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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)
PART 1
FUNDAMENTALS OF
TELECOMMUNICATIONS
NETWORKS
- 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)
I
Znformation and
its Lonveyance
All
The world about us brims with information. the time our ears, eyes, fingers, mouths and noses
sense theenvironmentaround us, continuallyincreasing our ‘awareness’,‘intelligence’ and
‘instructive knowledge’. Indeed these last two phrases are the heart of the Oxford Dictionary’s
at
definition of thewordinformation.Communication,ontheotherhand, is defined as ‘the
imparting, conveyance or exchange of ideas, knowledge or information’. It might be done by
word, image, instruction, motion, smell - or maybe just a wink! Telecommunication is com-
munication by electrical, radio or optical (e.g. laser) means. We introduce the basic capabilities
and terminology of telecommunications and networking in this chapter.
As hybrid words go, ‘telecommunications’ wins no prizes, but it is all we have to work
with. Thegreek ‘tele’ prefix means distant, and nothing communication, in the
else; sense
of information passed to and fro between human beings, and animals,is an activity that
goes back beyond recorded times. With a broad view, long distance communications
brings to mind Armada beacons, heliographs flashing between Frontier posts, empires
held together by relays of post-houses, whales singing to one another in the deep, and
the family dog which conducts its social by laying and following scent trails.For the
life
narrower purposes of this book telecommunications is going to mean the transfer of
information by electromagneticmeans,(andwiththis will goacertainamount of
accepted jargon). All systems have much in c o q n o n , whatever their age. In principle
each requires a transmitter, a carrying device or transmission medium, a receiver, and a
supply of information which be equally comprehensible at both ends. For lessons in
will
technique nothing should be disregarded, however ancient: for a cheap, speedy and
comprehensive message, what is there to beat a human wink?
In the science and business of telecommunications, a structured framework has been
created for conveying certain types of information across long distances, with little
respect for the barriers of geography. In this book we study this framework; first we
understand how the forcesof electricity, light and radiowaves may be tamed to provide
a basis for such communication; then we focus on the pragmatic operationof networks
and the quest for solutions to the business needs of information flow.
3
- 4 CONVEYANCE
INFORMATION AND ITS
Figure 1.1 illustrates a simple but powerful model for understanding and categorizing
various different means of communication. The model illustrates a number of different
ways in which a business may communicate either within itself, or with its external
environment of suppliers and customers.
Thus we introduce concept
the of an
‘informationenvironment’,acrosswhich information flows inone of anumber of
different forms.
The simplest form of information flow (illustrated by Figure 1.1) might be directly
from one person to another, by word of mouth or by a visual signal. Alternatively the
information could have been conveyed on paper orelectrically. The advantage of either
of the latter two methods is that the information in paper or electronic form may also
be readily stored for future reference.
In this book we shall use the model of Figure 1.1 twice. Here we use it to illustrate
how different methods of communication may be categorized into one of the three
broad types, and as a basis for explaining the prerequisite components of a telecom-
munications system. In Chapter43 it is used to illustrate the analysis, simplification and
planning of business information flows. This double theme runs throughout the book:
understandingtelecommunicationstechnology, and explainingitsexploitation in a
pragmatic business-oriented manner. Well-known examples of communications met-
hods that fall into the three categories of paper, person-to-person and electronic are
given in Figure 1.2.
Some types of communication are hybrids of the three basic methods. Modern fac-
simile machines, for example, are capable of relaying images of paper documents over
the telephone network and recreating them at a distant location. This would appear as
quite a complex information path on our model of Figure 1.1, as Figure 1.3 shows.
Firstapersonmustrecordtherelevantinformationonpaper(shownas(a)on
Figure 1.3), then he must feed it into the facsimilemachinewhichconvertsit into
n Business
I
Per s&
Paper
process
4
4 4
Paper
m Person
a
External
2. r. r f
storage
Figure 1.1 Categorizing information flows
- INFORMATION AND ITS CONVEYANCE 5
Mode Examples
One - t o - one Broadly
aimed
Paper Sending
a Advertising
letter board
Leavingnote
a
Person - t o - person Tal king Acting
Winking Television
Radio
Electronic c - Computernetworking
-
Figure 1.2 Categorizing simple communication methods
electronic format (b). Next the telephone network conveys the electronic information
(c), before the distant facsimile machine reconverts the information to paper (d) and
the receiver reads it (e).
Theexample we havechosen is ratherconvoluted,requiringseveral successive
conversions to take place, changing the format of the information from ‘personal’ to
‘paper’ to ‘electronic’ and back again. All these
conversions make the process
inefficient, and as we shall find out later, companieswho have recognized this fact have
already about
set converting their
all key businessinformation electronic
into
(computer)format,notonlyforconveyance,butalsoforstorageandprocessing
purposes.
Paper
process
(a) Information
typed
(b)Paper (dl Receiving
fed into facsimile
Per son facsimile machine Person
( sender ) machine Ireceiver)
telephone
Figure 1.3 Information conveyance by facsimile
- 6 CONVEYANCE
INFORMATION AND ITS
1.1 TYPES INFORMATION
OF
Put specifically in the context of telecommunications, information might be a page of
written text, a conversation or a television picture. The information usually requires
conversion intoan electricalsignal in orderto beconveyed by telecommunication
means. However, there are many different types of information, so can they all be
treated identically? The answer to this is ‘no’, because each type of information makes
slightly different demands on the telecommunication system.
Information conveyed over telecommunications systems is usually classed as either
an analogue signal information or as data (digital information). An analogue signal is
an electricalwaveformwhichhasashapedirectlyanalogous to the information it
represents (e.g. speech or a television picture). Data, on the other hand, is the word
given to describe information in the form of text, numbers or coded computer or video
information.
Different forms of data and analogue signal information require different treatment.
For example, when conversing with someone we expect their reply to follow shortly
after our own speech, but when we send a letter we do notexpect a reply for some days.
The analogy runs directly into telecommunications. Thus, an electrical representation
of conversation must allow the listener to respond instantly. However, in the case of
data communication, slightly more leeway exists, as a computer is prepared to accept
response times of several seconds. A human would find this length of delay intolerable
in everyday speech. Another difference between electrical representations designed for
different applications will be the speed with which information can be transferred. This
is normally referred to as the information rate, the bandwidth or the bitrate. A speech
circuit requires more bandwidth to carry the different voice tones than a telegraph wire
needs simply to carry the same information as text. Later chapters in this book discuss
the various methods of electrical representation and the technical standards used.
1.2 TELECOMMUNICATIONS
SYSTEMS
There are four essentials for effective information transfer between two points, all of
which are provided in well-designed telecommunications systems:
0 atransmitting device
0 atransport mechanism
e areceivingdevice
e the fourth requirement is that the conveyed information is coded in such a way as to
be compatible with, and comprehensible to, the receiver.
All four components together form a telecommunications system.
In the example of a communication system consisting of two people talking to one
another, the transmitting device is the mouth, the transport mechanism is the sound
through the air, and the receiving device is the other person’s ear. Provided that both
people talk the same language, then the fourth requirement has also been met, and
- A 7
conversation can continue. However, if the talker speaks English, and the listener only
understands French, then, despite the availability of the ‘physical components’ of the
system (i.e. mouth, ear and air), communication ineffective due to the incompatibility
is
of the information.
The coding and method of transfer of the information over the transport mechanism
is to said to be the protocol. In our example the protocol would be either the English
or the French language: the fact that the talker is English and the listener French is
an example of protocol incompatibility. Protocol also defines the procedure to be used.
An example of the procedural part of protocol is the use of the word ‘over’ to signify
the end of radio messages (for example ‘Come in Foxtrot, Over’). The protocol in this
case prompts a reply and prevents both parties speaking at once. The hardest part of
telecommunications system design is often the need to ensure the compatibility of the
protocol. In some cases, this necessitates the provision of interworking devices. In our
example, the interworking device might be a human English/French interpreter.
1.3 A BASIC TELECOMMUNICATIONS SYSTEM
Figure1.4illustratesthephysicalelements of asimpletelecommunicationssystem
including the transmitter, the receiver and the transport mechanism.
As alreadydiscussed,thephysicalelementshowninFigure 1.1 mustbecomple-
mented by the use of a compatible protocol between transmitter and receiver. Together
with such a protocol,we have all the means communication from point A to point
for B
in Figure 1.4. We do not, however, have the wherewithal for communication in reverse
(i.e. from B to A). Such single direction communication, or simplex operation as it is
called, may suffice for some purposes. For manymore examples of communication, two
way, or duplexoperation is normallyrequired. For duplexoperation,atransmitter
and a receiver must be provided at both endsof the connection, as shown in Figure 1.5.
A telephonehandset, for example,contains bothamicrophoneand anearphone.
Duplex operation allows both parties to talk at once and both to be able to (and have
to)listen.Thisallowsthe human listener tointerrupt,or twocomputerstosend
information to one another in both directions at the same time. Not all devices are
capable of talking and listening at the same time as required for duplex operation.
Information f l o w
*
Receiver Transport
transmitter
mechanism
A*0
Figure 1.4 Basic physical elements of a telecommunications system (simplex operation)
- 8 CONVEYANCE
INFORMATION AND ITS
Information flow
4
1 1
Transmitter 4Receiver
A 0
+ Receiver +
Transmitter
Figure 1.5 A basic duplex telecommunications system
There is also halfduplex operation, inwhichcommunication is possiblein both
directions, but not at the same time, as only one communications path is available. First
the talker must stop speaking, then the listener can reply.
The transport mechanism can be one of a range of different media, ranging from
sound waves passing through air to laser light pulses passing down the latest tech-
nology, optical fibre. Furthermore the transport mechanism may or may not comprise
an element of switching, as we describe later in the chapter.
Most transport mechanisms demand an encoding of the information or data into a
signal form suitable forconveyance over electrical transmission media. Chapters 2 to 5
describehow many of thecommonforms of information(e.g.speech, TV, telex,
computer data,facsimile, etc.) are converted into a transmittable signal carried either
in
‘analogue’ or ‘digital’ form. In Chapters 6 and 9 we discuss various methods of switch-
ing and in Chapter 8 we discuss a range of different transmission media (cables, radio
systems, etc.), describing how different ones provide the optimum balance of low cost
and good transmission performance for individual cases of application.
1.4 COMMON TYPES OF TELECOMMUNICATIONSSYSTEMS
Inorderto meetdifferingcommunicationsneeds,anumber of differenttypes of
telecommunications equipment have been developed time.
over These
include, in
chronological order:
e telegraph
e telephone
e telex
e data networks using either circuit-, packet-, frame- or cell-switched conveyance
e computer local urea networks (LANs),metropolitan ureu networks ( M A N S )and ”ide
area networks ( WANs)
- NETWORKS 9
0 integrated voice and data networks
0 multimedia networks
This book coversthe principles involved in each of theabovetelecommunications
types; it also aims to give adequate background to enable the reader to tackle basic
network planning of any of these types. The book covers networking from the simple
interconnection of two telephones right up to complex, globally spread, telecommu-
nications networks.
1.5 NETWORKS
Let us now consider
the ideal properties of thevarious
components of the
telecommunications system illustrated in Figure 1.5. If both stations A and B are
provided with telephones, then the transport mechanism need be no more than a single
transmission line, as illustrated in Figure 1.6.
The system can also be extended to include further parties. For example, if a third
station C wishes to be interconnected for private interconnection with either or both of
the other two stations (A and B) then this can be achieved by duplication of the simple
layout. In this way a triangular network between A, B and C is created, as shown in
Figure 1.7.
-
Telephone ‘circuit’ or ‘line’
A Telephone . Telephone B
Figure 1.6 A simple two station telephone system
Telephone Telephone
A B
c
Telephone . ~
Telephone
Telephone Telephone
C
Figure 1.7 Three stations interconnected by independent telephone lines
- 10 INFORMATION AND ITS CONVEYANCE
The configuration of Figure 1.7 is used today by some companies in their private
networks, wherededicated
a ‘private telephone’ operate a
line may overspecial
telephone line, leased from a telecommunications administration, to connect different
premises.However, foranetworkinterconnectingalargenumber of stations, the
configuration is uneconomical equipment.
in In thethree-station (A, B, C) case
illustrated, six telephones and three lines are needed to interconnect the stations, but
only two telephones and one line can ever be used at any one time (unless one of the
people is superhuman and can talk and listen on more than one telephone at a time).
As even more stations are introduced to the configuration, the relative inefficiency
grows. In a systemof N stations in which each has a direct linkto each other, a total of
i N ( N - 1) telephone lines willbe needed, together with N(N - 1) telephone sets. If
configured in the manner shown in Figure 1.7, the linking of 100 stations would need
5000 links (and 10000 telephones) and a 10000 station system would need 50 million
lines and 100 million telephones. We need to find a more efficient configuration!
Let us limit each station in Figure 1.7 to one telephone only. To make this possible
we install a switching device each station to enable appropriateline selection, so that
at
connection to the desired destination may be achieved demand.This is now a simple
on
switchednetwork, asFigure 1.8 shows. Nowthetransport mechanism (stylised in
Figure 1.5) is no longer just a single ‘line’, but is a more complex ‘switch’ and line
arrangement.
Let us develop Figure 1.8 further, by permitting more stations (telephones in this
case) to be connected to each of the three switches. Three more stations, A’, B’ and C’
are shown in Figure 1.9. The new configuration allows the idle lines Figure 1 .S (A-C
of
and B-C) to be put to use.
Cl
Telephone C
Figure 1.8 A simple three station switched network
- CONNECTION-ORIENTED
TRANSPORT
SERVICE AND CONNECTIONLESS
NETWORK
SERVICE 11
6 Switchpoint(apon 1
+ Switchpoint(active)
C C‘
Figure 1.9 A simple telephone network
Figure 1.9 illustrates simultaneous calls involving A and B, B’ and C’, A’ and C’. In
this example each of the switches (which are now labelled as exchanges) is shared by a
number of stations, each of which is switched and connected to theexchange by a local
line or local loop. Our examplenow resembles a publicswitchedtelephonenetwork
( P S T N ) . Because the lines between exchanges are correctly referred to as junctions or
trunks, they have been labelled accordingly.
In a real telephonenetworkthenumbers of exchanges and theirlocationsare
governed by the overall number and geographical density of stations (telephone users)
requiring interconnection. Similarly, number
the of junctions or trunks provided
between the various exchangeswill be made sufficient to cater for the normal telephone
call demand. In this way, far fewer junctions than stations need to be provided. This
affords a significant cost saving over the configuration of Figure 1.6.
Before leaving Figure 1.9, note how each of the exchanges has been drawn as an
array of individual switch points. This allows either of the telephones connected to the
exchange to access either of the junctions, and this is a so-calledfull availability switch
as any one the incoming lines may be connected to any one the available junctions.
of of
We shall come back to circuit theory of switching and availability in Chapter 6.
1.6 CONNECTION-ORIENTED TRANSPORT SERVICE (COTS)
AND
CONNECTIONLESS NETWORK SERVICE (CLNS)
In the example of the last section we justified on economic grounds alone the use of
switched as opposed to ‘transmission line only’ networks. The particular case that we
- 12 CONVEYANCE
INFORMATION AND ITS
have developed is an example of a circuit-switched network. Other important switched
network types, especially used for data transmission, are those of packet switching,
message switching and cell switching.
Circuit-switched and most packet-switched and cell-switched networks are examples
of connection-oriented switching or connection-oriented transport service (COTS). In a
connection-oriented switching technique a circuit,virtualcircuit,connection or virtual
connection ( V C ) is established between sender and receiver beforeinformation is
conveyed. Thus a telephone connection is first established by dialling, before the con-
versation takes place. This ensures the readiness of the receiver to receive information
before it is sent.(There is no pointintalking if nobody is listening). In contrast,
connectionless switchingtechniques or connectionless-networkservice ( C O N S ) allow
messages to be despatched, maybe even without checking the validity of the address.
Thus, for example, the postal service is analogous to a connectionless service. The sender
posts the entire message (envelope and contents) into the post box and forgets about it.
Sometime later, the receiver receives the message, either delivered directly through his
letter box or by picking it up from his local post office. Electronic mail, today’s com-
puterizedversionof thepostal service, is alsoa connectionlessnetworkservice for
sending letters directly between computers.
The advantage of connectionless service is that the sender need not waitforthe
receiver to be ready and the network need not be encumbered with the extra effort of
setting-up a connection. Thus neither sender nor network bother to keep redialling
need
when either the receiver is already busy on another call, asleep on the other side of the
world, disconnected, switched-off or otherwise unable to answer the call. Instead the
message is lodged in a temporary store or ‘postoffice’-like device. The disadvantage is
that the sender has no clear guarantee or confirmation of message delivery. He is left in
doubt: did the receiver not get the message or was he simply too lazy to reply?
Message switching networks are networks which deliver the message (e.g. letter or
document) in one go. Most message switching networks (including perhaps the best
known, Internet) are based on connectionless network service.
As an aside, sometimes the end-to-end communication is connectionless even though
each of the individuallinks in thephysical communication chain is a connection-oriented
connection. Thus both sender and receiver mighttelephone an electronic mail post
ofice to send and receive their mail. The connection between the two post offices may
be apermanentconnectionandbothtelephone calls (to deliver and pick upthe
message) are connection-oriented, but because the sender and receiver do not both
need to connect to their respective post offices at the same time, the end-to-end com-
munication is connectionless.
1.7 CIRCUIT-, PACKET- AND CELL-SWITCHED NETWORKS
The distinguishing property of a circuit-switched connection is the existence throughout
thecommunicationphaseofthe call, of an unbroken physical and electrical path
between origin and destination points. The pathis established at call set-up and cleared
afterthecall.Thepathmay offer eitheronedirection (simplex) or two direction
(duplex) use. Telephone networks are circuit-switched networks.
- PACKET-
CIRCUIT-, AND NETWORKS
CELL-SWITCHED 13
Conversely, although packet-switched networks are alsoconnection-oriented, an entire
physical path from origin to destination will not generally be established at any time
during communication. Instead, the total information be transmitted is broken down
to
into a numberof elemental ‘packets’, each of which is sent in turn. Ananalogy might be
sending the textof a large book through the post a large number envelopes. In each
in of
individualenvelopemight be just a single page. The envelopes caneither be sent
sequentially (say one each day), numberedso that the
or receiver can reassemble the pages
in order.Figure 1.10 illustratesthissimplepacket-switchedcommunicationsystem.
Should thereceiver in Figure 1.10 not receive any given numbered envelope, he may write
back over thereverse connection and re-request it. In this way,very accurate andreliable
communication may be established.
Packet-switched networks are usually connection-oriented. A connection set-up phase
confirms the readiness of the receiver to receive information and it determines the route
through the network which will be used to carry the packets. The connection which
results is actually termed a virtual connection, because though it appears to the two
end-users as though a dedicated path exists, the physical connection is actually shared
with other users. By breaking the information into packets, statistical multiplexing may
be used to increase the network throughput. Statistical multiplexing is the technique
of sendingpacketsfrom different users’ virtualconnections overthesame physical
connection(Figure 10). This is made possible by labelling eachpacket(pages of
Figure 1.10) with the identity of the virtual connection to which it belongs (separate
books could be sent simultaneously in Figure 1.10). The labelling allows packets to be
sent from any of the virtual connections, provided that the line is at that moment idle.
If the line is already busy, it may be that the new packets must wait a fraction of a
second before transmission is possible. This possibility of slight delay leads to another
description of packet-switching as a store-and-forward technique. Packet-switching is the
technique behind X.25, frame relay and many other computer network techniques.
Circuit-switched networksare generally necessary when very rapid or instantan-
eous interaction is required (as is the case with speech). Conversely, packet-switched
Received so far
Page number page number
r--------- l
I
I
-
Paae S Page L
I’
I
I Today’s i
I
post
Tomorrow’s
post ‘I
L ---A
Transportmechanism
l the postman )
Figure 1.10 A simple ‘message switched’ communication system
- 14 INFORMATION AND ITS CONVEYANCE
networks are more efficient when instantaneous reaction is not required, but when very
low ‘corruption’ (either through distortion or loss of information) is paramount.
Cell-switching is a specialized form of packet-switching in which the packet lengths
are standardized at a fixed length. As we shall see in Chapters 25 and 26, cell switching
or cell relay switching is the basis of multimedia networks, the broadband integrated
services digital network (B-ZSDN) and ATM (asynchronous transfer mode).
1.8 CONSIDERATIONS FOR NETWORK PLANNERS
We have established the economic value of switched networks (as opposed to direct
wire systems), and briefly addressed the needs for basic communication (the physical,
switching and protocol needs). What other factors need to be provided in order to
enable networks to function in a manner fit for purpose? The following list is a brief
summary of some of the factors which require consideration.
Transmission and configuration plan
A plan laying out guidelines according to which appropriate transmission media may be
arranged so that adequate end-to-end conveyanceof information is achieved. The plan
will include safeguards to ensure that thereceived signal is loud enough, clear enough
and free of noise and interference.
Numbering and routing plans
The numbering plan is crucial to the ability of the network to deliver communications
to the appropriate destination. Much like reading an address on an envelope, it is the
inspection of the destination number that permits the determinationof destination and
appropriate route within the network.
Usage monitoring plan
A usage monitoring plan is needed to ensure continuing and future suitability of the
network. Adequate measurements of network performance are required so that early
steps may be taken, as necessary, to adjust the network design, or expand its overall
throughput capacity to meet demand.
Charging and accounting plan
Publictelecommunicationoperators (PTOs) needreimbursementfortheir services.
Specific equipment may be required to monitor individuals’ use of the network, so that
bills can be generated.
Maintenance plan
A maintenance plan is needed to guarantee that the service level meets agreed targets,
that routine maintenance is carried out, and that problems are quickly seen to. Any of
these items may be appropriate to any type of telecommunications network, regardless
of what type of information it is carrying (for example, telephone, data or multimedia
network). Each is discussed more fully in later chapters.
- STANDARDS
TECHNICAL FOR TELECOMMUNICATION
SYSTEMS 15
1.9 TECHNICAL STANDARDS FOR
TELECOMMUNICATIONS SYSTEMS
As a way of promotinggreatercompatibility between varioustelecommunications
systems in different parts of the world, a number of bodies that define technical stand-
ards are active in various parts of the world. These are discussed in more detail in
Chapter 40. However,twoofthemostsignificant and influentialbodies are worth
mentioning now. These are:
0 the International Organization for Standardization (ISO)
0 the International Telecommunications Union (ITU), and specifically its sub-entities,
thestandardizationsector (ITU-T, formerly CCITT, consultativecommittee f o r
international telephones and telegraphs) and the radiocommunication sector (ITU-R,
formerly CCIR, consultative committee for international radiocommunication).
The standards and recommendations of these bodies commonly
two are used
throughout telecommunications, and are often referred to by this book.
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