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Token bus networks are not commonly used in office environments where ethernet and token ring networks predominate.. 19.5 TOKEN RING IEEE 802.5 The token ring standard is similar in oper

19 Local Area Networks

(LANs)

We have seen how packet switching has contributed greatly to the efficiency and flexibility of

‘wide area’ data networks, involving a large number of devices spread at geographically diverse locations Packet switching, however, is not so efficient for smaller scale networks, those limited

to linking personal computers within an office building; that is the realm of an alternative type of packet-switched-like network called a local area network or LAN for short In this chapter we discuss the concept of a LAN and the various technical realizations which are available

19.1 THE EMERGENCE OF LANs

LANs emerged in the late 1980s as the most important means of conveying data between different computers and computer peripheral devices (printer, file server, electronic mail server, fax gateway, host gateway, computer printer, scanner, etc.) within a single office, office building or small campus LANs are constrained by their mode of operation to a geographically limited area, but are ideally suited for short distance data transport

A high bit speed LAN can carry high volumes of data with rapid response times Such performance is crucial for most office applications, and has made them the ideal foundation for the new generation of ‘electronic offices’ comprising electronic work- stations, word processors, shared printers, electronic filing cabinets, electronic mail systems and so on

Most LANs conform to one of the different types specified in the Institution of Electrical and Electronic Engineers’ IEEE 802 series of standards All the types have been developed from proprietary LANs, developed earlier by individual companies or organizations, but have now achieved American and worldwide recognition, as I S 0

8802 standards

19.2 LAN TOPOLOGIES AND STANDARDS

The different types of LAN are characterized by their distinctive topologies They all comprise a single transmission path interconnecting all the data terminal devices, with a

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Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

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bit speed typically between 1 and 30 Mbit/s, together with appropriate protocols (called

the logical link control and the medium access control ( M A C ) ) to enable data transfer

The three most common topologies are illustrated in Figure 19.1, and are called the

star, ring and bus topologies

Slightly different protocol standards apply to the different topologies For example,

IEEE 802.3 defines a physical layer protocol called CSMAjCD (carrier sense multiple

access with collision detection) which may be used with a bus or star topology Used

with a bus form medium, such LANs are normally referred to as ethernets IEEE 802.4

( I S 0 8802.4) defines an alternative layer-l protocol for a token bus, again suitable for

either a bus or star topology IEEE 802.5 defines a layer 1 protocol suitable for use on a

token ring topology Finally, IEEE 802.2 ( I S 0 8802.2) defines a logical link control

protocol (equivalent to the OS1 layer 2) that can be used with any of the above This

provides for the transfer of information between any two devices connected to the

LAN The information to be transported (i.e information frame or packet) is submitted

- - -

O S 1 l a y e r 2 Logical link control

( I E E E 802.21

- - -

I

C S M A l C D Token

( I E E E

8 0 2 4 I i Token

r i n g ( I E E E

802.51

P h y s i c a l n e t w o r k ' B u s ' o r ' S t a r ' I 'Token ring'

I

CSMAjCD (IEEE 802.3, I S 0 8802.3): ETHERNET 369

to the logical link control (LLC) layer together with the address of the device to which it

is to be transmitted Much like HDLC in X.25 (Chapter 18), the LLC assures successful transfer, error detection, retransmit, etc Figure 19.2 shows the relationship of the various standards

Which physical layer protocol and which topology of LAN to use depend largely on individual preference and the compatibility of the existing computer kit needing to be connected to the LAN To a lesser degree, the geographic circumstances and the

network’s performance requirements are also factors All the possible protocols transfer data between the nodes, using a packet mode of transmission; they differ in how they prevent more than one terminal using the bus or ring at the same time The various protocols and their relative merits are now considered in turn

19.3 CSMA/CD (IEEE 802.3, I S 0 8802.3): ETHERNET

contention protocol On a CSMA/CD LAN the terminals do not request permission from a central controller before transmitting data on the transmission channel; they

contend for its use Before transmitting a packet of data, a sending terminal ‘listens’ to check whether the path is already in use, and if so it waits before transmitting its data Even when it starts to send data, it needs to continue checking the path to make sure that no other stations have started sending data at the same time If the sending

terminal’s output does not match that which it is simultaneously monitoring on the transmission path, it knows there has been a collision To receive data, the medium access control ( M A C ) or layer 1 software in each terminal monitors the transmission

path, decoding the destination address of each packet passing through to find out

whether it is the intended destination If it is, the data is read and decoded; if not, the data are ignored

The most important type of network that employs the CSMA/CD is called ethernet

Ethernet was originally a proprietary LAN standard (predating the IEEE 802.3 standard) developed by the Xerox corporation of USA The original design was based

on a length of coaxial cable, with ‘tee-offs’ to individual work stations, with a maximum

of around 500 stations The idea was to simplify the cabling needs of offices in which many personal computers were in use Simply by laying a single coaxial cable along each of the corridors and connecting all the cables together, a bus could be created over

which all the office computer devices could intercommunicate Each time a new device was installed, a new tee-off could be installed from the corridor into the particular office where the device was situated (Figure 19.3(a)) Meanwhile, no new cabling needed to be installed along the corridor, so saving space in the conduits and averting the constant removal and replacement of the ceiling tiles

The technology for basic ethernet (lObase5) developed rapidly First, clever devices

for the tee-off points were developed, which enabled new devices to be connected very quickly without first severing the main coaxial cable bus The devices pressed directly into the cable This reduced the time needed for new installations and reduced the disturbance to existing users Thin-ethernet (cheapernet or IObase2) appeared This

coaxial cable

0

-

-

baluns

bus

a) ethernet as coaxial cable bus

b) ethernet as structured twisted pair cabling

allowed the use of narrower gauge coaxial cable as the main bus in smaller networks, and helped to reduce the installation costs Meanwhile, the numbers of computer devices in offices were multiplying rapidly Multiple ethernets became necessary, and increased flexibility was demanded to enable users to move offices without major cabling disturbances This caused the development of LANs on structured cabling,

using LAN hubs and twisted pair telephone cabling, in a star configuration The

ethernet ZObaseT standard was born (Figure 19.3(b))

As Figure 3(a) illustrates, in the coaxial cable realisation, a single cable bus, usually

installed in the cable conduit in the office corridor provides the main network element Tee-offs into individual offices are installed as needed, either by teeing directly into the main bus, or by using pre-installed sockets and connectors A baluns, usually built into

the coaxial cable socket in the end location, provides for correct impedance matching

(50), whether or not the device is connected into the socket

When installed as part of a structured cabling scheme (nowadays the most common

realization of ethernet), twisted pair cabling provides for the transmission medium

Multiple twisted pair cables are usually installed in each individual office and near each

TOKEN BUS (IEEE 802.4, I S 0 8802.4) 371

individual desk during office renovation, and wired back to a wiring cabinet, of which there is usually one per floor, installed in an equipment room Usually next to the wir- ing cabinet, or even in the same rack, a LAN hub is installed The hub replaces the

coaxial cable backbone, so that the arrangement is sometimes referred to as a collapsed backbone topology The bus topology still exists, but now only within the hub itself, which provides for the interconnection of all the devices forming the LAN, ensuring

physical connection and appropriate electrical impedance matching

Should new devices need to be added, a spare cable can be patched through at the wiring cabinet and a new port card can be slid into the hub Should any of the devices need to be moved from one office to another this can be achieved by re-patching at the wiring cabinet The adds and changes are thus far less disruptive both to other LAN

users and the office furnishings In the structured cabling scheme, the baluns is no longer

needed, since the hub provides for this function

Ethernet LAN components are relatively cheap The bus topology is easy to realize

and manage and is resilient to transmission line failures As a result, ethernet has

become the predominant type of LAN The fact that any station may use the transmission path, so long as it was previously idle, means that fairly good use can be made of the LAN even when some destinations are unavailable because of a transmission path break, a capability which is not enjoyed by LANs employing more

sophisticated data transmission, as we shall see later

Theory suggests that the random collisions of a large number of competing devices all trying to communicate over the same CDMA/CDLAN lead to rapid network

performance degradation under heavy load In practice, however, the traffic is rarely

random, because most users communicate with the various main central server devices

within the network which regulate the communication However, should poor per- formance under heavy load be a problem, it can usually be overcome by subdivision into smaller, interconnected LANs

19.4 TOKEN BUS (IEEE 802.4, I S 0 8802.4)

A token bus LAN controls the transmission of data onto the transmission path by the

use of a single token Only the terminal with the token may transmit packets onto the

bus The token can be made available to any terminal wishing to transmit data When a terminal has the token it sends any data frames it has ready, and then passes the token

on to the next terminal To check that its successor has received the token correctly the terminal makes sure that the successor is transmitting data If not, the successor is assumed to be on a failed part of the network, and to prevent ‘lock-up’ of the LAN, the

original terminal creates a new successor by generating a new token Transmission faults in the LAN bus can therefore be circumvented to some extent However those

parts of the LAN that are isolated from the token remain cut-off

Token bus networks are not commonly used in office environments where ethernet and token ring networks predominate Token bus networks are most common in

manufacturing premises, often operating as broadband (high speed) networks for the tooling and control of complex robotic machines

19.5 TOKEN RING (IEEE 802.5)

The token ring standard is similar in operation to the token bus, using the token to pass the ‘right to transmit data’ around each terminal on the ring in turn The sequence of token passing is different: the token itself is used to carry the packet of data The transmitting terminal sets the token’sflag, putting the destination address in the header

to indicate that the token is full The token is then passed around the ring from one terminal to the next Each terminal checks whether the data is intended for it, and passes it on; sooner or later it reaches the destination terminal where the data is read Receipt of the data is confirmed to the transmitter by changing a bit value in the token’s flag When the token gets back to the transmitting terminal, the terminal is obliged to empty the token and pass it to the next terminal in the ring

The feature of IEEE 802.5 MAC protocol is its ability to establish priorities among the ring terminals This it does through a set of priority indicators in the token As the token is passed around the ring, any terminal may request its use on the next pass by putting a request of a given priority in the reservation field Provided no other station makes a higher priority request, then access to the token is given next time around The reservation field therefore gives a means of determining demand on the LAN at any

moment by counting the number of requests in the flag, and in addition the system of prioritization ensures that terminals with the highest pre-assigned authority have the

first turn High speed operation of certain pre-determined, time-critical devices is likely

to be crucial to the operation of the network as a whole, but they are unlikely to need the token on every pass, so that lower priority terminals have a chance to use the ring when the higher priority stations are not active

Token ring was developed by the IBM company, and is most common in office installations where large IBM mainframe and midrange computers (particularly AS400) are in use, in addition to large numbers of IBM PCs The original form required

specialized cabling (IBM type 1) and operated at 4 Mbit/s form The initial idea was that a single cable loop could be laid through all the offices on a floor or in a building and devices added on demand To avoid the disturbances and complications which might arise when connecting new devices to the ring (any break in the ring renders the LAN inoperative), IBM developed a sophisticated cabling system, including the various IBM special cables The cable loop was pre-fitted with a number of sockets at all possible user device locations The sockets ensured that when no device was connected, the ring was through-connected However, on plugging in a new device, the ring is diverted through that device (Figure 19.4) The special socket for early token ring

networks thus catered not only for correct impedance matching, but also for the ring

continuity

Token ring cards in the individual end user computer devices connected to token ring LANs also need to be designed to ensure ring continuity in the case that the device

is switched off Thus the card reverts to a ‘switched-through’ state when no power is applied, so that even though the end device itself plays no active part in token passing while switched off, the tokens nonetheless still have a complete ring available

The further development of the token ring technology (mainly by IBM) has brought about the ability to use twistedpair cabling, and the emergence of a 16 Mbit/s as well as

the original 4 Mbit/s version In the 16 Mbit/s version, higher quality cabling (e.g cate-

gory 5 cable, as discussed in Chapter 8) may be required

TOKEN RING (IEEE 802.5) 373

unconnected

Token ring LAN hubs have also developed alongside ethernet hubs, and allow for

similar collapsed backbone topologies in conjunction with structured cabling systems

Thus a token ring LAN today is difficult to distinguish from an ethernet LAN (Figure 19.3(b)) The ring topology is collapsed into the hub itself, and two sets of wires to each individual user station allow for the extension of the ring to each user device The switch-through function previously performed by the socket is also undertaken at the hub, so reducing the complexity and cost of individual sockets, so that standard telephone sockets may be used

The token ring L A N may differ from the ethernet L A N only in the port cards used

within the hub and the LAN cards used in the individual PCs Otherwise cabling, wiring

cabinet and LAN hub unit may be identical Indeed, in some companies, ethernet and token ring LANs exist alongside one another, without the user being aware to which type of LAN he is connected

Token rings, like ethernets, are common in office environments, linking personal computers for the purpose of data file transfer, electronic messaging, mainframe computer interaction or file sharing Some LAN administrators are emotional about

whether ethernet or token ring offers the best solution, but in reality for most office users

there is little to choose between them Token ring LANs perform better than ethernets

at near full capacity or durihg overload but can be more difficult and costly to install, especially when only a small number of users are involved

In most cases, the choice between ethernet and token ring comes down to the

recommendation of a user’s computer supplier, as hardware and software of a particular computer type may have been developed with one or other type of LAN in mind Thus

token ring remains the recommendation of the IBM company, whereas in all other

environments ethernet has gained the upper hand

Slotted ring and other types of LAN also exist, but are not covered in detail in this

book because they are rare I S 0 8802.7, for example, describes a slotted ring LAN used

primarily by the UK academic community

19.6 LOGICAL LINK CONTROL FOR LANs

A local area network ( L A N ) provides for the establishment of direct (OS1 layer 2)

connection between any two end devices directly connected to the LAN However, although various different physical forms and topologies are possible (e.g ethernet, token ring, etc.), it was quickly realized that all LANs are expected to be capable of the

same basic function: carriage of data between software or applications running on two

different computers It therefore made sense to define a standard interface between the LAN and the computer software intended to communicate across it This standard

interface is called the logical link control ( L L C ) protocol It is defined by IEEE standard

802.2 (IS0 8802.2)

The logical link control ( L L C ) provides a standard communication interface equiva-

lent to that provided by OS1 layer 2 to OS1 layer 3 (datalink service, see Chapter 9)

LLC in combination with the medium access control ( M A C ) protocol specific to the particular LAN (e.g ethernet, token bus, token ring) is equivalent to an OS1 layer 2

protocol

The information carried by LLC consists of four fields, which together are termed the

LLC protocol data unit ( P D U ) The four fields are

the destination service access point ( D S A P ) , an address which identifies the

application or software session to be activated in the destination computer to receive the packet

the source service access point ( S S A P ) , an address that identifies the application

which sent the packet

control information, which includes details of the type of connection (e.g

connection-mode, connectionless, acknowledged connectionless), the protocol in use

at the next higher layer (e.g TCP/IP, IPX, Appletalk, etc.)

the user data (i.e the raw data being transported)

19.7 LAN OPERATING SOFTWARE AND SERVERS

So far we have talked about the physical structure of LANs, and the logical procedures used to convey the information packets across them This alone, however, is not a sufficient basis for creation of an office LAN In addition, a L A N operating system

INTERCONNECTION OF LANS: BRIDGES, ROUTERS AND GATEWAYS 375

(software) is required At the start, a number of different manufacturers offered altern-

ative proprietary systems Over time, the systems in use have reduced to five: Novell Netware, IBM LAN Manager, Appletalk, Windows for workgroups and WindowsNT

LAN operating systems provide for the software sockets (i.e interface) between

normal computer operating software (e.g Microsoft DOS, Windows, Windows9.5, Apple Macintosh, etc.) and the new functions made possible by LAN networks (e.g file server, host gateway, fax server, common printer, etc.) LAN operating systems are closely linked to network protocols, many of which have a proprietary nature Thus, for

example, Novell Netware (network operating system software) in conjunction with the

Novell IPX network protocol allows the personal computer user to use various types of

‘network’ services For example, a PC on the LAN may be able to choose between any

of the printers connected to the LAN rather than being limited to the one directly connected to his computer Sometimes he might choose the fast black and white laser printer, whereas on other occasions the colour printer is more appropriate

Alternatively, a common file server might allow the LAN users to share a common

data filing system In this way each individual user has a wider choice of facilities, and overall less equipment is needed because the printers and other devices can be shared between large groups of users Equivalent functionality can be provided using the

UNIX operating system software in conjunction with TCP/IP or the Apple Macintosh system in conjunction with Appletalk

Servers are typically powerful and expensive computers, capable of faster processing

and additional functions useful to the workgroup as a whole Servers are connected to

the LAN, and usually remain in operation for 24 hours per day

Afile server is usually a computer with a large amount of storage capability which

may be rapidly accessed and easily backed up by specialist computer staff on a once per

day or once per week basis It provides for secure storage of information and easy sharing of information basis on a workgroup or defined closed user group basis

A mail server provides for the transmission of electronic mail letters between individual PC users connected to the LAN without the need for the users both to be connected to the network and have their PCs switched on at the time of sending or receiving the letter

A facsimile server allows individual PC users to send printed documents directly to a

remote facsimile machine without the need first to print the document to paper The facsimile server itself is a device like a PC which is connected simultaneously to the LAN and to a facsimile/telephone connection To the LAN, the facsimile server appears like a printer with LAN operating software, but instead of printing directly, the document is converted to facsimile (fax) format and transmitted over the telephone line

to the remote fax device

19.8 INTERCONNECTION OF LANs: BRIDGES,

ROUTERS AND GATEWAYS

The interconnection of numerous LANs, perhaps of different types, or the connection

of a LAN to a mainframe computer or other external network or device requires the use

of bridges, routers or gateways We discuss these in turn

A bridge is used to link two separate LANs together as if they were a single LAN,

typically enabling the maximum capacity of a single LAN to be surpassed, or two separate LANs in locations remote from one another to be connected as if they were a

single LAN (a so-called remote bridge) The bridge is an intelligent hardware connected

to the LAN, which examines the address in the LLC (logical link control) header of

each packet or frame For relevant frames, the packet is removed, passed across the bridge connection to the second bridge, where it is injected into the second LAN (Figure 19.5), which may have a different physical form (e.g ethernetltoken ring)

Either a table of the relevant addresses must be kept up to date in each of the bridges, to determine which packets must be transferred into the second LAN, or simply all

packets are bridged

A remote bridge differs from a local bridge only in that a wide area type connection

(e.g X.25 connection or leased line connection) is used to connect the bridges Usually only the packets destined for the remote LAN are bridged by a remote bridge, so that the lower bitrate of the bridge connection (typically 9600 bit/s) does not become a bottleneck

Although bridges provide for a relatively cheap means of interconnecting LANs, they are not to be recommended in large, complex networks, because they result in very complicated topologies which are extremely difficult to manage Thus, for example, a bridge network of three LANs could have three bridge connections, connecting the LANs in a triangle fashion The problem now is to ensure that the appropriate bridge connection (i.e the direct one) is used when transporting frames between any pair of the LANs In very large networks the chance of optimal path-finding is very low, so that there is a great risk of endless circular paths To overcome this problem, the router

appeared

Routers are much more intelligent devices than bridges They are designed to ‘learn’

the topology of complicated networks (even ones which are constantly growing or changing) and accordingly route frames or packets across them to the destination indicated in the header Routers learn about network changes through experience

Crudely put, if they receive a packet or message for a destination which they do not recognize, they choose a route at random and see if it is successful On following occasions, the previously successful route is selected In this way, communication is possible even across very complicated and cumbersome networks which have been built

by different parties and simply connected together

Many LAN protocols (e.g Novell’s ZPX, Appletalk, etc.) may be routed in their native (i.e raw) form, but it is nowadays increasingly common instead to use the transmission control protocollinternet protocol (TCPIZP) as the main protocol to

interconnect complicated LAN networks

Ethernet LAN