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Data Network Principles and Protocols We have described the binary form in which data are held by computer systems, and how such data are conveyed over digital line transmission systems

Data Network Principles

and Protocols

We have described the binary form in which data are held by computer systems, and how such data are conveyed over digital line transmission systems, but we shall need to know more than this before we can design the sort of devices which can communicate sensibly with one another in something equivalent to human conversation In this chapter we shall discuss in detail the conveyance of data between computer systems, the networks required, and the so-called

‘protocols’ they will need to ensure that they are communicating properly In the second half of the chapter some practical data and computer network topologies and the equipment needed to support them are described

Between 1950 and 1970 computers were large and unwieldy, severely limited in their power and capabilities, and rather unreliable Only the larger companies could afford

them, and they were used for batch-processing scientific, business or financial data on a

large scale Data storage in those days was laborious, limited in capacity, long in

preparation, not at all easy to manage Many storage mechanisms (e.g paper tape and punched cards) were very labour-intensive; they were difficult to store, and prone to damage Even magnetic tape, when it appeared, had its drawbacks; digging out some trivial information ‘buried’ in the middle of a long tape was a tedious business, and the tapes themselves had to be painstakingly protected against data corruption and loss caused by mechanical damage or nearby electrical and magnetic fields

Computing was for specialists Computer centre staff looked after the hardware, while software experts spent long hours improving their computer programmes,

squeezing every last drop of ‘power’ out of the computers’ relatively restricted capacity

By the mid-1970s all this began to change, and very rapidly Cheap semiconductors heralded the appearance of the microcomputer, which when packaged with the newly developed floppy diskette systems, opened a new era of cheap and widespread com-

puting activity Personal computers ( P G ) began to appear on almost every manager’s

177

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)

178 DATA NETWORK PRINCIPLES AND PROTOCOLS

desk, and many even invaded peoples’ homes Suddenly, computing was within reach of the masses, and the creation and storage of computer data was easy, cheap and fast All sorts of individuals began to prepare their own isolated databases and to write computer programmes for small scale applications, but these individuals soon recog- nized the need to share information and to pass data between different computers This could be done by transferring floppy diskettes from one machine to another, but as time went on that method on its own proved inadequate There was a growing demand for more geographically widespread, rapid, voluminous data transfer More recently, the demands of distributed processing computer networks comprising clients and servers

have created a boom in demand for data networks

A very simple computer or data network consists of a computer linked to a piece of

peripheral equipment, such as a printer The link is necessary so that the data in the computer’s memory can be reproduced on paper The problem is that a ‘wires-only’

direct connection of this nature is only suitable for very short connections, typically up

.to about 20 metres Beyond this range, some sort of line driver telecommunications

technique must be used A number of techniques are discussed here A long distance point-to-point connection may be made using modems A slightly more complex

computer network might connect a number of computer terminals in outlying buildings

back to a host (mainframe computer) in a specialized data centre Another network might be a Local Area Network (or L A N ) , used in an office to interconnect a number of

desktop computing devices, laser printers, data storage devices (e.g file servers), etc More complex computer networks might interconnect a number of large mainframe computers in the major financial centres of the world, and provide dealers with ‘up-to- the-minute’ market information

The basic principles of transmission, as set out in the early chapters of this book, apply equally to data which are communicated around computer networks So circuit-switched

networks or simple point-to-point lines may also be used for data communication Data communication, however, makes more demands on its underlying network than a voice

or analogue signal service, and additional measures are needed for coding the data in preparation for transmission, and in controlling the flow of data during transmission Computers do not have the same inherent ‘discipline’ to prevent them talking two at a

time For this reason special protocols are used in data communication to make quite

sure that information passing between computers is correct, complete and properly understood

9.2 BASIC DATA CONVEYANCE: INTRODUCING THE DTE

As we learned in Chapter 4, data are normally held in a computer or computer storage medium in a binary code format, as a string of digits with either value ‘0’ or value ‘l’

A series of such binary digits can be used to represent alphanumeric characters (e.g ASCII code), or graphical images, such as those transmitted by facsimile machines,

video or multimedia signals

In Chapter 5 we went on to discuss the principles of digital transmission, and found that it was ideal for the conveyance of binary data Digital transmission has become the

BASIC DATA CONVEYANCE: INTRODUCING THE DTE AND THE DCE 179

backbone of both private and public networks However, despite the increasing

availability and ideal suitability of digital transmission for data communication, it is unfortunately not always available In circumstances where it is not, digitally-oriented computer information must instead be translated into a form suitable for transmission across an analogue network This translation is carried out by a piece of equipment called a modulatorldemodulator, or modem for short Modems transmit data by imposing the binary (or digital) data stream onto an audio frequency carrier signal The process is very similar to that used in the frequency division multiplexing of voice channels described in Chapter 3

Figure 9.1 illustrates two possible configurations for data communication between two computers using either a digital or an analogue transmission link The configura- tions look very similar, comprising the computers themselves (these are specific examples of data terminal equipment ( D T E ) ) ; sandwiched between them in each case is a line and a pair of data circuit terminating equipments ( D C E )

A digital DCE (Figure 9.l(a)) connects the customer's digital DTE to a digital transmission line, perhaps provided by the public telecommunications operator (PTO) The DCE provides several network functions In the transmit direction, it regenerates the digital signal provided by the DTE and converts it into a standardized format, level and line code suitable for transmission on the digital line More complex DCEs may also interpret the signals sent by the DTE to the network to indicate the address desired

180 DATA NETWORK PRINCIPLES AND PROTOCOLS

when establishing a connection, and help in setting up the call In the receive direction, the DCE establishes a reference voltage for use of the DTE and reconverts the received line signal into a form suitable for passing to the DTE In addition, it uses the clocking signal (i.e the exact bit rate of the received signal) as the basis for its transmitting bit rate The received clocking signal is used because this is derived from the highly

accurate master clock in the PTO’s network Two interfaces need to be standardized

These are the DTE/DCE interface and the DCE/DCE interface

Digital DCEs can be used to provide various digital bit speeds, from as low as 2.4 kbit/s, through the standard channel of 64 kbit/s, right up to higher order systems such as 1.544Mbit/s (called T1 or DSl), 2.048 Mbit/s (called El), 45 Mbit/s (called T3

or DS3) or higher When bit speeds below the basic channel bit rate of 64 kbit/s are required by the DTE, then the DCE has an additional function to carry out, breaking down the line bandwidth of 64 kbit/s into smaller units in a process called sub-rate multiplexing The process derives a number of lower bit rate channels, such as 2.4 kbit/s,

4.8 kbit/s, 9.6 kbit/s, etc., some or all of which may be used by a number of different

LINES USING A MODEM

Three basic datamodulation techniques are used in modems for conveying digital end user information across DCE/DCE interfaces comprising analogue lines or radio systems, but there are more sophisticated versions of each type and even hybrid versions, combining the various techniques Each modem is in reality a specialized device for a specific line or radio system type The three techniques are illustrated in Figure 9.2 and described below

HIGH BIT RATE MODEMS 181

of tone Modems using frequency modulation are more commonly called FSK, or

frequency shift key modems A common form of FSK modem uses four different

frequencies (or tones), two for the transmit direction and two for the receive This allows simultaneous sending and receiving (i.e duplex transmission) of data by the modem, using only a two-wire circuit

Phase modulation

In phase modulation (Figure 9.2(c)), the carrier signal is advanced or retarded in its phase cycle by the modulating bit stream At the beginning of each new bit, the signal will either retain its phase or change its phase Thus in the example shown the initial signal phase represents the value ‘l’ The change of phase by 180” represents next bit ‘0’

In the third bit period the phase does not change, so the value transmitted is ‘l’ Phase modulation (often called phase sh$t keying or P S K ) is conducted by comparing the

signal phase in one time period to that in the previous period; thus it is not the absolute value of the signal phase that is important, rather the phase change that occurs at the beginning of each time period

The transmission of high bit rates can be achieved by modems in one of two ways One is

t o modulate the carrier signal at a rate equal to the high bit rate of change of the

182 DATA NETWORK PRINCIPLES AND PROTOCOLS

Figure 9.3 Multilevel transmission and lower baud rate

modulating signal Now the rate (or frequency) at which we fluctuate the carrier signal is

called the baud rate, and the disadvantage of this first method of high bit rate carriage is

the equally high baud rate that it requires The difficulty lies in designing a modem capable of responding to the line signal changes fast enough Fortunately an alternative method is available in which the baud rate is lower than the bit rate of the modulating bit stream The lower baud rate is achieved by encoding a number of consecutive bits from the modulating stream to be represented by a single line signal state The method is

called multilevel transmission, and is most easily explained using a diagram Figure 9.3

illustrates a bit stream of 2 bits per second (2 bit/s being carried by a modem which uses

four different line signal states The modem is able to carry the bit stream at a baud rate

of only 1 per second (1 Baud))

The modem used in Figure 9.3 achieves a lower baud rate than the bit rate of the data transmitted by using each of the line signal frequencies f 1, f2, f 3 and f4 to represent two consecutive bits rather than just one This means that the line signal always has to

be slightly in delay over the actual signal (by at least one bit as shown), but the benefit is that the receiving modem will have twice as much time to detect and interpret the

datastream represented by the received frequencies Multi-level transmission is invari-

ably used in the design of very high bit rate modems

At this point we introduce the concept of modem constellation diagrams, because these assist in the explanation of more complex amplitude and phase-shift-keyed ( P S K ) modems Figure 9.4 illustrates a modem constellation diagram composed of four dots

Each dot on the diagram represents the relative phase and amplitude of one of the pos- sible line signal states generated by the modem The distance of the dot from the origin

( c ) Signal phase,cammencing at 90' ( d ) The four line signals

Figure 9.4 Modem constellation diagram

of the diagram axes represents the amplitude of the signal, and the angle subtended between the X-axis and a line from the point of origin represents the signal phase relative to the signal state in the preceding instant of time

Figure 9.4(b) and (c) together illustrate what we mean by signal phase Figure 9.4(b)

shows a signal of 0" phase, in which the time period starts with the signal at zero amplitude and increases to maximum amplitude Figure 9.4(c), by contrast, shows a signal of 90" phase, which commences further on in the cycle (in fact, at the 90" phase angle of the cycle) The signal starts at maximum amplitude but otherwise follows a similar pattern Signal phases, for any phase angle between 0 and 360" could similarly

be drawn Returning to the signals represented by the constellation of Figure 9.4(a) we can now draw each of them, as shown in Figure 9.4(d) (assuming that each of them was preceded in the previous time instant by a signal of 0 phase) The phase angles in this case are 45", 135", 225", 315"

We are now ready to discuss a complicated but common modem modulation

technique known as quadrature amplitude modulation, or Q A M Q A M is a technique

using a complex hybrid of phase (or quadrature) as well as amplitude modulation,

hence the name Figure 9.5 shows a simple eight-state form of QAM in which each line

signal state represents a three-bit signal (values nought to seven in binary can be represented with only three bits) The eight signal states are a combination of four different relative phases and two different amplitude levels The table of Figure 9.5(a) relates the individual three-bit patterns to the particular phases and amplitudes of the signals that represent them Note: Figure 9.5(b) illustrates the actual line signal pattern that would result if we sent the signals in the table consecutively as shown Each signal

184 DATA NETWORK PRINCIPLES AND PROTOCOLS

I o w high

l o w

high I Phase

shift 0 0 90'

9 0' 180'

180'

270' 270'

l

000 010 011 111 .l00 001 110 ' 101 l 0 1

(b) Typical line signal

( a ) B i t combinations and line signal attributes

High Amplitude

t

2 70'

( c ) Modem constellation

Figure 9.5 Quadrature amplitude modulation

is shown in the correct phase state relative to the signal in the previous time interval

(unlike Figure 9.4 where we assumed that each signal individually had been preceded by one of 0 phase) Thus in Figure 9.5(b) the same signal state is not used consistently to convey the same three-bit pattern, because the phase difference with the previous time period is what counts, not the absolute signal phase Hence the eighth and ninth time periods in Figure 9.5(b) both represent the pattern 'lOl', but a different line is used, 180" phase shifted

Finally, Figure 9.5(c) shows the constellation of this particular modem

To finish off the subject of modem constellations, Figure 9.6 presents, without

discussion, the constellation patterns of a couple of very sophisticated modems, specified

by ITU-T recommendations V.22 bis and V.32 as a DCE/DCE interface As in Figure 9.5, the constellation pattern would allow the interested reader to work out the respective 16 and 32 line signal states Finally, Table 9.1 lists some of the common modem types and their uses When reading the table, bear in mind that synchronous and asynchronous operation is to be discussed later in the chapter, and that half-duplex means that two-way transmission is possible but in only one direction at a time This differs from simplex operation (as discussed in Chapter 1) where one-way transmission only is possible

MODEM ‘CONSTELLATIONS’ 185

( a ) V 2 2 bis ( 3 a m p l i t u d e s , ( b ) V 3 2 15 amplitudes,

12 2 8 p h a s e s )

p h a s e s )

Figure 9.6 More modem constellations

Table 9.1 Common modem types

(ITU-T Modulation Bit speed or asynchronous Full or half Circuit type

recommendation) type (bit/s) (A) operation duplex required

to asynchronous format

-

~

Full Full Full Full Half Full Half Half Half Full Full Full Full Full Full Full

~~~~~~

2w telephone line 2w telephone line 2w telephone line 4w leaseline 2w telephone line 4w leaseline 2w telephone line 2w leaseline 2w leaseline 4w leaseline 2w telephone line 2w telephone line 2w telephone line groupband leaseline groupband leaseline groupband leaseline Error correcting protocol

Data compression technique

186 DATA NETWORK PRINCIPLES AND PROTOCOLS

9.6 COMPUTER-TO-NETWORK INTERFACES

Returning now to Figure 9 I , we can see that we have discussed in some detail the method

of data conveyance over the telecommunications line between one DCE and the other (or between one modem and the other) but what about the interface that connects a computer

to a DCE or modem? This interface is of a generic type (DTE/DCE, i.e between DTE and DCE) and can conform to any one of a number of standards, as specified by various organizations The interface controls the flow of data between DTE and DCE, making sure that the DCE has sufficient instructions to deliver the data correctly It also allows the DCE to prepare the distant DTE and confirm receipt of data if necessary Physically, the interface usually takes the form of a multiple pin connection (plug and socket) typically with 9,15,25 or 37 pins arranged in a ‘D-formation’ (so-called D-socket or sub-D-socket)

Computer users will be familar with the type of sockets shown in Figure 9.7

The interface itself is designed either for parallel data transmission or serial data transmission, the latter being the more common The two different methods of trans- mission differ in the way each eight-bit data pattern is conveyed Internally, computers operate using the parallel transmisssion method, employing eight parallel circuits to

carry one bit of information each Thus during one time interval all eight bits of the data pattern are conveyed The advantage of this method is the increased computer process- ing speed which is made possible The disadvantage is that it requires eight circuits instead of one Parallel data transmission is illustrated in Figure 9.8(a), where the

pattern ‘10101 110’ is being conveyed over the eight parallel wires of a computer’s data bus Note: nowadays 16-bit, and even 32-bit data buses are used in the most advanced

computers Examples of parallel interfaces are those specified by ITU-T recommenda- tions V.19 and V.20

Serial transmission requires only one transmission circuit, and so is far more cost- effective for data transmisssion on long links between computers The parallel data on the computer’s bus is converted into a serial format simply by ‘reading’ each line of the bus in turn The same pattern ‘10101 1 10’ is being transmitted in a serial manner in Figure 9.8(b) Note that the baud rate needed for serial transmission is much higher

than for the equivalent parallel transmission interface

Many DTE-to-DCE (or even short direct DTE-to-DTE connections) use a serial transmission interface conforming to one of ITU-T’s suites of recommendations, either X.21 or X.2lbis The basic functions of all types of DTE/DCE interface are similar:

\??

0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

25-pin (female) D-socket (IS0 21 10) 15-pin (female) sub-D-socket (IS0 4903)

connection (RS232 or V.24N.28) connection (X.21, V.10 or V l l )

Figure 9.7 Plug and socket arrangement for common DTE/DCE cables

COMPUTER-TO-NETWORK INTERFACES 187

0 clocking and synchronizing the data transfer

0 regulating the bit rate, so that the receiving device does not become swamped with data

In Figure 9.9 we summarize the complete suite of ITU-T recommendations detailing the

most important physical layer DTE/DCE interfaces These are those defined by ITU-T

recommendations X.21 and X.21 bis as shown

C i r c u i t voltage

f \

C o m p u t e r ( i n t e g r a t e d

Direction o f transmission

m 0 1 1 1 0 1 0 1

( b ) S e r i a l t r a n s m i s s i o n Figure 9.8 ‘Parallel’ and ‘serial’ transmission

188 DATA NETWORK PRINCIPLES AND PROTOCOLS

Figure 9.9 Common interfaces for DTE/DCE attachment

The ITU-T recommendations listed in Figure 9.9 define for each of the interfaces: the mechanical nature of the plugs and sockets (i.e their physical structure), the

electrical currents and voltages to be used, the functional purpose or meaning of the various circuits which appear on the various pins of the plug/socket connection, and the valid pin combinations and order (or procedure) in which the various functions may be used

There are two broad groups of physical layer interfaces The first broad group are the

group sometimes loosely referred to as V.24 (RS-232) or X.21 bis which were developed

in conjunction with modems and are relatively low speed devices (up to about 28 kbit/s and about 15 m cable length) allowing computer data to be transferred to modems for carriage across analogue telephone lines The second group is the X.21 suite (sometimes only the subsets X.24 or V.ll are used) This second group was developed later for higher speed lines, which became possible in the advent of digital telephone leaslines (64 kbit/s and above and up to 100 m cable length)

In Europe, the standard interfaces offered by modems and digital leaseline DCEs

(variously called line terminating unit ( L T U ) , network terminating unit ( N T U ) , channel service unit ( C S U ) , D S U (data service unit)) are V.24 for analogue leaselines and X.21

(or the subsets X.24 or V 11) for digital lines In North America RS-232 (V.24) is widely used for analogue lines, but V.35 and V.36 predominate over X.21 for digital lines

SYNCHRONIZATION 189

You might wonder how you connect a V.35 DTE in the USA to an X.21 DTE in

Europe The answer is quite straightforward You order an international digital leaseline with V.35 DCE interface in the USA and X.21 interface in Europe The fact that the two DTE-DCE connections have different forms is immaterial

9.7 SYNCHRONIZATION

An important component of any data communications system is the clocking device The data consists of a square, ‘tooth-like’ signal, continuously changing between state

‘0’ and state ‘l’, as we recall in Figure 9.10

The successful transmission of data depends not only on the accurate coding of the transmitted signal (Chapter 4), but also on the ability of the receiving device to decode

the signal correctly This calls for accurate knowledge (or synchronization) of where

each bit and each message begins and ends

The receiver usually samples the communication line at a rate much faster than that

of the incoming data, thus ensuring a rapid response to any change in line signal state,

as Figure 9 l 1 shows Theoretically it is only necessary to sample the incoming data at a rate equal to the nominal bit rate, but this runs a risk of data corruption If we choose

to sample near the beginning or end of each bit we might lose or duplicate data as Figures 9.1 l(a) and (b) show, simply as the result of a slight fluctuation in the time duration of individual bits Much faster sampling ensures rapid detection of the start of

each ‘0’ to ‘1’ or ‘1’ to ‘0’ transition, as Figure 9 l l(c) shows The signal transitions are

interpreted as bits

Variations in the time duration of individual bits come about because signals are liable to encounter time shifts during transmission, which may or may not be the same for all bits within the total message These variations are usually random and they

combine to create an effect known as jitter, which can lead to incorrect decoding of incoming signals when the receiver sampling rate is too low, as Figure 9.12 shows

Jitter, together with a slight difference in the timing of the incoming signal and the

receiver’s low sampling rate have created an error in the received signal, inserting an

S t a t e ‘ 1 ’ State ‘0’

Figure 9.10 Typical data signal

t t t t t t t t t t t t

( a ) L a t e s a m p l e ( b l Early sample ( c ) High sample rate

Figure 9.11 Effect of sample rate

190 DATA NETWORK PRINCIPLES AND PROTOCOLS

Actual data pattern :

Figure 9.12 The effect of jitter

extra ‘l’ Thus the signal at the top is intended to represent only a ten bit pattern, but

because the bit durations are not exactly correct, (due to jitter), the pattern has been

misinterpreted An extra bit has been inserted by the receiving device

T o prevent an accumulation of errors over a period of time, we use a sampling rate

higher than the nominal data transfer rate as already discussed; we also need to carry

out a periodic synchronization of the transmitting and receiving end equipments

The purpose of synchronization is to remove all short, medium and long term time

effects In the very short term, synchronization between transmitter and receiver takes

place at a bit level, by bit synchronization, which keeps the transmitting and receiving

clocks in step, so that bits start and stop at the expected moments In the medium term

it is also necessary to ensure character or word synchronization, which prevents any

confusion between the last few bits of one character and the first few bits of the next If

we interpret the bits wrongly, we end up with the wrong characters Finally there is

frame synchronization, which ensures data reliability (or integrity) over even longer time

periods

Figure 9.13 shows two different pulse transmission schemes used for bit synchroniza-

tion The first, Figure 9.13(a), is a non-return to zero ( N R Z ) code which looks like a

Non-return to zero ( N R 2 1 code

CHARACTER SYNCHRONIZATION: SYNCHRONOUS AND ASYNCHRONOUS DATA TRANSFER 191

string of ‘1’ and ‘0’ pulses, sent in the manner with which we are already familiar

In contrast Figure 9.13(b) is a return-to-zero ( R Z ) code, in which ‘l’s are represented by

a short pulse which returns to zero at the midpoint RZ code therefore provides extra 0

to 1 and 1 to 0 transitions, most noticeably within a string of consecutive ‘l’s By so doing, it better maintains synchronization of clock speeds (or bit synchronization) between the transmitter and the receiver A number of alternative techniques also exist

The technique used in a given instance will depend on the design of the equipment and the accuracy of bit synchronization required More often than not it is the interface standard that dictates which type will be used The synchronization code used widely on

IBM SDLC networks is called N R Z I (non-return to zero inverted) These and a number

of other line codes are illustrated in Chapter 5

9.9 CHARACTER SYNCHRONIZATION: SYNCHRONOUS AND

ASYNCHRONOUS DATA TRANSFER

Data conveyance over a transmission link can be either synchronous (in which

individual data characters (or at least a predetermined number of bits) are transmitted

at a regular periodic rate) or asynchronous mode (in which the spacing between the characters or parts of a message need not be regular) In asynchronous data transfer

each data character (represented say by an eight-bit ‘byte’) is preceded by a few

additional bits, which are sent to mark (or delineate) the start of the eight-bit string to the receiving end This assures that character synchronization of the transmitting and

receiving devices is maintained

Between characters on an asynchronous transmission system the line is left in a

quiescent state, and the system is programmed to send a series of ‘l’s during this period

to ‘exercise’ the line and not to generate spurious start bits (value ‘0’) When a character (consisting of eight bits) is ready to be sent, the transmitter precedes the eight bit

pattern with an extra start bit (value ‘O’), then it sends the eight bits, and finally it suffixes the pattern with two ‘stop bits’ both set to ‘l’ The total pattern appears as in Figure 9.14, where the user’s eight bit pattern 11010010 is being sent (Note: usually nowadays, only one stop bit is used This reduces the overall number of bits which need

to be sent to line to convey the same information by 9%.)

In asynchronous transmission, the line is not usually in constant use The idle period

between character patterns (the quiescent period) is filled by a string of 1 S The receiver can recognize the start of each character when sent by the presence of the start bit transition (from state ‘1’ to state ‘0’) The following eight bits then represent the

character pattern

The advantage of asynchronous transmission lies in its simplicity The start and stop

bits sent between characters help to maintain synchronization without requiring very accurate clocking, so that devices can be quite simple and cheap Asynchronous transmission is quite widely used between computer terminals and the computers themselves because of the simplicity of terminal design and its consequent cheapness Given that human operators type at indeterminate speeds and sometimes leave long pauses between characters, it is ideally suited to this use The disadvantage of asynchronous transmission lies in its relatively inefficient use of the available bit speed

192 DATA NETWORK PRINCIPLES AND PROTOCOLS

1 1 0 1 0 0 1 0

Quiescent period,

or next character Figure 9.14 Asynchronous data transfer

SY N User d a t a ( m a n y bytes) SYN SYN

Figure 9.15 Synchronous data transfer

As we can see from Figure 9.14, out of eleven bits sent along the line, only eight represent useful information

In synchronous data transfer, the data are clocked at a steady rate A highly accurate

clock is used at both ends, and a separate circuit may be used to transmit the timing between the two Provided that all the data bit patterns are of an equal length, the start

of each is known to follow immediately the previous character The advantage of synchronous transmission is that much greater line efficiency is achieved (because no start bits need be sent), but its more complex arrangements do increase the cost as compared with asynchronous transmission equipment Byte synchronization is established at the very beginning of the transmission or after a disturbance or line break using a special synchronization (SYN) pattern, and only minor adjustments are needed thereafter Usually an entire user’s data field is sent between the synchronization (SYN) patterns, as Figure 9.15 shows

The SYN byte shown in Figure 9.15 is a particular bit pattern, used to distinguish it from other user data

9.10 HANDSHAKING

We cannot leave the subject of modems without at least a brief word about the Hayes command set (nowadays also called the A T command set), for many readers will at

some time find themselves faced with the question of whether a modem is ‘Hayes

compatible’ By this, we ask whether it uses the Hayes command set, the procedure by

PROTOCOLS FOR TRANSFER OF DATA 193

which the link is set up and the data transfer is controlled; if you like, the handshake

and etiquette of conversation It allows, for example, a PC to instruct a modem to dial a given telephone number and confirm when the connection is ready The Hayes

command set has become the de facto standard for personal computer communication

over telephone lines An alternative scheme is today offered by ITU-T recommendation V.25 bis

Unfortunately the X.21, V.24/V.28 (RS-232) and RS449 standards and their inter-DCE equivalents (i.e the line transmission standards used by modems or digital links such as V.22, V.32 or V.34) are still not enough in themselves to ensure the controlled flow of data across a network A number of additional layered mechanisms are necessary, to indicate the coding of the data, to control the message flow between the end devices and

to provide for error correction of incomprehensible information Unlike human beings

engaged in conversation, machines can make no sense at all of corrupted messages, and they need hard and fast rules of procedure to be able to cope with any eventuality When they are given a clear procedure they are able, unlike most human beings, to carry on several ‘conversations’ at once These rules of procedure are laid out in

protocols

Now, sit down and make yourself comfortable

Once upon a time protocols were relatively unsophisticated like the simple computer- to-terminal networks which they supported and they were contained within other

computer application programmes Thus the computer, besides its main processing function, would be controlling the line transmission between itself and its associated terminals and other peripheral equipment However, as organizations grew in size and data networks became more sophisticated and far-flung, the supporting communica- tions software and hardware developed to such an extent as to be unwieldy and unmaintainable Many computer items (particularly different manufacturers’ equip- ments) were incompatible Against this background the concept of layered protocols developed with the objective of separating out the overall telecommunications functions

into a layered set of sub-functions, each layer performing a distinct and self-contained

task but being dependent on sub-layers Thus complex tasks would comprise several layers, while simple ones would need only a few Each layer’s simple function would comprise simple hardware and software realization and be independent of other layer functions In this way we could change either the functions or the realization of one functional layer with only minimal impact on the software and hardware implementa- tions of other layers For example, a change in the routing of a message (i.e the topology of a network) could be carried out without affecting the functions used for correcting any corrupted data (or errors) introduced on the line between the end terminals

Most data transfer protocols in common use today use a stack of layered protocols

By studying such a protocol stuck we have a good idea of the whole range of functions

that are needed for successful data transfer We need to consider the functions of each protocol layer as laid out in the international standards organization’s (ISO’s) open