Mạng và viễn thông P19

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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 computerswithin 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. ...

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  1. 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) 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 computerswithin 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 computerperipheral 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 constrainedby their mode of operation to a geographically limited area, but areideally suited for shortdistance 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,wordprocessors,sharedprinters,electronic filing cabinets,electronicmail 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 TOPOLOGIESANDSTANDARDS The different types of LAN are characterized by their distinctive topologies. They all comprise a single transmission path interconnecting all the dataterminal devices, with a 367
  2. 368 (LANS) LOCAL AREA NETWORKS *c> (a) Star ( b ) Ring Figure 19.1 AlternativeLANtopologies [ c ) Bus 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 logicallinkcontrol 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 transported (i.e. information frameor packet) is submitted be ------ Logicallinkcontrol O S 1 layer 2 ( I E E E 802.21 ------ I P By s i'o rrkS t a r ' nh uw c a ' ' et s o l CSMAlCD Token (IEEE 802.4 I I ring' I 'Token Figure 19.2 The IEEE 802 LAN standards i Token ring (IEEE 802.51
  3. CSMAjCD (IEEE 802.3, I S 0 8802.3): ETHERNET 369 to the logical link control (LLC) layer together with the address of thedevice to which it is to be transmitted. Much like HDLC in X.25 (Chapter 18), the LLC assures successful transfer,errordetection,retransmit,etc.Figure 19.2 showstherelationship 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 connectedtotheLAN.Toa lesser degree,thegeographiccircumstancesandthe network’s performance requirements are also factors. the possible protocols transfer All 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 CSMAICD standsfor carriersensemultipleaccesswith collision detection. It is a 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 pathis 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 thatnootherstations havestartedsendingdataatthesametime.Ifthesending 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 called ethernet. is Ethernet was originallyproprietary a LANstandard(predatingtheIEEE 802.3 standard) developed by the Xerox corporation of USA. The original design was based on a length coaxial cable, with of ‘tee-offs’ to individualwork 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 bus each of the corridors and connecting the cables together, a could be created over all 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 cabling needed tobe new 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 existingusers. Thin-ethernet(cheapernet or IObase2) appeared.This
  4. 370 NETWORKS AREA LOCAL (LANS) coaxial cable bus 0 baluns - tee-off - W- a) ethernet as coaxial cable bus b) ethernet as structured twisted pair cabling Figure 19.3 Typical coaxial cable and twisted pair wiring configurations for Ethernet allowed the use of narrower gauge coaxial cable as the main bus in smaller networks, and helped to reduce theinstallationcosts.Meanwhile,thenumbers of computer devices in offices were multiplying rapidly. Multiple ethernets became necessary, and increasedflexibilitywasdemanded to enableuserstomove offices withoutmajor cablingdisturbances.Thiscausedthedevelopment of LANs on structuredcabling, using LAN hubs and twistedpair telephonecabling, inastarconfiguration.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
  5. 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 transmissionlinefailures. As aresult,ethernethas become predominant the type of LAN. The fact that station any 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 destinations unavailable some are 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 alltrying tocommunicateoverthesame CDMA/CDLAN lead to rapidnetwork performance degradation under heavy load. In practice, however, the traffic is rarely random, because most users communicate with the various main central server devices withinthenetworkwhichregulate thecommunication.However,shouldpoorper- 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 canbe 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’ the LAN, the of originalterminalcreatesa new successor by generatinga 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 andtoken ringnetworkspredominate.Tokenbusnetworksaremostcommonin manufacturing premises, often operating as broadband (high speed) networks for the tooling and control of complex robotic machines.
  6. 372 NETWORKS AREA LOCAL (LANS) 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 the header in 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 thedata is confirmed to the transmitter changing a bit value in the token’s by 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 tokengiven next time around. The is 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-criticaldevices 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 ringwasdeveloped by theIBMcompany,and is mostcommon in office installations where large IBM mainframe and midrange computers (particularly AS400) are inuse,in additiontolargenumbers of IBMPCs.Theoriginalformrequired 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 specialcables. 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 divertedthroughthatdevice(Figure 19.4). The specialsocket forearlytokenring 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 alsoneed 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 (mainlyby IBM) has brought about the ability to twistedpair cabling, and the emergence of 16 Mbit/s as well as use a 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.
  7. TOKEN RING (IEEE 802.5) 373 0 . unconnected Figure 19.4 Socket design in token ring LANs to ensure ring continuity 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 hubitself, 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 reducingthecomplexity and cost of individualsockets, so thatstandard telephone sockets may be used. The token ring LAN may differ from the ethernet LAN only in the port cards used within the hub and the LAN cards in the individualPCs. Otherwise cabling, wiring used 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,linkingpersonal computersforthepurpose 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, butin 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.
  8. 374 AREA LOCAL NETWORKS (LANS) Inmostcases,thechoicebetweenethernetandtokenringcomesdowntothe recommendation of a user’s computer supplier, as hardware and software a particular of computer type may have been developed with one or other type LAN in mind. Thus of 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 ringLAN used primarily by the UK academic community. 19.6 LOGICAL LINK CONTROL FOR LANs A localarea 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 differentphysicalforms 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 carriedby LLC consists of four fields, which togetherare termed the LLC protocol data unit (PDU). The four fields are the destination service access point ( D S A P ) , an address which identifies the application or software sessionto 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 thephysical structure of LANs, and thelogical 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 LAN operating system
  9. 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 forthe software sockets (i.e. interface) between normal computer operatingsoftware(e.g. Microsoft DOS, Windows, Windows9.5, Apple Macintosh, etc.) and the new functions made possible by LAN networks (e.g. file server, hostgateway, fax server, common printer, etc.). LAN operating systems are closely linked to network protocols, manyof which have a proprietary nature. Thus, for example, Novell Netware (network operating system software) in conjunction with the Novell IPX network protocolallows the personal computer user to use various types of ‘network’ services. For example, a PC on the LANmay 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 occasionscolour on other the printer is moreappropriate. 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 largegroups of users.Equivalentfunctionality can be providedusingthe 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 the for 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 LANandto afacsimile/telephoneconnection. 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 toa mainframe computer or other external network or device requires the use of bridges, routers or gateways. We discuss these in turn.
  10. 376 (LANS) NETWORKS AREA LOCAL A bridge is used to link two separate LANs together as if they were a single LAN, typicallyenablingthe maximum capacity of asingle 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 bridgeconnectiontothesecondbridge,whereit is injectedinto the second LAN (Figure 19.5), which may haveadifferentphysicalform(e.g.ethernetltokenring). Either a table of the relevant addresses must be kept to date each of the bridges, to up in determinewhichpacketsmust be transferredintothesecondLAN,orsimplyall 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 meansof 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) used when transporting framesbetween any pairof the is 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 complicatednetworks (even oneswhich are constantly growing or changing)and accordingly routeframes or packetsacrossthem to thedestination indicatedintheheader.Routerslearnaboutnetworkchangesthroughexperience. Crudely put, if they receive a packet or message for a destination which they do not recognize,theychoosea 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 complicatedand cumbersome networkswhich have been built by different parties and simply connected together. ManyLANprotocols (e.g. Novell’s ZPX, Appletalk, etc.)may be routedintheir native(i.e.raw)form,butit is nowadaysincreasingly common instead to use the transmission controlprotocollinternetprotocol (TCPIZP) as main the protocol to interconnect complicated LAN networks. / bridge Ethernet LAN Figure 19.5. LAN bridge
  11. INTERCONNECTION OF LANS: BRIDGES, ROUTERS AND GATEWAYS 377 3270 I h && m G z A W SNA network Figure 19.6 Replacement of dumb terminals using a LAN and 3270 gateway TCP/IP uses a worldwide standardized addressing scheme (Internet addressing) to identifyenduserstationsuniquely.Thisprovidestheability to connectall LANs worldwide into a single common network, the Internet, thus extending the information sharing and electronic capabilities mail of single LANs the to world computer community as a whole. TCP/IP originally developed as a transport protocolbefore was the OS1 layer 4 was fully defined. Its development was sponsored by the US government and military, particularly to provide for widescale interconnection of UNIX computers and workstations. The problemwith networksof multiple routers (including the Internet itself) is that the individual routes through the network are difficult to monitor and manage. It difficult is to know which networks are being transitted along the way, so that optimal network loadingandthe security of informationcannot be guaranteed, though emerging standards will address these shortcomings. We discuss the Internet and TCPjIP protocol in Chapter 22. A L A N gareway provides access for a LAN user to an external service, such as a mainframe computer.Typically a gateway consists of a Personal Computer (PC) on the LAN used entirely to rungateway software. An exampleof a widely used LANgateway is a 3270- or SNA-gateway. IBM 3270 is the communication protocol used between the host computer and the terminal of an IBM mainframe. The protocol (part of SNA, Chapter 18) allows the computer to interpret keyboard interaction at the terminal and control the exact image appearing on the terminal screen, without there needing to be ‘intelligence’ in the terminal. Thus3270 type terminals are sometimes described as dumb terminals. The conversion necessary to make anintelligent terminal (such as a personal computer) appear to talk to a host computer like a dumb terminal is carried out by terminal emulation or 3270 emulation software. Where this software resides in a LAN gateway, then it is called a 3270 gateway (Figure 19.6).
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