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.
Saturday, January 16, 2010
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.
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