appears as either a repeating hexadecimal 5 or A sequence. The corrupted, partially transmitted messages are often referred to as collision fragments or runts. Normal collisions are less than 64 octets in length and therefore fail both the minimum length test and the FCS checksum test.
The next page will define different types of collisions.
6.2.5 This page will describe collisions and how they are handled on a network.
The most common error condition on Ethernet networks are collisions. Collisions are the mechanism for resolving contention for network access. A few collisions provide a smooth, simple, low overhead way for network nodes to arbitrate contention for the network resource. When network contention becomes too great, collisions can become a significant impediment to useful network operation.
Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput.
The considerable majority of collisions occur very early in the frame, often before the SFD. Collisions occurring before the SFD are usually not reported to the higher layers, as if the collision did not occur. As soon as a collision is detected, the sending stations transmit a 32-bit “jam” signal that will enforce the collision. This is done so that any data being transmitted is thoroughly corrupted and all stations have a chance to detect the collision.
In Figure two stations listen to ensure that the cable is idle, then transmit. Station 1 was able to transmit a significant percentage of the frame before the signal even reached the last cable segment. Station 2 had not received the first bit of the transmission prior to beginning its own transmission and was only able to send several bits before the NIC sensed the collision. Station 2 immediately truncated the current transmission, substituted the 32-bit jam signal and ceased all transmissions. During the collision and jam event that Station 2 was experiencing, the collision fragments were working their way back through the repeated collision domain toward Station 1. Station 2 completed transmission of the 32-bit jam signal and became silent before the collision propagated back to Station 1 which was still unaware of the collision and continued to transmit. When the collision fragments finally reached Station 1, it also truncated the current transmission and substituted a 32-bit jam signal in place of the remainder of the frame it was transmitting. Upon sending the 32-bit jam signal Station 1 ceased all transmissions.
A jam signal may be composed of any binary data so long as it does not form a proper checksum for the portion of the frame already transmitted. The most commonly observed data pattern for a jam signal is simply a repeating one, zero, one, zero pattern, the same as Preamble. When viewed by a protocol analyzer this pattern appears as either a repeating hexadecimal 5 or A sequence. The corrupted, partially transmitted messages are often referred to as collision fragments or runts. Normal collisions are less than 64 octets in length and therefore fail both the minimum length test and the FCS checksum test.
The next page will define different types of collisions.
Interframe spacing and backoff
Interframe spacing and backoff
6.2.4 This page explains how spacing is used in an Ethernet network for data transmission.
The minimum spacing between two non-colliding frames is also called the interframe spacing. This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame.
After a frame has been sent, all stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame. On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter. This interval is referred to as the spacing gap. The gap is intended to allow slow stations time to process the previous frame and prepare for the next frame.
A repeater is expected to regenerate the full 64 bits of timing information, which is the preamble and SFD, at the start of any frame. This is despite the potential loss of some of the beginning preamble bits because of slow synchronization. Because of this forced reintroduction of timing bits, some minor reduction of the interframe gap is not only possible but expected. Some Ethernet chipsets are sensitive to a shortening of the interframe spacing, and will begin failing to see frames as the gap is reduced. With the increase in processing power at the desktop, it would be very easy for a personal computer to saturate an Ethernet segment with traffic and to begin transmitting again before the interframe spacing delay time is satisfied.
After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing), then the stations that collided must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame. The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions. This is accomplished in part by expanding the interval from which the random retransmission time is selected on each retransmission attempt. The waiting period is measured in increments of the parameter slot time.
If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer. Such an occurrence is fairly rare and would happen only under extremely heavy network loads, or when a physical problem exists on the network.
The next page will discuss collisions.
6.2.4 This page explains how spacing is used in an Ethernet network for data transmission.
The minimum spacing between two non-colliding frames is also called the interframe spacing. This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame.
After a frame has been sent, all stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame. On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter. This interval is referred to as the spacing gap. The gap is intended to allow slow stations time to process the previous frame and prepare for the next frame.
A repeater is expected to regenerate the full 64 bits of timing information, which is the preamble and SFD, at the start of any frame. This is despite the potential loss of some of the beginning preamble bits because of slow synchronization. Because of this forced reintroduction of timing bits, some minor reduction of the interframe gap is not only possible but expected. Some Ethernet chipsets are sensitive to a shortening of the interframe spacing, and will begin failing to see frames as the gap is reduced. With the increase in processing power at the desktop, it would be very easy for a personal computer to saturate an Ethernet segment with traffic and to begin transmitting again before the interframe spacing delay time is satisfied.
After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing), then the stations that collided must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame. The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions. This is accomplished in part by expanding the interval from which the random retransmission time is selected on each retransmission attempt. The waiting period is measured in increments of the parameter slot time.
If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer. Such an occurrence is fairly rare and would happen only under extremely heavy network loads, or when a physical problem exists on the network.
The next page will discuss collisions.
Ethernet timing
Ethernet timing
6.2.3 This page explains the importance of slot times in an Ethernet network.
The basic rules and specifications for proper operation of Ethernet are not particularly complicated, though some of the faster physical layer implementations are becoming so. Despite the basic simplicity, when a problem occurs in Ethernet it is often quite difficult to isolate the source. Because of the common bus architecture of Ethernet, also described as a distributed single point of failure, the scope of the problem usually encompasses all devices within the collision domain. In situations where repeaters are used, this can include devices up to four segments away.
Any station on an Ethernet network wishing to transmit a message first “listens” to ensure that no other station is currently transmitting. If the cable is quiet, the station will begin transmitting immediately. The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next. Because of the delay and latency, it is possible for more than one station to begin transmitting at or near the same time. This results in a collision.
If the attached station is operating in full duplex then the station may send and receive simultaneously and collisions should not occur. Full-duplex operation also changes the timing considerations and eliminates the concept of slot time. Full-duplex operation allows for larger network architecture designs since the timing restriction for collision detection is removed.
In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble. The sending station will then transmit the following information:
• Destination and source MAC addressing information
• Certain other header information
• The actual data payload
• Checksum (FCS) used to ensure that the message was not corrupted along the way
Stations receiving the frame recalculate the FCS to determine if the incoming message is valid and then pass valid messages to the next higher layer in the protocol stack.
10 Mbps and slower versions of Ethernet are asynchronous. Asynchronous means that each receiving station will use the eight octets of timing information to synchronize the receive circuit to the incoming data, and then discard it. 100 Mbps and higher speed implementations of Ethernet are synchronous. Synchronous means the timing information is not required, however for compatibility reasons the Preamble and Start Frame Delimiter (SFD) are present.
For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes how a transmission may be no smaller than the slot time. Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or 64 octets. Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512 octets. Slot time is calculated assuming maximum cable lengths on the largest legal network architecture. All hardware propagation delay times are at the legal maximum and the 32-bit jam signal is used when collisions are detected.
The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected. For the system to work the first station must learn about the collision before it finishes sending the smallest legal frame size. To allow 1000-Mbps Ethernet to operate in half duplex the extension field was added when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return. This field is present only on 1000-Mbps, half-duplex links and allows minimum-sized frames to be long enough to meet slot time requirements. Extension bits are discarded by the receiving station.
On 10-Mbps Ethernet one bit at the MAC layer requires 100 nanoseconds (ns) to transmit. At 100 Mbps that same bit requires 10 ns to transmit and at 1000 Mbps only takes 1 ns. As a rough estimate, 20.3 cm (8 in) per nanosecond is often used for calculating propagation delay down a UTP cable. For 100 meters of UTP, this means that it takes just under 5 bit-times for a 10BASE-T signal to travel the length the cable.
For CSMA/CD Ethernet to operate, the sending station must become aware of a collision before it has completed transmission of a minimum-sized frame. At 100 Mbps the system timing is barely able to accommodate 100 meter cables. At 1000 Mbps special adjustments are required as nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. For this reason half duplex is not permitted in 10-Gigabit Ethernet.
The next page defines interframe spacing and backoff.
6.2.3 This page explains the importance of slot times in an Ethernet network.
The basic rules and specifications for proper operation of Ethernet are not particularly complicated, though some of the faster physical layer implementations are becoming so. Despite the basic simplicity, when a problem occurs in Ethernet it is often quite difficult to isolate the source. Because of the common bus architecture of Ethernet, also described as a distributed single point of failure, the scope of the problem usually encompasses all devices within the collision domain. In situations where repeaters are used, this can include devices up to four segments away.
Any station on an Ethernet network wishing to transmit a message first “listens” to ensure that no other station is currently transmitting. If the cable is quiet, the station will begin transmitting immediately. The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next. Because of the delay and latency, it is possible for more than one station to begin transmitting at or near the same time. This results in a collision.
If the attached station is operating in full duplex then the station may send and receive simultaneously and collisions should not occur. Full-duplex operation also changes the timing considerations and eliminates the concept of slot time. Full-duplex operation allows for larger network architecture designs since the timing restriction for collision detection is removed.
In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble. The sending station will then transmit the following information:
• Destination and source MAC addressing information
• Certain other header information
• The actual data payload
• Checksum (FCS) used to ensure that the message was not corrupted along the way
Stations receiving the frame recalculate the FCS to determine if the incoming message is valid and then pass valid messages to the next higher layer in the protocol stack.
10 Mbps and slower versions of Ethernet are asynchronous. Asynchronous means that each receiving station will use the eight octets of timing information to synchronize the receive circuit to the incoming data, and then discard it. 100 Mbps and higher speed implementations of Ethernet are synchronous. Synchronous means the timing information is not required, however for compatibility reasons the Preamble and Start Frame Delimiter (SFD) are present.
For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes how a transmission may be no smaller than the slot time. Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or 64 octets. Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512 octets. Slot time is calculated assuming maximum cable lengths on the largest legal network architecture. All hardware propagation delay times are at the legal maximum and the 32-bit jam signal is used when collisions are detected.
The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected. For the system to work the first station must learn about the collision before it finishes sending the smallest legal frame size. To allow 1000-Mbps Ethernet to operate in half duplex the extension field was added when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return. This field is present only on 1000-Mbps, half-duplex links and allows minimum-sized frames to be long enough to meet slot time requirements. Extension bits are discarded by the receiving station.
On 10-Mbps Ethernet one bit at the MAC layer requires 100 nanoseconds (ns) to transmit. At 100 Mbps that same bit requires 10 ns to transmit and at 1000 Mbps only takes 1 ns. As a rough estimate, 20.3 cm (8 in) per nanosecond is often used for calculating propagation delay down a UTP cable. For 100 meters of UTP, this means that it takes just under 5 bit-times for a 10BASE-T signal to travel the length the cable.
For CSMA/CD Ethernet to operate, the sending station must become aware of a collision before it has completed transmission of a minimum-sized frame. At 100 Mbps the system timing is barely able to accommodate 100 meter cables. At 1000 Mbps special adjustments are required as nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. For this reason half duplex is not permitted in 10-Gigabit Ethernet.
The next page defines interframe spacing and backoff.
MAC / MAC rules and collision detection/backoff
MAC
6.2.1 This page will define MAC and provide examples of deterministic and non-deterministic MAC protocols.
MAC refers to protocols that determine which computer in a shared-media environment, or collision domain, is allowed to transmit data. MAC and LLC comprise the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. The two broad categories of MAC are deterministic and non-deterministic.
Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment since only one host can transmit at a time.
Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for the absence of a signal on the media and begins to transmit. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit.
Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows:
• Ethernet – uses a logical bus topology to control information flow on a linear bus and a physical star or extended star topology for the cables
• Token Ring – uses a logical ring topology to control information flow and a physical star topology
• FDDI – uses a logical ring topology to control information flow and a physical dual-ring topology
The next page explains how collisions are avoided in an Ethernet network.
MAC rules and collision detection/backoff
6.2.2 This page describes collision detection and avoidance in a CSMA/CD network.
Ethernet is a shared-media broadcast technology. The access method CSMA/CD used in Ethernet performs three functions:
• Transmitting and receiving data packets
• Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
• Detecting errors within data packets or on the network
In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must first check to see whether the networking media is busy. If the node determines the network is busy, the node will wait a random amount of time before retrying. If the node determines the networking media is not busy, the node will begin transmitting and listening. The node listens to ensure no other stations are transmitting at the same time. After completing data transmission the device will return to listening mode.
Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all nodes detect the collision. When all nodes have detected the collision, the backoff algorithm is invoked and transmission stops. The nodes stop transmitting for a random period of time, determined by the backoff algorithm. When the delay periods expire, each node can attempt to access the networking media. The devices that were involved in the collision do not have transmission priority.
The Interactive Media Activity shows the procedure for collision detection in an Ethernet network.
The next page will discuss Ethernet timing.
6.2.1 This page will define MAC and provide examples of deterministic and non-deterministic MAC protocols.
MAC refers to protocols that determine which computer in a shared-media environment, or collision domain, is allowed to transmit data. MAC and LLC comprise the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. The two broad categories of MAC are deterministic and non-deterministic.
Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment since only one host can transmit at a time.
Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for the absence of a signal on the media and begins to transmit. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit.
Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows:
• Ethernet – uses a logical bus topology to control information flow on a linear bus and a physical star or extended star topology for the cables
• Token Ring – uses a logical ring topology to control information flow and a physical star topology
• FDDI – uses a logical ring topology to control information flow and a physical dual-ring topology
The next page explains how collisions are avoided in an Ethernet network.
MAC rules and collision detection/backoff
6.2.2 This page describes collision detection and avoidance in a CSMA/CD network.
Ethernet is a shared-media broadcast technology. The access method CSMA/CD used in Ethernet performs three functions:
• Transmitting and receiving data packets
• Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
• Detecting errors within data packets or on the network
In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must first check to see whether the networking media is busy. If the node determines the network is busy, the node will wait a random amount of time before retrying. If the node determines the networking media is not busy, the node will begin transmitting and listening. The node listens to ensure no other stations are transmitting at the same time. After completing data transmission the device will return to listening mode.
Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all nodes detect the collision. When all nodes have detected the collision, the backoff algorithm is invoked and transmission stops. The nodes stop transmitting for a random period of time, determined by the backoff algorithm. When the delay periods expire, each node can attempt to access the networking media. The devices that were involved in the collision do not have transmission priority.
The Interactive Media Activity shows the procedure for collision detection in an Ethernet network.
The next page will discuss Ethernet timing.
Saturday, January 16, 2010
Ethernet frame structure / Ethernet frame fields
Ethernet frame structure
6.1.6 This page will describe the frame structure of Ethernet networks.
At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are very different. Each speed has a distinct set of architecture design rules.
In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start-of-Frame (SOF) Delimiter were combined into a single field. The binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version since both uses were common.
The Ethernet II Type field is incorporated into the current 802.3 frame definition. When a node receives a frame it must examine the Length/Type field to determine which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal, 1536 decimal, then the contents of the Data Field are decoded according to the protocol indicated.
The next page will discuss the information included in a frame.
Ethernet frame fields
6.1.7 This page defines the fields that are used in a frame.
Some of the fields permitted or required in an 802.3 Ethernet frame are as follows:
• Preamble
• SOF Delimiter
• Destination Address
• Source Address
• Length/Type
• Header and Data
• FCS
• Extension
The preamble is an alternating pattern of ones and zeros used to time synchronization in 10 Mbps and slower implementations of Ethernet. Faster versions of Ethernet are synchronous so this timing information is unnecessary but retained for compatibility.
A SOF delimiter consists of a one-octet field that marks the end of the timing information and contains the bit sequence 10101011.
The destination address can be unicast, multicast, or broadcast.
The Source Address field contains the MAC source address. The source address is generally the unicast address of the Ethernet node that transmitted the frame. However, many virtual protocols use and sometimes share a specific source MAC address to identify the virtual entity.
The Length/Type field supports two different uses. If the value is less than 1536 decimal, 0x600 hexadecimal, then the value indicates length. The length interpretation is used when the LLC layer provides the protocol identification. The type value indicates which upper-layer protocol will receive the data after the Ethernet process is complete. The length indicates the number of bytes of data that follows this field.
The Data field and padding if necessary, may be of any length that does not cause the frame to exceed the maximum frame size. The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified amount of data is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. This extra data is called a pad. Ethernet requires each frame to be between 64 and 1518 octets.
A FCS contains a 4-byte CRC value that is created by the device that sends data and is recalculated by the destination device to check for damaged frames. The corruption of a single bit anywhere from the start of the Destination Address through the end of the FCS field will cause the checksum to be different. Therefore, the coverage of the FCS includes itself. It is not possible to distinguish between corruption of the FCS and corruption of any other field used in the calculation.
This page concludes this lesson. The next lesson will discuss the functions of an Ethernet network. The first page will introduce the concept of MAC.
6.1.6 This page will describe the frame structure of Ethernet networks.
At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are very different. Each speed has a distinct set of architecture design rules.
In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start-of-Frame (SOF) Delimiter were combined into a single field. The binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version since both uses were common.
The Ethernet II Type field is incorporated into the current 802.3 frame definition. When a node receives a frame it must examine the Length/Type field to determine which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal, 1536 decimal, then the contents of the Data Field are decoded according to the protocol indicated.
The next page will discuss the information included in a frame.
Ethernet frame fields
6.1.7 This page defines the fields that are used in a frame.
Some of the fields permitted or required in an 802.3 Ethernet frame are as follows:
• Preamble
• SOF Delimiter
• Destination Address
• Source Address
• Length/Type
• Header and Data
• FCS
• Extension
The preamble is an alternating pattern of ones and zeros used to time synchronization in 10 Mbps and slower implementations of Ethernet. Faster versions of Ethernet are synchronous so this timing information is unnecessary but retained for compatibility.
A SOF delimiter consists of a one-octet field that marks the end of the timing information and contains the bit sequence 10101011.
The destination address can be unicast, multicast, or broadcast.
The Source Address field contains the MAC source address. The source address is generally the unicast address of the Ethernet node that transmitted the frame. However, many virtual protocols use and sometimes share a specific source MAC address to identify the virtual entity.
The Length/Type field supports two different uses. If the value is less than 1536 decimal, 0x600 hexadecimal, then the value indicates length. The length interpretation is used when the LLC layer provides the protocol identification. The type value indicates which upper-layer protocol will receive the data after the Ethernet process is complete. The length indicates the number of bytes of data that follows this field.
The Data field and padding if necessary, may be of any length that does not cause the frame to exceed the maximum frame size. The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified amount of data is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. This extra data is called a pad. Ethernet requires each frame to be between 64 and 1518 octets.
A FCS contains a 4-byte CRC value that is created by the device that sends data and is recalculated by the destination device to check for damaged frames. The corruption of a single bit anywhere from the start of the Destination Address through the end of the FCS field will cause the checksum to be different. Therefore, the coverage of the FCS includes itself. It is not possible to distinguish between corruption of the FCS and corruption of any other field used in the calculation.
This page concludes this lesson. The next lesson will discuss the functions of an Ethernet network. The first page will introduce the concept of MAC.
Layer 2 framing
Layer 2 framing
6.1.5 This page will explain how frames are created at Layer 2 of the OSI model.
Encoded bit streams, or data, on physical media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen. Framing provides essential information that could not be obtained from coded bit streams alone. This information includes the following:
• Which computers are in communication with each other
• When communication between individual computers begins and when it ends
• Which errors occurred while the computers communicated
• Which computer will communicate next
Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit.
A voltage versus time graph could be used to visualize bits. However, it may be too difficult to graph address and control information for larger units of data. Another type of diagram that could be used is the frame format diagram, which is based on voltage versus time graphs. Frame format diagrams are read from left to right, just like an oscilloscope graph. The frame format diagram shows different groupings of bits, or fields, that perform other functions.
There are many different types of frames described by various standards.A single generic frame has sections called fields. Each field is composed of bytes. The names of the fields are as follows:
• Start Frame field
• Address field
• Length/Type field
• Data field
• Frame Check Sequence (FCS) field
When computers are connected to a physical medium, there must be a way to inform other computers when they are about to transmit a frame. Various technologies do this in different ways. Regardless of the technology, all frames begin with a sequence of bytes to signal the data transmission.
All frames contain naming information, such as the name of the source node, or source MAC address, and the name of the destination node, or destination MAC address.
Most frames have some specialized fields. In some technologies, a Length field specifies the exact length of a frame in bytes. Some frames have a Type field, which specifies the Layer 3 protocol used by the device that wants to send data.
Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message to be sent, or user application data. Extra bytes may be added so frames have a minimum length for timing purposes. LLC bytes are also included with the Data field in the IEEE standard frames. The LLC sublayer takes the network protocol data, which is an IP packet, and adds control information to help deliver the packet to the destination node. Layer 2 communicates with the upper layers through LLC.
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded.
Because the source cannot detect that the frame has been discarded, retransmission has to be initiated by higher layer connection-oriented protocols providing data flow control. Because these protocols, such as TCP, expect frame acknowledgment, ACK, to be sent by the peer station within a certain time, retransmission usually occurs.
There are three primary ways to calculate the FCS number:
• Cyclic redundancy check (CRC) – performs calculations on the data.
• Two-dimensional parity – places individual bytes in a two-dimensional array and performs redundancy checks vertically and horizontally on the array, creating an extra byte resulting in an even or odd number of binary 1s.
• Internet checksum – adds the values of all of the data bits to arrive at a sum.
The node that transmits data must get the attention of other devices to start and end a frame. The Length field indicates where the frame ends. The frame ends after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter.
The next page will discuss the frame structure of an Ethernet network.
6.1.5 This page will explain how frames are created at Layer 2 of the OSI model.
Encoded bit streams, or data, on physical media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen. Framing provides essential information that could not be obtained from coded bit streams alone. This information includes the following:
• Which computers are in communication with each other
• When communication between individual computers begins and when it ends
• Which errors occurred while the computers communicated
• Which computer will communicate next
Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit.
A voltage versus time graph could be used to visualize bits. However, it may be too difficult to graph address and control information for larger units of data. Another type of diagram that could be used is the frame format diagram, which is based on voltage versus time graphs. Frame format diagrams are read from left to right, just like an oscilloscope graph. The frame format diagram shows different groupings of bits, or fields, that perform other functions.
There are many different types of frames described by various standards.A single generic frame has sections called fields. Each field is composed of bytes. The names of the fields are as follows:
• Start Frame field
• Address field
• Length/Type field
• Data field
• Frame Check Sequence (FCS) field
When computers are connected to a physical medium, there must be a way to inform other computers when they are about to transmit a frame. Various technologies do this in different ways. Regardless of the technology, all frames begin with a sequence of bytes to signal the data transmission.
All frames contain naming information, such as the name of the source node, or source MAC address, and the name of the destination node, or destination MAC address.
Most frames have some specialized fields. In some technologies, a Length field specifies the exact length of a frame in bytes. Some frames have a Type field, which specifies the Layer 3 protocol used by the device that wants to send data.
Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message to be sent, or user application data. Extra bytes may be added so frames have a minimum length for timing purposes. LLC bytes are also included with the Data field in the IEEE standard frames. The LLC sublayer takes the network protocol data, which is an IP packet, and adds control information to help deliver the packet to the destination node. Layer 2 communicates with the upper layers through LLC.
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded.
Because the source cannot detect that the frame has been discarded, retransmission has to be initiated by higher layer connection-oriented protocols providing data flow control. Because these protocols, such as TCP, expect frame acknowledgment, ACK, to be sent by the peer station within a certain time, retransmission usually occurs.
There are three primary ways to calculate the FCS number:
• Cyclic redundancy check (CRC) – performs calculations on the data.
• Two-dimensional parity – places individual bytes in a two-dimensional array and performs redundancy checks vertically and horizontally on the array, creating an extra byte resulting in an even or odd number of binary 1s.
• Internet checksum – adds the values of all of the data bits to arrive at a sum.
The node that transmits data must get the attention of other devices to start and end a frame. The Length field indicates where the frame ends. The frame ends after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter.
The next page will discuss the frame structure of an Ethernet network.
Naming
Naming
6.1.4 This page will discuss the MAC addresses used by Ethernet networks.
An address system is required to uniquely identify computers and interfaces to allow for local delivery of frames on the Ethernet. Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer. MAC addresses are sometimes referred to as burned-in MAC addresses (BIAs) because they are burned into ROM and are copied into RAM when the NIC initializes.
At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. The data from upper layers is encapsulated within the data link frame, between the header and trailer, and then sent out on the network.
The NIC uses the MAC address to determine if a message should be passed on to the upper layers of the OSI model. The NIC does not use CPU processing time to make this assessment. This enables better communication times on an Ethernet network.
When a device sends data on an Ethernet network, it can use the destination MAC address to open a communication pathway to the other device. The source device attaches a header with the MAC address of the intended destination and sends data through the network. As this data travels along the network media the NIC in each device checks to see if the MAC address matches the physical destination address carried by the data frame. If there is no match, the NIC discards the data frame. When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers. On an Ethernet network, all nodes must examine the MAC header.
All devices that are connected to the Ethernet LAN have MAC addressed interfaces. This includes workstations, printers, routers, and switches. The next page will focus on Layer 2 frames.
6.1.4 This page will discuss the MAC addresses used by Ethernet networks.
An address system is required to uniquely identify computers and interfaces to allow for local delivery of frames on the Ethernet. Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer. MAC addresses are sometimes referred to as burned-in MAC addresses (BIAs) because they are burned into ROM and are copied into RAM when the NIC initializes.
At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. The data from upper layers is encapsulated within the data link frame, between the header and trailer, and then sent out on the network.
The NIC uses the MAC address to determine if a message should be passed on to the upper layers of the OSI model. The NIC does not use CPU processing time to make this assessment. This enables better communication times on an Ethernet network.
When a device sends data on an Ethernet network, it can use the destination MAC address to open a communication pathway to the other device. The source device attaches a header with the MAC address of the intended destination and sends data through the network. As this data travels along the network media the NIC in each device checks to see if the MAC address matches the physical destination address carried by the data frame. If there is no match, the NIC discards the data frame. When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers. On an Ethernet network, all nodes must examine the MAC header.
All devices that are connected to the Ethernet LAN have MAC addressed interfaces. This includes workstations, printers, routers, and switches. The next page will focus on Layer 2 frames.
Ethernet and the OSI model
6.1.3 This page will explain how Ethernet relates to the OSI model.
Ethernet operates in two areas of the OSI model. These are the lower half of the data link layer, which is known as the MAC sublayer, and the physical layer.
Data that moves from one Ethernet station to another often passes through a repeater. All stations in the same collision domain see traffic that passes through a repeater. A collision domain is a shared resource. Problems that originate in one part of a collision domain will usually impact the entire collision domain.
A repeater forwards traffic to all other ports. A repeater never sends traffic out the same port from which it was received. Any signal detected by a repeater will be forwarded. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal.
To guarantee minimum bandwidth and operability, standards specify the maximum number of stations per segment, maximum segment length, and maximum number of repeaters between stations. Stations separated by bridges or routers are in different collision domains.
Figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. Ethernet at Layer 1 involves signals, bit streams that travel on the media, components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations. Layer 2 addresses these limitations.
Data link sublayers contribute significantly to technological compatibility and computer communications. The MAC sublayer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.
Figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. While there are other varieties of Ethernet, the ones shown are the most widely used.
IEEE Ethernet naming rules
IEEE Ethernet naming rules
6.1.2 This page focuses on the Ethernet naming rules developed by IEEE.
Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet.
When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. An abbreviated description, called an identifier, is also assigned to the supplement.
The abbreviated description consists of the following elements:
• A number that indicates the number of Mbps transmitted
• The word base to indicate that baseband signaling is used
• One or more letters of the alphabet indicating the type of medium used. For example, F = fiber optical cable and T = copper unshielded twisted pair
Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. The data signal is transmitted directly over the transmission medium.
In broadband signaling, the data signal is no longer placed directly on the transmission medium. Ethernet used broadband signaling in the 10BROAD36 standard. 10BROAD36 is the IEEE standard for an 802.3 Ethernet network using broadband transmission with thick coaxial cable running at 10 Mbps. 10BROAD36 is now considered obsolete. An analog or carrier signal is modulated by the data signal and then transmitted. Radio broadcasts and cable TV use broadband signaling.
IEEE cannot force manufacturers to fully comply with any standard. IEEE has two main objectives:
• Supply the information necessary to build devices that comply with Ethernet standards
• Promote innovation among manufacturers
6.1.2 This page focuses on the Ethernet naming rules developed by IEEE.
Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet.
When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. An abbreviated description, called an identifier, is also assigned to the supplement.
The abbreviated description consists of the following elements:
• A number that indicates the number of Mbps transmitted
• The word base to indicate that baseband signaling is used
• One or more letters of the alphabet indicating the type of medium used. For example, F = fiber optical cable and T = copper unshielded twisted pair
Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. The data signal is transmitted directly over the transmission medium.
In broadband signaling, the data signal is no longer placed directly on the transmission medium. Ethernet used broadband signaling in the 10BROAD36 standard. 10BROAD36 is the IEEE standard for an 802.3 Ethernet network using broadband transmission with thick coaxial cable running at 10 Mbps. 10BROAD36 is now considered obsolete. An analog or carrier signal is modulated by the data signal and then transmitted. Radio broadcasts and cable TV use broadband signaling.
IEEE cannot force manufacturers to fully comply with any standard. IEEE has two main objectives:
• Supply the information necessary to build devices that comply with Ethernet standards
• Promote innovation among manufacturers
Friday, January 8, 2010
Introduction to Ethernet
Introduction to Ethernet
6.1.1 This page provides an introduction to Ethernet. Most of the traffic on the Internet originates and ends with Ethernet connections. Since it began in the 1970s, Ethernet has evolved to meet the increased demand for high-speed LANs. When optical fiber media was introduced, Ethernet adapted to take advantage of the superior bandwidth and low error rate that fiber offers. Now the same protocol that transported data at 3 Mbps in 1973 can carry data at 10 Gbps.
The success of Ethernet is due to the following factors:
• Simplicity and ease of maintenance
• Ability to incorporate new technologies
• Reliability
• Low cost of installation and upgrade
The introduction of Gigabit Ethernet has extended the original LAN technology to distances that make Ethernet a MAN and WAN standard.
The original idea for Ethernet was to allow two or more hosts to use the same medium with no interference between the signals. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii. A system called Alohanet was developed to allow various stations on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere. This work later formed the basis for the Ethernet access method known as CSMA/CD.
The first LAN in the world was the original version of Ethernet. Robert Metcalfe and his coworkers at Xerox designed it more than thirty years ago. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX). Metcalfe wanted Ethernet to be a shared standard from which everyone could benefit, so it was released as an open standard. The first products that were developed from the Ethernet standard were sold in the early 1980s. Ethernet transmitted at up to 10 Mbps over thick coaxial cable up to a distance of 2 kilometers (km). This type of coaxial cable was referred to as thicknet and was about the width of a small finger.
In 1985, the IEEE standards committee for Local and Metropolitan Networks published standards for LANs. These standards start with the number 802. The standard for Ethernet is 802.3. The IEEE wanted to make sure that its standards were compatible with the International Standards Organization (ISO) and OSI model. To do this, the IEEE 802.3 standard had to address the needs of Layer 1 and the lower portion of Layer 2 of the OSI model. As a result, some small modifications to the original Ethernet standard were made in 802.3.
The differences between the two standards were so minor that any Ethernet NIC can transmit and receive both Ethernet and 802.3 frames. Essentially, Ethernet and IEEE 802.3 are the same standards.
The 10-Mbps bandwidth of Ethernet was more than enough for the slow PCs of the 1980s. By the early 1990s PCs became much faster, file sizes increased, and data flow bottlenecks occurred. Most were caused by the low availability of bandwidth. In 1995, IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for Gigabit Ethernet in 1998 and 1999.
All the standards are essentially compatible with the original Ethernet standard. An Ethernet frame could leave an older coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps Ethernet fiber link, and end up at a 100-Mbps NIC. As long as the packet stays on Ethernet networks it is not changed. For this reason Ethernet is considered very scalable. The bandwidth of the network could be increased many times while the Ethernet technology remains the same.
The original Ethernet standard has been amended many times to manage new media and higher transmission rates. These amendments provide standards for new technologies and maintain compatibility between Ethernet variations.
The next page explains the naming rules for the Ethernet family of networks.
6.1.1 This page provides an introduction to Ethernet. Most of the traffic on the Internet originates and ends with Ethernet connections. Since it began in the 1970s, Ethernet has evolved to meet the increased demand for high-speed LANs. When optical fiber media was introduced, Ethernet adapted to take advantage of the superior bandwidth and low error rate that fiber offers. Now the same protocol that transported data at 3 Mbps in 1973 can carry data at 10 Gbps.
The success of Ethernet is due to the following factors:
• Simplicity and ease of maintenance
• Ability to incorporate new technologies
• Reliability
• Low cost of installation and upgrade
The introduction of Gigabit Ethernet has extended the original LAN technology to distances that make Ethernet a MAN and WAN standard.
The original idea for Ethernet was to allow two or more hosts to use the same medium with no interference between the signals. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii. A system called Alohanet was developed to allow various stations on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere. This work later formed the basis for the Ethernet access method known as CSMA/CD.
The first LAN in the world was the original version of Ethernet. Robert Metcalfe and his coworkers at Xerox designed it more than thirty years ago. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX). Metcalfe wanted Ethernet to be a shared standard from which everyone could benefit, so it was released as an open standard. The first products that were developed from the Ethernet standard were sold in the early 1980s. Ethernet transmitted at up to 10 Mbps over thick coaxial cable up to a distance of 2 kilometers (km). This type of coaxial cable was referred to as thicknet and was about the width of a small finger.
In 1985, the IEEE standards committee for Local and Metropolitan Networks published standards for LANs. These standards start with the number 802. The standard for Ethernet is 802.3. The IEEE wanted to make sure that its standards were compatible with the International Standards Organization (ISO) and OSI model. To do this, the IEEE 802.3 standard had to address the needs of Layer 1 and the lower portion of Layer 2 of the OSI model. As a result, some small modifications to the original Ethernet standard were made in 802.3.
The differences between the two standards were so minor that any Ethernet NIC can transmit and receive both Ethernet and 802.3 frames. Essentially, Ethernet and IEEE 802.3 are the same standards.
The 10-Mbps bandwidth of Ethernet was more than enough for the slow PCs of the 1980s. By the early 1990s PCs became much faster, file sizes increased, and data flow bottlenecks occurred. Most were caused by the low availability of bandwidth. In 1995, IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for Gigabit Ethernet in 1998 and 1999.
All the standards are essentially compatible with the original Ethernet standard. An Ethernet frame could leave an older coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps Ethernet fiber link, and end up at a 100-Mbps NIC. As long as the packet stays on Ethernet networks it is not changed. For this reason Ethernet is considered very scalable. The bandwidth of the network could be increased many times while the Ethernet technology remains the same.
The original Ethernet standard has been amended many times to manage new media and higher transmission rates. These amendments provide standards for new technologies and maintain compatibility between Ethernet variations.
The next page explains the naming rules for the Ethernet family of networks.
Module 6: Ethernet Fundamentals
Overview of Module 6 Ethernet Fundamentals
Ethernet is now the dominant LAN technology in the world. Ethernet is a family of LAN technologies that may be best understood with the OSI reference model. All LANs must deal with the basic issue of how individual stations, or nodes, are named. Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations. However, the basic frame format and address scheme is the same for all varieties of Ethernet.
Various MAC strategies have been invented to allow multiple stations to access physical media and network devices. It is important to understand how network devices gain access to the network media before students can comprehend and troubleshoot the entire network.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Describe the basics of Ethernet technology
• Explain naming rules of Ethernet technology
• Explain how Ethernet relates to the OSI model
• Describe the Ethernet framing process and frame structure
• List Ethernet frame field names and purposes
• Identify the characteristics of CSMA/CD
• Describe Ethernet timing, interframe spacing, and backoff time after a collision
• Define Ethernet errors and collisions
• Explain the concept of auto-negotiation in relation to speed and duplex
Ethernet is now the dominant LAN technology in the world. Ethernet is a family of LAN technologies that may be best understood with the OSI reference model. All LANs must deal with the basic issue of how individual stations, or nodes, are named. Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations. However, the basic frame format and address scheme is the same for all varieties of Ethernet.
Various MAC strategies have been invented to allow multiple stations to access physical media and network devices. It is important to understand how network devices gain access to the network media before students can comprehend and troubleshoot the entire network.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Describe the basics of Ethernet technology
• Explain naming rules of Ethernet technology
• Explain how Ethernet relates to the OSI model
• Describe the Ethernet framing process and frame structure
• List Ethernet frame field names and purposes
• Identify the characteristics of CSMA/CD
• Describe Ethernet timing, interframe spacing, and backoff time after a collision
• Define Ethernet errors and collisions
• Explain the concept of auto-negotiation in relation to speed and duplex
Summary of Module 5
Summary
Ethernet is the most widely used LAN technology and can be implemented on a variety of media. Ethernet technologies provide a variety of network speeds, from 10 Mbps to Gigabit Ethernet, which can be applied to appropriate areas of a network. Media and connector requirements differ for various Ethernet implementations.
The connector on a network interface card (NIC) must match the media. A bayonet nut connector (BNC) connector is required to connect to coaxial cable. A fiber connector is required to connect to fiber media. The registered jack (RJ-45) connector used with twisted-pair wire is the most common type of connector used in LAN implementations. Ethernet
When twisted-pair wire is used to connect devices, the appropriate wire sequence, or pinout, must be determined as well. A crossover cable is used to connect two similar devices, such as two PCs. A straight-through cable is used to connect different devices, such as connections between a switch and a PC. A rollover cable is used to connect a PC to the console port of a router.
Repeaters regenerate and retime network signals and allow them to travel a longer distance on the media. Hubs are multi-port repeaters. Data arriving at a hub port is electrically repeated on all the other ports connected to the same network segment, except for the port on which the data arrived. Sometimes hubs are called concentrators, because hubs often serve as a central connection point for an Ethernet LAN.
A wireless network can be created with much less cabling than other networks. The only permanent cabling might be to the access points for the network. At the core of wireless communication are devices called transmitters and receivers. The transmitter converts source data to electromagnetic (EM) waves that are passed to the receiver. The receiver then converts these electromagnetic waves back into data for the destination. The two most common wireless technologies used for networking are infrared (IR) and radio frequency (RF).
There are times when it is necessary to break up a large LAN into smaller, more easily managed segments. The devices that are used to define and connect network segments include bridges, switches, routers, and gateways.
A bridge uses the destination MAC address to determine whether to filter, flood, or copy the frame onto another segment. If placed strategically, a bridge can greatly improve network performance.
A switch is sometimes described as a multi-port bridge. Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.
Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router controls broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure.
Computers typically communicate with each other by using request/response protocols. One computer issues a request for a service, and a second computer receives and responds to that request. In a peer-to-peer network, networked computers act as equal partners, or peers. As peers, each computer can take on the client function or the server function. In a client/server arrangement, network services are located on a dedicated computer called a server. The server responds to the requests of clients.
WAN connection types include high-speed serial links, ISDN, DSL, and cable modems. Each of these requires a specific media and connector. To interconnect the ISDN BRI port to the service-provider device, a UTP Category 5 straight-through cable with RJ-45 connectors, is used. A phone cable and an RJ-11 connector are used to connect a router for DSL service. Coaxial cable and a BNC connector are used to connect a router for cable service.
In addition to the connection type, it is necessary to determine whether DTE or DCE connectors are required on internetworking devices. The DTE is the endpoint of the user’s private network on the WAN link. The DCE is typically the point where responsibility for delivering data passes to the service provider. When connecting directly to a service provider, or to a device such as a CSU/DSU that will perform signal clocking, the router is a DTE and needs a DTE serial cable. This is typically the case for routers. However, there are cases when the router will need to be the DCE.
Ethernet is the most widely used LAN technology and can be implemented on a variety of media. Ethernet technologies provide a variety of network speeds, from 10 Mbps to Gigabit Ethernet, which can be applied to appropriate areas of a network. Media and connector requirements differ for various Ethernet implementations.
The connector on a network interface card (NIC) must match the media. A bayonet nut connector (BNC) connector is required to connect to coaxial cable. A fiber connector is required to connect to fiber media. The registered jack (RJ-45) connector used with twisted-pair wire is the most common type of connector used in LAN implementations. Ethernet
When twisted-pair wire is used to connect devices, the appropriate wire sequence, or pinout, must be determined as well. A crossover cable is used to connect two similar devices, such as two PCs. A straight-through cable is used to connect different devices, such as connections between a switch and a PC. A rollover cable is used to connect a PC to the console port of a router.
Repeaters regenerate and retime network signals and allow them to travel a longer distance on the media. Hubs are multi-port repeaters. Data arriving at a hub port is electrically repeated on all the other ports connected to the same network segment, except for the port on which the data arrived. Sometimes hubs are called concentrators, because hubs often serve as a central connection point for an Ethernet LAN.
A wireless network can be created with much less cabling than other networks. The only permanent cabling might be to the access points for the network. At the core of wireless communication are devices called transmitters and receivers. The transmitter converts source data to electromagnetic (EM) waves that are passed to the receiver. The receiver then converts these electromagnetic waves back into data for the destination. The two most common wireless technologies used for networking are infrared (IR) and radio frequency (RF).
There are times when it is necessary to break up a large LAN into smaller, more easily managed segments. The devices that are used to define and connect network segments include bridges, switches, routers, and gateways.
A bridge uses the destination MAC address to determine whether to filter, flood, or copy the frame onto another segment. If placed strategically, a bridge can greatly improve network performance.
A switch is sometimes described as a multi-port bridge. Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.
Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router controls broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure.
Computers typically communicate with each other by using request/response protocols. One computer issues a request for a service, and a second computer receives and responds to that request. In a peer-to-peer network, networked computers act as equal partners, or peers. As peers, each computer can take on the client function or the server function. In a client/server arrangement, network services are located on a dedicated computer called a server. The server responds to the requests of clients.
WAN connection types include high-speed serial links, ISDN, DSL, and cable modems. Each of these requires a specific media and connector. To interconnect the ISDN BRI port to the service-provider device, a UTP Category 5 straight-through cable with RJ-45 connectors, is used. A phone cable and an RJ-11 connector are used to connect a router for DSL service. Coaxial cable and a BNC connector are used to connect a router for cable service.
In addition to the connection type, it is necessary to determine whether DTE or DCE connectors are required on internetworking devices. The DTE is the endpoint of the user’s private network on the WAN link. The DCE is typically the point where responsibility for delivering data passes to the service provider. When connecting directly to a service provider, or to a device such as a CSU/DSU that will perform signal clocking, the router is a DTE and needs a DTE serial cable. This is typically the case for routers. However, there are cases when the router will need to be the DCE.
Setting up console connections
Setting up console connections
5.2.7 This page will explain how console connections are set up.
To initially configure the Cisco device, a management connection must be directly connected to the device. For Cisco equipment this management attachment is called a console port. The console port allows monitoring and configuration of a Cisco hub, switch, or router.
The cable used between a terminal and a console port is a rollover cable, with RJ-45 connectors. The rollover cable, also known as a console cable, has a different pinout than the straight-through or crossover RJ-45 cables used with Ethernet or the ISDN BRI. The pinout for a rollover is as follows:
1 to 8
2 to 7
3 to 6
4 to 5
5 to 4
6 to 3
7 to 2
8 to 1
To set up a connection between the terminal and the Cisco console port, perform two steps. First, connect the devices using a rollover cable from the router console port to the workstation serial port. An RJ-45-to-DB-9 or an RJ-45-to-DB-25 adapter may be required for the PC or terminal. Next, configure the terminal emulation application with the following common equipment (COM) port settings: 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
The AUX port is used to provide out-of-band management through a modem. The AUX port must be configured by way of the console port before it can be used. The AUX port also uses the settings of 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
5.2.7 This page will explain how console connections are set up.
To initially configure the Cisco device, a management connection must be directly connected to the device. For Cisco equipment this management attachment is called a console port. The console port allows monitoring and configuration of a Cisco hub, switch, or router.
The cable used between a terminal and a console port is a rollover cable, with RJ-45 connectors. The rollover cable, also known as a console cable, has a different pinout than the straight-through or crossover RJ-45 cables used with Ethernet or the ISDN BRI. The pinout for a rollover is as follows:
1 to 8
2 to 7
3 to 6
4 to 5
5 to 4
6 to 3
7 to 2
8 to 1
To set up a connection between the terminal and the Cisco console port, perform two steps. First, connect the devices using a rollover cable from the router console port to the workstation serial port. An RJ-45-to-DB-9 or an RJ-45-to-DB-25 adapter may be required for the PC or terminal. Next, configure the terminal emulation application with the following common equipment (COM) port settings: 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
The AUX port is used to provide out-of-band management through a modem. The AUX port must be configured by way of the console port before it can be used. The AUX port also uses the settings of 9600 bps, 8 data bits, no parity, 1 stop bit, and no flow control.
Routers and ISDN BRI connections / Routers and DSL connections / Routers and cable connections
Routers and ISDN BRI connections
5.2.4 This page will help students understand ISDN BRI connections.
With ISDN BRI, two types of interfaces may be used, BRI S/T and BRI U. Determine who is providing the Network Termination 1 (NT1) device in order to determine which interface type is needed.
An NT1 is an intermediate device located between the router and the service provider ISDN switch. The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop. In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device.
It may be necessary to provide an external NT1 if the device is not already integrated into the router. Reviewing the labeling on the router interfaces is usually the easiest way to determine if the router has an integrated NT1. A BRI interface with an integrated NT1 is labeled BRI U. A BRI interface without an integrated NT1 is labeled BRI S/T. Because routers can have multiple ISDN interface types, determine which interface is needed when the router is purchased. The type of BRI interface may be determined by looking at the port label. To interconnect the ISDN BRI port to the service-provider device, use a UTP Category 5 straight-through cable.
Routers and DSL connections
5.2.5 This page describes routers and DSL connections.
The Cisco 827 ADSL router has one asymmetric digital subscriber line (ADSL) interface. To connect an ADSL line to the ADSL port on a router, do the following:
• Connect the phone cable to the ADSL port on the router.
• Connect the other end of the phone cable to the phone jack.
To connect a router for DSL service, use a phone cable with RJ-11 connectors. DSL works over standard telephone lines using pins 3 and 4 on a standard RJ-11 connector.
The next page will discuss cable connections.
Routers and cable connections
5.2.6 This page will explain how routers are connected to cable systems.
The Cisco uBR905 cable access router provides high-speed network access on the cable television system to residential and small office, home office (SOHO) subscribers. The uBR905 router has a coaxial cable, or F-connector, interface that connects directly to the cable system. Coaxial cable and an F connector are used to connect the router and cable system.
Use the following steps to connect the Cisco uBR905 cable access router to the cable system:
• Verify that the router is not connected to power.
• Locate the RF coaxial cable coming from the coaxial cable (TV) wall outlet.
• Install a cable splitter/directional coupler, if needed, to separate signals for TV and computer use. If necessary, also install a high-pass filter to prevent interference between the TV and computer signals.
• Connect the coaxial cable to the F connector of the router. Hand-tighten the connector, making sure that it is finger-tight, and then give it a 1/6 turn with a wrench.
• Make sure that all other coaxial cable connectors, all intermediate splitters, couplers, or ground blocks, are securely tightened from the distribution tap to the Cisco uBR905 router.
5.2.4 This page will help students understand ISDN BRI connections.
With ISDN BRI, two types of interfaces may be used, BRI S/T and BRI U. Determine who is providing the Network Termination 1 (NT1) device in order to determine which interface type is needed.
An NT1 is an intermediate device located between the router and the service provider ISDN switch. The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop. In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device.
It may be necessary to provide an external NT1 if the device is not already integrated into the router. Reviewing the labeling on the router interfaces is usually the easiest way to determine if the router has an integrated NT1. A BRI interface with an integrated NT1 is labeled BRI U. A BRI interface without an integrated NT1 is labeled BRI S/T. Because routers can have multiple ISDN interface types, determine which interface is needed when the router is purchased. The type of BRI interface may be determined by looking at the port label. To interconnect the ISDN BRI port to the service-provider device, use a UTP Category 5 straight-through cable.
Routers and DSL connections
5.2.5 This page describes routers and DSL connections.
The Cisco 827 ADSL router has one asymmetric digital subscriber line (ADSL) interface. To connect an ADSL line to the ADSL port on a router, do the following:
• Connect the phone cable to the ADSL port on the router.
• Connect the other end of the phone cable to the phone jack.
To connect a router for DSL service, use a phone cable with RJ-11 connectors. DSL works over standard telephone lines using pins 3 and 4 on a standard RJ-11 connector.
The next page will discuss cable connections.
Routers and cable connections
5.2.6 This page will explain how routers are connected to cable systems.
The Cisco uBR905 cable access router provides high-speed network access on the cable television system to residential and small office, home office (SOHO) subscribers. The uBR905 router has a coaxial cable, or F-connector, interface that connects directly to the cable system. Coaxial cable and an F connector are used to connect the router and cable system.
Use the following steps to connect the Cisco uBR905 cable access router to the cable system:
• Verify that the router is not connected to power.
• Locate the RF coaxial cable coming from the coaxial cable (TV) wall outlet.
• Install a cable splitter/directional coupler, if needed, to separate signals for TV and computer use. If necessary, also install a high-pass filter to prevent interference between the TV and computer signals.
• Connect the coaxial cable to the F connector of the router. Hand-tighten the connector, making sure that it is finger-tight, and then give it a 1/6 turn with a wrench.
• Make sure that all other coaxial cable connectors, all intermediate splitters, couplers, or ground blocks, are securely tightened from the distribution tap to the Cisco uBR905 router.
Routers and serial connections
Routers and serial connections 5.2.3 This page will describe how routers and serial connections are used in a WAN.
Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router contains broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure. In order to provide these services the router must be connected to the LAN and WAN.
In addition to determining the cable type, it is necessary to determine whether DTE or DCE connectors are required. The DTE is the endpoint of the user’s device on the WAN link. The DCE is typically the point where responsibility for delivering data passes into the hands of the service provider.
When connecting directly to a service provider, or to a device such as a CSU/DSU that will perform signal clocking, the router is a DTE and needs a DTE serial cable. This is typically the case for routers. However, there are cases when the router will need to be the DCE. When performing a back-to-back router scenario in a test environment, one of the routers will be a DTE and the other will be a DCE.
When cabling routers for serial connectivity, the routers will either have fixed or modular ports. The type of port being used will affect the syntax used later to configure each interface.
Interfaces on routers with fixed serial ports are labeled for port type and port number.
Interfaces on routers with modular serial ports are labeled for port type, slot, and port number. The slot is the location of the module. To configure a port on a modular card, it is necessary to specify the interface using the syntax “port type slot number/port number”. Use the label “serial 1/0”, when the interface is serial, the slot number where the module is installed is slot 1, and the port that is being referenced is port 0.
The next page discusses routers and ISDN BRI connections.
Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router contains broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure. In order to provide these services the router must be connected to the LAN and WAN.
In addition to determining the cable type, it is necessary to determine whether DTE or DCE connectors are required. The DTE is the endpoint of the user’s device on the WAN link. The DCE is typically the point where responsibility for delivering data passes into the hands of the service provider.
When connecting directly to a service provider, or to a device such as a CSU/DSU that will perform signal clocking, the router is a DTE and needs a DTE serial cable. This is typically the case for routers. However, there are cases when the router will need to be the DCE. When performing a back-to-back router scenario in a test environment, one of the routers will be a DTE and the other will be a DCE.
When cabling routers for serial connectivity, the routers will either have fixed or modular ports. The type of port being used will affect the syntax used later to configure each interface.
Interfaces on routers with fixed serial ports are labeled for port type and port number.
Interfaces on routers with modular serial ports are labeled for port type, slot, and port number. The slot is the location of the module. To configure a port on a modular card, it is necessary to specify the interface using the syntax “port type slot number/port number”. Use the label “serial 1/0”, when the interface is serial, the slot number where the module is installed is slot 1, and the port that is being referenced is port 0.
The next page discusses routers and ISDN BRI connections.
WAN serial connections
WAN serial connections
5.2.2 This page will discuss WAN serial connections.
For long distance communication, WANs use serial transmission. This is a process by which bits of data are sent over a single channel. This process provides reliable long distance communication and the use of a specific electromagnetic or optical frequency range.
Frequencies are measured in terms of cycles per second and expressed in Hz. Signals transmitted over voice grade telephone lines use 4 kHz. The size of the frequency range is referred to as bandwidth. In networking, bandwidth is a measure of the bits per second that are transmitted.
For a Cisco router, physical connectivity at the customer site is provided by one of two types of serial connections. The first type is a 60-pin connector. The second is a more compact ‘smart serial’ connector. The provider connector will vary depending on the type of service equipment.
If the connection is made directly to a service provider, or a device that provides signal clocking such as a channel/data service unit (CSU/DSU), the router will be a data terminal equipment (DTE) and use a DTE serial cable. Typically this is the case. However, there are occasions where the local router is required to provide the clocking rate and therefore will use a data communications equipment (DCE) cable. In the curriculum router labs one of the connected routers will need to provide the clocking function. Therefore, the connection will consist of a DCE and a DTE cable.
The next page will discuss routers and serial connections
5.2.2 This page will discuss WAN serial connections.
For long distance communication, WANs use serial transmission. This is a process by which bits of data are sent over a single channel. This process provides reliable long distance communication and the use of a specific electromagnetic or optical frequency range.
Frequencies are measured in terms of cycles per second and expressed in Hz. Signals transmitted over voice grade telephone lines use 4 kHz. The size of the frequency range is referred to as bandwidth. In networking, bandwidth is a measure of the bits per second that are transmitted.
For a Cisco router, physical connectivity at the customer site is provided by one of two types of serial connections. The first type is a 60-pin connector. The second is a more compact ‘smart serial’ connector. The provider connector will vary depending on the type of service equipment.
If the connection is made directly to a service provider, or a device that provides signal clocking such as a channel/data service unit (CSU/DSU), the router will be a data terminal equipment (DTE) and use a DTE serial cable. Typically this is the case. However, there are occasions where the local router is required to provide the clocking rate and therefore will use a data communications equipment (DCE) cable. In the curriculum router labs one of the connected routers will need to provide the clocking function. Therefore, the connection will consist of a DCE and a DTE cable.
The next page will discuss routers and serial connections
Cabling WANs / WAN physical layer
WAN physical layer
5.2.1 This page describes the WAN physical layer.
The physical layer implementations vary based on the distance of the equipment from each service, the speed, and the type of service. Serial connections are used to support WAN services such as dedicated leased lines that run PPP or Frame Relay. The speed of these connections ranges from 2400 bps to T1 service at 1.544 Mbps and E1 service at 2.048 Mbps.
ISDN offers dial-on-demand connections or dial backup services. An ISDN Basic Rate Interface (BRI) is composed of two 64 kbps bearer channels (B channels) for data, and one delta channel (D channel) at 16 kbps used for signaling and other link-management tasks. PPP is typically used to carry data over the B channels.
As the demand for residential broadband high-speed services has increased, DSL and cable modem connections have become more popular. Typical residential DSL service can achieve T1/E1 speeds over the telephone line. Cable services use the coaxial cable TV line. A coaxial cable line provides high-speed connectivity that matches or exceeds DSL. DSL and cable modem service will be covered in more detail in a later module.
The next page will describe WAN serial connections.
5.2.1 This page describes the WAN physical layer.
The physical layer implementations vary based on the distance of the equipment from each service, the speed, and the type of service. Serial connections are used to support WAN services such as dedicated leased lines that run PPP or Frame Relay. The speed of these connections ranges from 2400 bps to T1 service at 1.544 Mbps and E1 service at 2.048 Mbps.
ISDN offers dial-on-demand connections or dial backup services. An ISDN Basic Rate Interface (BRI) is composed of two 64 kbps bearer channels (B channels) for data, and one delta channel (D channel) at 16 kbps used for signaling and other link-management tasks. PPP is typically used to carry data over the B channels.
As the demand for residential broadband high-speed services has increased, DSL and cable modem connections have become more popular. Typical residential DSL service can achieve T1/E1 speeds over the telephone line. Cable services use the coaxial cable TV line. A coaxial cable line provides high-speed connectivity that matches or exceeds DSL. DSL and cable modem service will be covered in more detail in a later module.
The next page will describe WAN serial connections.
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