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Chapter 4 - LAN Technologies © 1996, BICSI LAN Design Manual - CD-ROM, Issue 11

Overview

LAN technologies defined

LANs have flourished over the past decade to become an integral part of the officeenvironment. Many LAN technologies have been introduced, but only a few have proventhemselves and become readily accepted.

In this section, some of the more popular LAN technologies will be examined. Each one willbe described according to its history, features and traditional configuration. The features of

each will be described according to the LAN architecture features described in Chapter 1—transmission medium, topology, access control, transmission technique and speed.

Under the heading of traditional LAN technologies, the following will be examined:

• ARCnet.

• Ethernet.

• Token-ring.

• AppleTalk.

The world of LANs is dynamic, and while certain technologies have earned wide acceptancein the marketplace, newer ones are always being introduced.

… LAN technologies defined,continued


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Under the heading of emerging LAN technologies, the following will be discussed:

• FDDI/TP-PMD.

• High-speed Ethernets.

• ATM.


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TRADITIONAL LAN TECHNOLOGIES

ARCnet

History

ARCnet (Attached Resource Computing Network) was developed by John Murphy atDatapoint Corporation in 1977. At that time, it was introduced as a LAN solution for ownersof Datapoint computers. The company began licensing its technology to other manufacturers

in the early 1980’s, which led to PC-based ARCnet LANs.

ARCnet’s initial popularity was due to its use of the same type of coax cabling as IBM 3270terminals. Customers could purchase PCs, place them on users’ desks, remove the cablefrom the terminal, plug it into the ARCnet interface card in the PC and take away theterminal. In this manner, the migration to a LAN could be accomplished without recabling. Bycontrast, Ethernet required a different type of coax, so migrating to Ethernet was a

significantly more costly affair for the same customer.

ARCnet predated the IEEE 802 committee for LANs, and Datapoint did not participate in themeetings of the group. As a result, ARCnet was not an IEEE-sanctioned LAN. This, coupledwith Datapoint’s relatively minor status in the world computing industry, made largecompanies reluctant to use ARCnet in their corporate networks.

… ARCnet history, continued


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In September 1989, ARCnet Plus was announced. It promised an eight-fold increase in bothspeed and the number of stations on a single network segment. More significant, stationsequipped with ARCnet Plus interface cards could coexist on the same segment as PCs witholder ARCnet cards. Clients could upgrade their network stations selectively, instead of beingobliged to spend money for every station to obtain better network performance.

In October 1992, after many years of lobbying by the ARCnet Trade Association, ANSIrecognized ARCnet as a LAN standard. By this time, however, it had long been surpassed insales volume by both Ethernet and Token-ring.

Today, with an installed base of well over four million stations worldwide, ARCnet is a proventechnology with a reputation for reliability in all types of environments, from the factory floorto the corporate boardroom.

Characteristics

Some of the characteristic features defining ARCnet are as follows:

• A reliable LAN solution where there are only a small number of stations—typically

numbering in the tens rather than hundreds.

• Very efficient for small-packet communications where short messages are sentbetween stations.

• It provides for a very flexible configuration—both in terms of topology and cablingmedia.

• Connectivity is simplified by the use of hubs—eases management andtroubleshooting.


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Terminology

• Token-bus

An access control mechanism which requires a station to have a token beforetransmitting. The message is then broadcast to all other stations over a bus

topology.

Summary of features

Transmission medium

Traditional ARCnet LANs were cabled using RG-62 coaxial cable. RG-62 coax is a twin-conductor, shielded cable with a 93-ohm impedance. It was the use of this type of coaxthat made ARCnet an inexpensive investment for many early LAN users—this is the same

type of coax used in the most popular mainframe environment—the IBM 3270. Not havingto install new cable represented significant cost savings.

RG-62 coax has the advantage of low attenuation at ARCnet’s frequency—5 MHz—permitting cable runs between hubs of up to 610 m (2000 ft).

ARCnet has evolved to support other transmission media—unshielded twisted-pair,shielded twisted-pair, and optical fiber.

… ARCnet summary of features,continued


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Topology

ARCnet’s traditional configuration is referred to as clustered star. Individual LAN stationsare connected to an active or a passive hub. These hubs are then connected daisy-chainstyle, one to the other. Alternately, stations may be connected to one another, with the last

station connected to a port on an active hub.Segments daisy-chained to an active hub can extend to a maximum of 305 m (1000 ft).Segments using passive hubs are limited to a maximum of 30 m (100 ft).

Typically, a hub supports four or eight stations with a maximum network size of 255stations.



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FIGURE 4.1: TRADITIONAL ARCNET COMPONENTS AND CONFIGURATION

Active Hub

Active Hub

Up to 610 m(2000 ft)

Passive Hub

93-ohmTerminator

Up to 610 m (2000 ft)

Up to 30 m (100 ft)

Up to 305 m (1000 ft)for a linear segment

Up to 30 m (100 ft)



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Access control

ARCnet closely resembles the IEEE 802.4 Token-bus standard. While IEEE 802.4 is usedmostly in the manufacturing environment, ARCnet is found both in factory floor and officeenvironments.

The principle behind ARCnet’s access to the media is based on token-passing. Anelectronic token is required for a network station to gain access to the transmissionchannel. Once a station has possession of the token it attaches the message to betransmitted to the token. At this point the message is broadcast over the network. Allstations are listening, and the destination station recognizes its address and accepts themessage.

If a station has no message to transmit, the token is passed to the LAN station with thenext higher network address. For this reason, the logical topology of the network differsfrom its physical topology—the next station in the chain may be next door or two floorsbelow. The station with the highest address passes the token to the station with the lowestaddress, creating a loop in the network.

Each Network Interface Card (NIC) knows its own address and the address of the station

to which it will pass the token.



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The procedure for sending a token from station to station differs from traditional token-passing:

• The station sending the token listens on the transmission media to see if the tokenhas in fact been received.

• It is assumed that if the token is received by the next station it will either transmit amessage or pass the token on to the next station in line.

• If the transmission channel is quiet—no messages are being transmitted and thetoken is not circulating—a second token is sent to the same intended station.

• If there is still no response, the original sending station broadcasts a message onthe channel asking for the address of the next station in line.

• This next station in line broadcasts its address and receives a token.

• The apparently inactive station is bypassed.

Transmission technique

ARCnet operates as a baseband network. Transmissions are broadcast by one networkstation to another specific network station.

ARCnet uses 5 MHz of bandwidth. Two cycles are used to signal the passage of a singlebit (1 or 0). The throughput is therefore 0.5 x 5 = 2.5 Mbps. ARCnet Plus uses the sameamount of bandwidth. However, one cycle is used to signal the passage of four bits. Thethroughput is therefore 4 x 5 = 20 Mbps.



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Speed

Traditional ARCnet operated at 2.5 Mbps. A newer technology, ARCnet Plus operates at20 Mbps.

The two forms of ARCnet are compatible and an ARCnet Plus segment can be added to a

traditional ARCnet LAN.

Future

ARCnet will most likely continue to be a popular solution for smaller office and especiallymanufacturing environments. Because of its delay in becoming a recognized standard, it has

not been readily adopted by corporate users.

In response to general LAN trends, ARCnet is evolving. ARCnet Plus operates at 20 Mbps.One vendor offers a modified version of ARCnet which operates at 100 Mbps.

ARCnet Plus offers several improvements over traditional ARCnet:

• It has a maximum packet size of 4224 bytes versus 516 bytes of ARCnet.

• It is able to support 2047 stations on the network versus 255 for ARCnet.

• It supports IEEE 802.2 globally-administered 48-bit addressing, whereas ARCnetuses locally-administered, 8-bit addressing.


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Ethernet

History

Ethernet was jointly developed by Xerox Corporation, Digital Equipment Corporation andIntel Corporation.

In the early 1970’s, Xerox Corporation’s Palo Alto Research Center (PARC) began work onwhat was then called Experimental Ethernet. The original Ethernet specification wasdeveloped at PARC by Bob Metcalfe who subsequently went on to found 3Com Corporation.

The first Ethernet LAN adapter for personal computers was shipped in September 1982 by3Com. Currently, most computer system vendors offer Ethernet connections for theirproducts.

While many variations of Ethernet existed in its early days, the most popular implementationtoday is based on the IEEE 802.3 specification. All variations, however, shared two commonfeatures:

• A contention-based access scheme.

• A linear-bus logical topology.

Ethernet design is discussed in detail in later chapters.

… Ethernet history, continued


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Characteristics

Some of the characteristic features of Ethernet are as follows:

• It is the most widely used LAN technology in the world.

• Due to its popularity in the computer industry, it may be the best means to create anetwork using equipment obtained from multiple vendors.

• It is a proven technology for environments where relatively few stations areresponsible for the majority of network transmissions.

• It continues to evolve in response to changes in technology and user needs, asdiscussed later in this chapter.


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Terminology

• AUI

Attachment Unit Interface. Mostly used when referring to the 15-pin D-typeconnector and cable used to connect stations to an Ethernet transceiver in a

10Base5 (Thicknet) environment. The connector is sometimes referred to as DIX(named for Digital, Intel and Xerox).

• Barrel adapter

A barrel-shaped connector used to attach two lengths of thick Ethernet coaxialcable, using N-connectors.

• BNC connector

The connectors used with thin Ethernet (Thinnet). These connectors, usedthroughout the cable length, attach to T-connectors, which in turn connect tostations.

BNC stands for Bayonet Navel Connector. A twist lock connector used with thincoaxial cable.

• Drop cable

A four-pair cable connecting the network station to the transceiver attached to themain trunk cable. Also referred to as AUI cable or transceiver cable.

• Jabber

An overly long data frame sometimes caused by a defective NIC.

… Ethernet terminology, continued


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• Runt

A short data frame typically caused by a collision.

• SQE

Signal Quality Error, also referred to as heartbeat. Used by a transceiver toperiodically inform a network control unit of its status.

• Tap

A connection point created in Thicknet coaxial trunk cable for the purpose ofattaching a transceiver.

• Terminating resistor (Terminator)

A device used at the ends of coaxial cable segments to prevent signals from beingreflected (or echoed back) onto the cable, which would cause signal interference.

• Transceiver

A unit designed to connect station(s) to the Ethernet trunk cabling. The unitprovides both transmitter and receiver functions to allow the station(s) to join the

network. May be found on the station’s NIC (Thinnet) or as a separate unit(Thicknet).


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Summary of features

Transmission medium

Coaxial cable-based EthernetThe original Ethernet was designed to use a thick coaxial main trunk cable, with a 10 mm(0.4 in) diameter. Later a more flexible, thinner coaxial cable—RG-58 with a 5 mm (0.2 in)diameter—was introduced for Ethernet LANs. Use of this thinner coax resulted in thenetwork being referred to as Thinnet or Cheapernet—due to the lower cost of the thincoax. The original Ethernet then became known as Thicknet.

While both forms of the coaxial cable support data rates of 10 Mbps, there are significantdifferences in the total number of stations attached to the cable segment. The thinnercable suffers from greater attenuation and less resistance to electromagnetic interference,therefore, it is capable of supporting fewer connected stations. The following tablesummarizes some of the differences between Ethernets created using the two coaxialcable types.

… Ethernet summary of features,continued


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Unshielded twisted-pair Ethernet

Migration to less expensive transmission media continued with the formalization of10Base-T Ethernet in 1990. This is an IEEE extension to the Ethernet standard. Itspecifies the use of unshielded twisted-pair cabling (UTP) as the transmission media.

10Base-T Ethernet is the first LAN standard to acknowledge the recommendations madein a cabling system standard. Many of the specifications for 10Base-T cabling are thesame as those for structured cabling in ANSI/TIA/EIA-568-A. For example, 10Base-T isdesigned to operate over a maximum end-to-end cable length of 100 m (328 ft)—thehorizontal link distance recommended by the cabling system standard.



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Topology

Thicknet Ethernet follows a bus topology where all network stations are attached totransceivers connected to a single length of coaxial cable using twisted-pair transceivercable.

The transceiver cable acts as the interface between the NIC and the transceiver. Atransceiver cable is made up of four individually shielded pairs of wires.

• One pair for transmit.

• One pair for receive.

• One pair for powering.

• One pair for collision detection.

In Thinnet, the transceiver is placed onto the NIC, therefore, there is no need for aseparate transceiver cable. The coaxial cable runs from station to station, forming the bus.

In 10Base-T or 10Base-F, the bus is reduced in size and placed inside a hub. Stationsconnect to the hub using UTP or optical fiber cabling.


F 4 2 T E


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FIGURE 4.2: THICKNET ETHERNET COMPONENTS AND CONFIGURATION


50-ohmterminator withground

Transceiver up to 50 m (164 ft)at least 2.5 m

(8 ft)

up to 500 m (1640 ft)

AUI cableThicknetcoaxial

cable

Terminator

FIGURE 4 3 THINNET ETHERNET COMPONENTS AND CONFIGURATION


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FIGURE 4.3: THINNET ETHERNET COMPONENTS AND CONFIGURATION


50-ohm

terminator withground

up to 185 m (607 ft)

Thinnet

coaxial cable

FIGURE 4 4: 10BASE T ETHERNET COMPONENTS AND CONFIGURATION


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FIGURE 4.4: 10BASE-T ETHERNET COMPONENTS AND CONFIGURATION


Hub

up to 100 m(328 ft)

FIGURE 4 5: 10BASE F ETHERNET COMPONENTS AND CONFIGURATION


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FIGURE 4.5: 10BASE-F ETHERNET COMPONENTS AND CONFIGURATION


Hub

up to 2000 m

(1.25 mi)

Access control


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Access control

Ethernet’s mechanism for accessing the transmission channel to transmit a message isbest described in the January 1991 issue of BYTE Magazine :

“Ethernet is based on the same etiquette that makes for a politeconversation: ‘listen before talking’. Of course, even when people aretrying not to interrupt each other, there are those embarrassingmoments when two people start talking at the same time.”

This method of access control is a form of contention. That is, each station monitors the

network and if no transmission is detected the station transmits its message. If atransmission is detected, the station waits. When the transmission channel is clear it isable to transmit its message.

The specific access control scheme used by Ethernet is known as Carrier-Sense MultipleAccess with Collision Detection (CSMA/CD), discussed in an earlier chapter.


Ethernet most often uses baseband transmission, although a broadband version ofEthernet is available.

Transmissions are broadcast over the network by the sending station, using Manchesterencoding. That is, transmissions can be heard by all attached network stations. While all

stations can hear the transmission, only the station for which it was intended will recognizeit and acknowledge it.


Speed


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Speed

The most common transmission speed for Ethernet is 10 Mbps. There is a version ofEthernet specified which transmits at 1 Mbps.

Future

With the introduction of the 10Base-T specification, there was a renewed interest in Ethernet.Choosing it provided the user with a reliable, structured and cost-effective LAN solution,often using the existing cabling infrastructure. Development has not stopped with 10Base-T.Different configurations and types of Ethernet have been recently introduced. They arediscussed later in this chapter.


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Token-ring

History

Token-ring technology was brought to market in 1985 by IBM as a connectivity solution for itsvaried computing environments.

IBM was the first computer systems vendor to recognize the need for a structured cablingsystem. The IBM Cabling System introduced in 1984 and used by traditional Token-ringLANs, favors shielded twisted-pair cable, referred to as STP.

Token-ring specifications have been formalized by the IEEE as the 802.5 standard.

Token-ring design will be discussed in detail in later chapters.

Characteristics

Some of the characteristic features of Token-ring are as follows:

• It is a very robust and highly fault-tolerant LAN technology.

• It is a proven technology for environments where all stations require equal accessto network resources.

• It allows for easy expansion, with little degradation in performance as new stationsare added.

Terminology


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Terminology

• Lobe

The section of cable attaching the NIC in a station to the MAU.

• MAU (or MSAU)

Multistation Access Unit. A passive (not powered) hub to which all stations on the

ring are attached. Found at the center of the Token-ring star-wired ring topology.

• CAU

Controlled Access Unit. Similar in function to a MAU, but is an active (powered)unit, which regenerates the incoming signal before forwarding it to the next station

on the ring.

• Media filter

A device used to connect UTP cable to a traditional Token-ring NIC, whichaccommodates STP cabling only. The more recent Token-ring NICs offer both UTPand STP connections, eliminating the need for such a device.

Summary of features


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Summary of features

Transmission medium

Token-ring was originally designed to operate at both 4 and 16 Mbps over a two-pair 150 Ω

shielded twisted-pair cable—known as IBM Type 1 cable. At the time of its introduction,the performance of unshielded twisted-pair cable was uncertain for high-speed datatransmissions.

IBM Type 1 cable was able to offer guaranteed high data transmission rates over extendeddistances.

Today, Token-ring networks operate over 100 W unshielded twisted-pair cable and optical

fiber cable, as well as STP.

… Token-ring summary of features,continued

Topology


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p gy

Token-ring follows a star-wired ring topology. While the physical topology resembles astar, the logical topology is that of a ring.

In this configuration, each station is directly wired to a central unit known as a Multistation

Access Unit (MAU). The MAU links the stations internally to create a ring between theconnected stations.

Using a MAU makes for a reliable, flexible and easily configured network. MAUs removethe unreliability of a traditional ring topology. If a network station fails or if a cable isbroken, the MAU reconfigures the ring, bypassing the error-causing link.

MAUs are usually passive devices. Each is often equipped with 8 or 16 ports for station

connections plus two additional ports—Ring-In and Ring-Out—for connections to otherMAUs, to provide ring growth.


FIGURE 4.6: TRADITIONAL TOKEN-RING COMPONENTS AND CONFIGURATION


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Logical Ring

Physical Star topology

Multistation Access Unit

Multistation Access Unit

Access control


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Token-ring uses a token-passing access control mechanism. A Token-ring token is anelectronic signal, 24 bits in length. A station in possession of the token has an exclusiveright to transmit.

Token-passing is a deterministic method of access control. It is possible to determine theprobability of a station possessing the token based on the number of stations on the ring.Therefore, it is possible to estimate how often a station will have the ability to transmit andwhat the level of traffic on the network will be, prior to implementation.

Requiring a token to transmit classifies Token-ring access control as a collision-avoidancemethod.

The token is created by one station on the network—the token manager. If that station isshut off, or fails for some reason, another station assumes the token creation task. Thelogic for the token generation process is built into the network adapter cards.




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The transmission technique used by Token-ring is baseband—only one station is able totransmit at a time. Possession of the token permits a station to transmit, using DifferentialManchester encoding.

Data transmission occurs as follows:• Once a station has possession of a token, it adds data and control fields to the

token, creating a frame.

• The frame passes from station to station until it reaches its destination.

• The destination station recognizes its own address and copies the frame.

• The destination station returns the frame to the network, where it continues tocirculate from station to station until it returns to the transmitting station.

• The transmitting station is responsible for removing the frame and for releasing anew token.

• As a frame passes from station to station, each station is responsible for

regenerating the transmission.


Speed


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The first Token-ring system (1985) operated at 4 Mbps. In 1988, a 16 Mbps version wasintroduced. Both speeds are currently available.

FutureToken-ring installations will likely continue to be very popular with users who demandreliability. The introduction of UTP-based Token-ring has further increased its popularity.

As the demand for bandwidth has increased, pressure has been put on vendors to develophigher-speed versions of their products. Token-ring technology is expected to evolve to meet

these requirements.

AppleTalk


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AppleTalk

History

When Apple Computer introduced the Macintosh computer in 1984, it included local areanetworking capability in each device through a combination of hardware—a built-in NetworkInterface Card—and software—built into the operating system.

The following year, the company introduced the LaserWriter, a high-quality laser printerwhich produced text and graphics output far superior to that of any dot-matrix printer. Thecost of the LaserWriter made it difficult for companies to purchase one for every Macintoshuser, therefore, a means of sharing this device was needed.

Since the LaserWriter was also equipped with a built-in network port, Macintosh networkingwas a simple matter. Initially, Apple referred to its network of Macintoshes and printers asAppleTalk. Later, this term was assigned to the software portion of the network—the built-inhardware became known as LocalTalk.

All other types of personal computers and printers began as stand-alone devices and overtime developed local area networking capabilities. The Macintosh, due to its initial design,

networked effortlessly from the day of its introduction.

In June 1989, Apple updated the AppleTalk protocols to AppleTalk Phase 2. This was done topermit Macintosh networks to grow more easily, as well as to allow for simpler integrationwith other types of networks and computers.

Characteristics


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Some of the characteristic features defining AppleTalk are as follows:

• A proprietary (not recognized by IEEE Project 802) set of networking protocolsdesigned to provide communications between devices in the Macintosh family of

products.• Allows for simple connectivity with little configuration required for network

operations.

• Has evolved to allow for communications with non-Apple systems—other types ofpersonal computers, minicomputers and mainframes.

Terminology


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• AppleTalk

The name given to the series of specifications which define how communicationsare established, maintained and terminated between devices over a network.

• LocalTalk

The name given to Apple Computer’s LAN hardware built into each device in theMacintosh family of products. Its specifications are discussed below.

• EtherTalk

Apple Computer’s implementation of IEEE 802.3 Ethernet in the Macintosh

environment. Permits the integration of Macintoshes into an Ethernet network.

• TokenTalk

Apple Computer’s implementation of IEEE 802.5 Token-ring in the Macintoshenvironment. Permits the integration of Macintoshes into a Token-ring network.

Summary of features


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Transmission medium

LocalTalk uses shielded twisted-pair cable. It requires only one pair for communications

between devices. With the introduction of EtherTalk and TokenTalk, support for coaxial,unshielded twisted-pair and optical fiber cabling has become available.

Topology

LocalTalk uses a bus topology. Stations may be attached anywhere along the length of the

bus using their built-in connectors. The maximum length of the bus is 305 m (1000 ft).

In the original AppleTalk protocol, referred to as AppleTalk Phase 1, the maximum numberof stations supported on a single network is 32. These networks can be linked together toa maximum of 254 devices, due to an 8-bit address field. In AppleTalk Phase 2, themaximum theoretical number of linked devices becomes more than 16 million forEtherTalk and TokenTalk networks, due to a 24-bit extended address field. LocalTalk,however, remains limited to 254 devices.

… AppleTalk summary of features,continued

FIGURE 4.7: TRADITIONAL LOCALTALK COMPONENTS AND CONFIGURATION


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up to 305 m (1000 ft)

cable


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When a station wishes to transmit, it uses the following process:

Th t ti i hi t t it h k th t i i di t if it i idl


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• The station wishing to transmit checks the transmission medium to see if it is idle.

• The transmission medium must be idle for at least 400 microseconds, at whichtime the transmitting station waits for an additional random amount of time.

• The transmitting station sends a Request to Send control packet to the intendeddestination station.

• The destination station sends a Clear to Send reply to the transmitting station.

• The data is then transmitted.

• The destination station receives the broadcast message, which was ignored by all

other stations on the network.If the sending station did not receive the Clear to Send, it assumes that there has been acollision and the process begins again.


LocalTalk uses baseband transmission, allowing only one station to transmit at a time.Transmissions use a broadcast technique, with EIA-422A signaling and FM-0 encoding. Inthe OSI model, this would be classified as a Layer 1—or physical layer—protocol.


Speed

L lT lk d i i f 230 4 Kb Thi i b i ll l


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LocalTalk operates at a data transmission rate of 230.4 Kbps. This is substantially slowerthan EtherTalk at 10 Mbps or TokenTalk at either 4 or 16 Mbps.

Due to the slow rate of transmission, servers on LocalTalk networks may be busy for

extended periods of time and therefore unable to respond to requests. For this reason,requests often need to be rebroadcast, further congesting the network.

If the number of devices to be connected is greater than 15, it is advisable to useEtherTalk or TokenTalk.

Future

AppleTalk continues to evolve in its role as the predominant networking environment forApple Computer products. In the beginning, it provided means of creating Macintosh

LANs. Today, it allows for the integration of the Macintosh into other network technologiessuch as Ethernet and Token-ring. As well, Apple is actively pursuing Macintosh integrationinto the emerging LAN environments, discussed in the following pages.

EMERGING LAN TECHNOLOGIES


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ANSI X3T9.5 — FDDI and TP-PMD

History

Fiber Distributed Data Interface

Fiber Distributed Data Interface (FDDI) is considered to be one of the newer LAN

technologies. However, the initial proposal for FDDI was made in October 1982.FDDI was originally designed to provide a means of interconnecting mainframe andminicomputers systems, as a very high-speed backbone network. FDDI specifies the useof optical fiber cable and benefits from fiber’s many advantages—higher transmissionrates, longer link lengths, immunity to electromagnetic noise and increased security.

In 1984, FDDI was modified to incorporate LANs and in 1986, FDDI was described in a

draft standard by the ANSI X3T9.5 committee. Final approval came in 1990.

FDDI design is discussed in detail in a later chapter.

… FDDI and TP-PMD history,continued

Twisted-Pair Physical Medium Dependent

Twisted-pair Physical Medium Dependent (TP-PMD) is a more recent technology which


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Twisted-pair Physical Medium Dependent (TP-PMD) is a more recent technology whichhas been adopted as an extension to the ANSI X3T9.5 standard. It has been referred to asFDDI over copper. Other than the transmission medium used, there is no significant

difference between FDDI and TP-PMD operations.

Due to the higher cost of installing optical fiber and its associated components, a lower-cost solution using copper cabling was seen as an attractive way to bring a high-speedLAN to the ever-powerful desktop station.

TP-PMD was the first technology to illustrate the feasibility of very high-speed LANoperations over copper twisted-pair cabling.

Characteristics

Some of the characteristic features defining ANSI X3T9.5 are as follows:

• Designed to provide high-speed connections over long distances.

• It is a standards-based technology providing a flexible, robust and high-

performance solution, using proven Token-ring access technology.

• ANSI X3T9.5 complements existing LAN standards and is often used tointerconnect other LAN environments as a backbone LAN.

Terminology

Twisted Pair Physical Medium Dependent (TP PMD) technology has


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• Twisted-Pair—Physical Medium Dependent (TP-PMD) technology hasalso been referred to by the following names:

– CDDI - Copper Distributed Data Interface.

– TPDDI - Twisted-Pair Distributed Data Interface.

– S(TP)DDI - Shielded (Twisted-Pair) Distributed Data Interface.

– UTPDDI - Unshielded Twisted-Pair Distributed Data Interface.

• Dual-Attachment Station (DAS)

Defined as a Class A station, which incorporates connections for both the primaryand the secondary (backup) ring.

• Single-Attachment Station (SAS)

Defined as a Class B station which incorporates connections for the primary ringalone.

Summary of features


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Transmission medium

All recommendations for the transmission medium are made in the ANSI X3T9.5 PhysicalMedium Dependent (PMD) sublayer specification which, together with the Physicalsublayer specification, makes up the ISO layer 1 (Physical) protocols. The PMDspecification outlines requirements for transmission media and all physical connections tothe transmission medium used.

Physical layer specifications for FDDI are detailed for the following:

• The physical characteristics of the optical fiber to use—including loss, bandwidth

and dispersion.

• The physical characteristics of the optical fiber connectors.

• The characteristics of the optical bypass switch—used by Dual-AttachmentStations.

• The characteristics of optical transmitters and receivers—power, sensitivity,

waveforms and center wavelength.• The mechanisms to establish a link between neighboring stations, and to engage

the optical bypass switch in the event of neighboring station failure.

… FDDI and TP-PMD summary of features, continued

FDDI recommends the use of 62.5/125 µm multimode optical fiber, although othermultimode sizes or even single-mode fiber may be used. The standard recommends using

li ht itti di d (LED) ti t 1300 th t itt


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a light-emitting diode (LED) operating at 1300 nm as the transmitter.

Additions made to the PMD specification has resulted in TP-PMD, which provides for

FDDI to operate over STP cable as well as Category 5 UTP cable.


Topology

The traditional FDDI/TP-PMD topology is one of dual counter-rotating rings Fault


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The traditional FDDI/TP PMD topology is one of dual counter rotating rings. Faultisolation and recovery is automatic, since one ring acts as a backup to the other.

In a dual-ring architecture, there are two optical fiber rings connecting stations. Counter-rotating means that the direction of transmission is opposite for the two rings. One of therings is active, while the second acts as a backup.

In the case of a broken cable or a nonresponsive station on the network, the ring wrapsaround on itself on both sides of the fault to correct the problem. This permits the LAN tocontinue operating without disruption of service to the other stations on the ring.

Fault isolation and recovery is under the control of the Station Management (SMT)

specification. It determines how ring data is collected and configured at setup or after afailure. SMT is responsible for seeing that there is a problem and implementing the faultprotection features. SMT was defined to:

• Ensure that the FDDI/TP-PMD network could be easily managed.

• Monitor network operations.

• Quickly identify any exception to normal operations.An FDDI network can have up to 500 dual-attached stations (attachments to both rings)with a maximum distance of 2 kilometers (1.25 miles) between stations. The FDDInetwork can span a total of 100 kilometers (62.5 miles).

The TP-PMD environment operates using Category 5 twisted-pair cable to a maximum of100 m (328 ft) between stations. It is was designed to provide 100 Mbps performance over

the largest possible horizontal cabling link, as defined by structured cabling standards.


FIGURE 4.8: FDDI COMPONENTS AND CONFIGURATION


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Concentrator

Primary ring

Secondary

ring

DAS

DAS

DAS

SAS

SAS

SAS

DAS = Dual-Attachment Station (Class A node)

SAS = Single-Attachment Station (Class B node)

Access control

The access control method used by FDDI/TP-PMD is much like that for Token-ring. It is a


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y gtoken-passing scheme that is only slightly different from the IEEE 802.5 specification—atoken is made available to the ring immediately after a station transmits, without waitingfor an acknowledgment from the recipient. This provides for a high degree of network

availability.

The Media Access Control (MAC) specifies the access mechanism used byFDDI/TP-PMD stations on the ring for transmitting and receiving data. The MACspecification defines the:

• Format and structure of the FDDI/TP-PMD frame.

• Time Token Rotation Protocol, which determines access time for transmittinginformation on the ring.

• Ring monitoring functions.


FDDI and TP-PMD both use baseband transmission, with 4B/5B encoding used torepresent the data being transmitted. This encoding scheme uses a 125 MHz bandwidth,which is multiplied by 4 and divided by 5 to derive the 100 Mbps bit rate.

FDDI uses the less sophisticated and more reliable encoding scheme of NRZI whileTP-PMD had to adopt a sophisticated encoding scheme known as MLT-3 to avoid violatingFCC emission regulations.


Speed

FDDI/TP-PMD was designed from the beginning to operate at 100 Mbps. It was the first


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g g g p pvery high-speed LAN technology available.

FutureFDDI is a proven and popular LAN backbone technology. TP-PMD has not been adopted bya large number of users as a replacement for existing Ethernet and Token-ring technologies.More recently, other technologies claiming to be as fast or faster—and lower in cost—haveappeared, as discussed next.


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IEEE 802.3 Fast Ethernet

In November 1992, Grand Junction Networks submitted its proposal for 100 Mbps


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Ethernet to the IEEE 802.3 committee. Its proposal was based on a technology allowingmigration from 10 Mbps Ethernet to 100 Mbps Ethernet on existing cabling systems. Thetechnology proposes using a frame format identical to that used in 10 Mbps Ethernet.

This technology is referred to as 100Base-T—although it has been called 100BaseX.

Since making its proposal, Grand Junction has formed a consortium of more than 60organizations interested in the advancement and interoperability of Fast Ethernettechnology.

As of March 1995, the specification was considered to be technically complete and

proceeding through the final formal standardization process.

IEEE 802.12 Demand Priority Ethernet

This second 100 Mbps version of Ethernet was first proposed by a consortium led byHewlett-Packard and AT&T. It is under review by a different IEEE committee because itstechnology is incompatible with existing Ethernet. That is, it is not based on a CSMA/CD

approach.

This technology is referred to as 100VG—although it has been called 100VG-AnyLAN,VG, AnyLAN, or 100Base-VG.

100VG is considered to be a unifying technology. It was designed to offer migration fromboth Ethernet and Token-ring networks.

… High-speed Ethernets introduction, continued

Switched Ethernet

Along with these two new Ethernet specifications, switching technologies have been


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recently introduced to the LAN environment. Implemented in centralized hubs, switchingallows for collision-free Ethernets—each station has its own segment, with the switchinstantaneously connecting any two stations internally.

Station links can be at the traditional Ethernet speed of 10 Mbps or at either of the two 100Mbps technologies described above. As well, the possibility of collision is nonexistent insuch a configuration, since a station can transmit and receive signals simultaneously,potentially doubling the link throughput to 20 or 200 Mbps. This is referred to as full-duplexEthernet. Traditional Ethernet is half-duplex—a station either transmits or receives, butnot both simultaneously.

Characteristics

Some of the characteristic features defining high-speed Ethernets are as follows:

• They are a response to the demands for greater bandwidth by Ethernet users—numbering over 40 million stations worldwide.

• They allow for a steady migration using a familiar technology, without the need formore expensive FDDI/TP-PMD or ATM technologies.

• In conjunction with switching, 100 Mbps Ethernet technologies can readilyaccommodate video services between users on a LAN, such as videoconferencingor multimedia file sharing.

… High-speed Ethernets characteristics, continued

Transmission medium

IEEE 802 3 Fast Ethernet


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100 Base-T is an extension of the existing Ethernet standard. It supports threetransmission media specifications or physical layers. These three are referred to as

follows:

• 100Base-TX

This is the specification supporting 100 Mbps Ethernet over two twisted-pairs—either 2-pair Category 5 UTP or 2-pair STP cabling.

• 100Base-T4

This is the specification supporting 100 Mbps Ethernet over four-pair UTPcabling—either Category 3, Category 4 or Category 5.

• 100Base-FX

This is the specification supporting 100 Mbps Ethernet over a two-fiber optical fiber

cabling system.

IEEE 802.12 Demand priority Ethernet

100VG (Voice Grade) accommodates both 4-pair unshielded twisted-pair cabling(Category 3 and Category 5) and optical fiber cabling. The difference between thespecifications for the two media are for maximum distances between hubs and stations.


Topology

IEEE 802 3 Fast Ethernet


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The specifications are essentially the same as for 10Base-T and the design of the cablingsystem is based on the recommendations made in ANSI/TIA/EIA-568-A. It is specified

that the system should follow a star topology.

However, to provide the high throughput in 100Base-T, the size of the collision domainmust be decreased from the limits specified in 10Base-T. That is, the total length of cableconnecting any two stations must be limited, in order to permit any attached station todetect collisions in time to delay transmitting its data. To meet these requirements, the

following distance modification is made:

The total length of UTP cabling from one station connected to ahub to another station connected to another hub, cannot exceed220 m (722 ft). In 10Base-T, this distance would be 500 m (1640 ft).

It should be noted that, while the collision domain of 100Base-T is smaller that that of10Base-T, all the distances specified by 100Base-T fall within the limits recommendedby ANSI/TIA/EIA-568-A. Any organization having installed a cabling system compliantwith ANSI/TIA/EIA-568-A can be assured that the horizontal cabling system will support100Base-T.


FIGURE 4.9: 100BASE-T FAST ETHERNET CONFIGURATION

H b


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HubHub

Hub Hub


The 100VG standard recommends a star topology that calls for a root hub which provideslinks to other hubs The collection of hubs can be cascaded to a maximum of three levels


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links to other hubs. The collection of hubs can be cascaded to a maximum of three levels.This implies a hierarchical structure for 100VG networks.

Maximum network cable length depends on the transmission media used, the number of

hubs used and the location of the hubs. Some examples of acceptable distances in the100VG environment include:

• Category 3 cable can support a maximum of three levels of hubs with a maximumdistance of 100 m (328 ft) between hubs. This results in a maximum station-to-station distance of 600 m (1968 ft).

• Category 5 cable can support a maximum of three levels of hubs with a maximumdistance of 150 m (492 ft) between hubs. This results in a maximum station-to-station distance of 900 m (2952 ft).

• If optical fiber connections are used, the station-to-station distance can be as longas 5000 m (16400 ft).


FIGURE 4.10: 100VG DEMAND PRIORITY ETHERNET CONFIGURATION

Level 1


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Root

Level 1

Level 2

Level 3

Access control



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The IEEE 802.3 Media Access Control (MAC) sublayer is unchanged.

The difference in performance level is attributed to how often data frames are transmitted.

With a 10 Mbps data rate, each frame takes 67.2 microseconds to betransmitted—57.6 microseconds to transmit the frame itself and agap of 9.6 microseconds between two frames. This translates into amaximum of 14880 frames being transmitted per second.(i.e., 67.2 microseconds x 14880 = 1 second) At 100 Mbps data rate,the frame format is unchanged, but the gap between two frames isreduced to 0.96 microseconds—one-tenth of what it is at 10 Mbps.This means that frames can be sent ten times as often—equaling148800 frames per second.



100VG does not provide for collision detection. Instead, it works with a deterministicaccess control method. There is no need for multiple access or for collision detection.


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p

The 100VG hub polls each attached station to see if it has data to transmit. If the stationhas data to transmit, it can do so in two ways—normal or priority mode. Stations are

polled in order of initial connection.

Both Ethernet and Token-ring frame types can be accommodated by the 100VGtechnology. Therefore, it can be used as an upgrade path for both.

100VG supports isochronous transmission—the ability to send time-sensitive frames, suchas those which carry voice or motion video signals.


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Transmission medium and speed

Because ATM is mainly a switching interface, its transmission can be implemented overany transmission medium—twisted-pair, optical fiber or coaxial cabling, as well as


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y p p gwireless links. The choice of media for ATM depends largely on the desired transmissionspeed and the type of environment—LAN or WAN.

The different transmission media and transmission speed choices appear to complicateATM implementation. However, ATM technology is scalable and the design intent is topermit communicating stations to operate at different speeds over their respectivenetworks yet still link transparently to each other as needed.

Overview ..................................................................................1

LAN technologies defined ............................................................. 1N

T S

•


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TRADITIONAL LAN TECHNOLOGIES ................................... 3

ARCnet .....................................................................................3

History................................................................................................ 3

Characteristics ................................................................................. 4

Terminology ...................................................................................... 5

Summary of features ....................................................................... 5Transmission medium ....................................................................... 5Topology ............................................................................................ 6Access control ................................................................................... 8Transmission technique .................................................................... 9Speed .............................................................................................. 10

Future ............................................................................................... 10

•

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O F C

O N T E N T S

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EMERGING LAN TECHNOLOGIES ..................................... 41

ANSI X3T9 5 FDDI d TP PMD 41N T S

•


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ANSI X3T9.5 — FDDI and TP-PMD .....................................41

History.............................................................................................. 41

Fiber Distributed Data Interface ................................................... 41Twisted-Pair Physical Medium Dependent ................................... 42

Characteristics ............................................................................... 42

Terminology .................................................................................... 43

Summary of features ..................................................................... 44Transmission medium ..................................................................... 44

Topology .......................................................................................... 46 Access control ................................................................................ 48Transmission technique .................................................................. 48Speed .............................................................................................. 49

Future ............................................................................................... 49

•

T A B L E

O F C

O N T E N T S

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O F

C O

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L O GI E S

High-speed Ethernets .......................................................... 50

Introduction ..................................................................................... 50IEEE 802 3 Fast Ethernet 51N

T S

•


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IEEE 802.3 Fast Ethernet ............................................................... 51IEEE 802.12 Demand Priority Ethernet .......................................... 51

Switched Ethernet ........................................................................... 52Characteristics ............................................................................... 52

Transmission medium ..................................................................... 53IEEE 802.3 Fast Ethernet............................................................. 53IEEE 802.12 Demand priority Ethernet ........................................ 53

Topology .......................................................................................... 54

IEEE 802.3 Fast Ethernet............................................................. 54

IEEE 802.12 Demand Priority Ethernet........................................ 56Access control ................................................................................. 58

IEEE 802.3 Fast Ethernet............................................................. 58IEEE 802.12 Demand Priority Ethernet........................................ 59

Asynchronous Transfer Mode (ATM) ................................. 60

Introduction ..................................................................................... 60Characteristics ............................................................................... 61

Transmission medium and speed ................................................... 62

•

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L O GI E S

Figure 4.1: Traditional ARCnet components

and configuration ..................................................... 7

Fi 4 2 Thi k t Eth t tR E S

•


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Figure 4.2: Thicknet Ethernet components

and configuration ................................................... 19

Figure 4.3: Thinnet Ethernet componentsand configuration ................................................... 20

Figure 4.4: 10Base-T Ethernet componentsand configuration ................................................... 21

Figure 4.5: 10Base-F Ethernet components


Figure 4.6: Traditional Token-ring components


Figure 4.7: Traditional LocalTalk componentsand configuration ................................................... 37

Figure 4.8: FDDI components and configuration .................... 47

Figure 4.9: 100Base-T Fast Ethernet configuration ............... 55

Figure 4.10: 100VG Demand Priority Ethernet configuration .. 57 •

F I G U R E S •

F I G U R E S

•

F I G U R E S

• F

I G U R

C

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E S

•

T bl 4 1 A i f thi k d thi Eth t 16


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•

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T A B L E S

•

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• T

A B L E

C

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T E C H N O

L O GI E S

Table 4.1: A comparison of thick and thin Ethernets ..................... 16

chapter4 lan technology

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