Xem mẫu
- 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
- 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
- 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
- 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
- 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.
- 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.
- 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.
- 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
- 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.
- 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
- 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).
nguon tai.lieu . vn
