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.
Tuesday, January 26, 2010
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.
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