Mạng và viễn thông P9

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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, but we shall need to know more than this before we can design the sort ofdevices which can communicatesensibly with one another in somethingequivalent to human conversation.Inthischapter we shalldiscuss indetailthe networks required, the and so-called conveyance of data between computer systems, the ‘protocols’ they will need to ensure that they are communicating properly....

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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) 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 ofdevices which can communicatesensibly with one another in somethingequivalent to human conversation.Inthischapter we shalldiscuss indetailthe conveyance of data between computer systems, the networks required, the and 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. 9.1 COMPUTER NETWORKS 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 largescale. Data storageinthosedayswaslaborious,limitedincapacity,longin 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 lookedafterthe hardware, while software experts spent hours long improving computer their 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
  2. 178 PRINCIPLES NETWORK DATA AND PROTOCOLS desk, and manyeven invaded peoples’ homes. Suddenly, computing waswithin 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 machineto another, but as time went on that method on its own proved inadequate. Therewas 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-pointconnectionmay 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 principlesof transmission, asset out in the early chapters this book, apply of So equally to datawhich are communicated around computer networks. circuit-switched networks or simple point-to-point lines may also beused for data communication. Data communication, however, makes more demandson 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 ANDTHE DCE 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 suchbinarydigitscanbe used to representalphanumericcharacters (e.g. ASCII code), or graphical images, such as those transmitted facsimile machines, by 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. Digitaltransmission has become the
  3. CONVEYANCE: DATA BASIC INTRODUCING THE DTE AND THE DCE 179 backbone of both private and public networks. However, despite increasing the 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 forshort.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 thefrequency 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- tionslook very similar,comprising computers the themselves (these are specific examples o data terminal equipment ( D T E ) ) ;sandwiched between them in each caseis a f line and a pair of data circuit terminating equipments (DCE). A digital DCE (Figure9.l(a)) connectsthecustomer'sdigital DTE to adigital 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 DT E DTE 2 ' TCE/ Digital line DCE 2 Computer , Digital information (a) D i g i t a l Line connection D i g i t ai ln f o r m a t i o n DTE DTE DCE modula ted DCE analoguesignal Computer Modem < Modem I Computer - 2 way analogue transmission link Figure 9.1 Point-to-point connection of computers. DTE, Data Terminal Equipment; DCE, Data Circuit Terminating Equipment
  4. 180 PRINCIPLES NETWORK DATA 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 forpassing 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 clockingsignal is usedbecausethis is derivedfromthe 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. DigitalDCEscan be used toprovidevariousdigitalbitspeeds,fromas 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 numberof lower bit rate channels, such as kbit/s, 2.4 4.8 kbit/s, 9.6 kbit/s, etc., some or all of which may be used by a number of different DTEs. In their various guises, digital DCEs go by different names. In the United Kingdom, BritishTelecom’s Kilostream digitalprivateline service uses a DCE called a NTU (networkterminatingunit). Meanwhile,intheUnitedStates,AT&T’s digitaldate system ( D D S ) and similar digital line services are provided by means of channel service units ( C S U ) or data service units ( D S U ) . Sometimes the term LTU (line terminating unit) is used. In the caseof analogue transmission lines, the DCE (Figure 9.1 (b)) cannot on the rely network to provide accurate clocking information, because the bit rate of the received signal is not accurately set by the PTO’s analogue network. For this reason, an internal clock is needed within the DCE in order to maintain an accurate transmitting bit rate. The other major difference between analogue and digital DCEs is that analogue DCEs (modems) need to convert and reconvert digital signals received from the DTE into an analogue signal suitable for line or radio transmission. 9.3 MODULATION OF DIGITAL INFORMATION OVER ANALOGUE LINESUSINGAMODEM Three basic datamodulation techniques are used in modems for conveying digital end user informationacrossDCE/DCE interfaces comprising analoguelines or radio systems, buttherearemoresophisticated versions of eachtype and even hybrid versions,combiningthevarioustechniques.Each modem is inrealityaspecialized device for a specific line or radio system type. The three techniques are illustrated in Figure 9.2 and described below. Amplitude modulation Modems employing amplitudemodulation altertheamplitude of thecarriersignal between a set valueand zero (effectively ‘on’ and ‘off ’) according to the respective value ‘1’ or ‘0’ of the modulating bit stream. Alternatively two different, non-zero values of amplitude may be used to represent ‘1’ and ‘0’.
  5. MS HIGH BIT RATE 181 Phase Phase 180’ change 0’ change [ a ) Amplitude modulation ( C ) Phase modulation l b) Frequency modulation Figure 9.2 Datamodulationtechniques Frequency modulation Infrequency modulation (Figure 9.2(b)), it is the frequency of the carrier signal that is altered to reflect the value ‘1’ or ‘0’ of the modulating bit stream. The amplitude and phase of the carrier signal is otherwise unaffected by the modulation process. If the number of bits transmitted per second is low, then the signal emitted by a frequency modulated modem is heard as a ‘warbling’ sound, alternating between two frequencies of tone. Modems usingfrequencymodulation are more commonly called FSK, or frequency shift key modems.A commonform of FSK modem uses four different frequencies(ortones),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 thevalue ‘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. 9.4 HIGH BIT RATE MODEMS 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
  6. 182 PRINCIPLES NETWORK DATA AND PROTOCOLS I I Bit rate ( 2 bl t per second1 Bit stream 1 Pair of b i t s Iv 10 10 v 11 00 v 10 01 I ' Modemllne signal f , (10) (frequency Baud r a t e and 2 - b i t fz(01) value) I lsec I 00 ( lsecond per change ) Figure 9.3 Multilevel transmission and lower baud rate modulating signal. Now the rate (or frequency) which we fluctuate the carriersignal is at 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 inwhich the baud rate lower than thebit rate of the modulatingbit is 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 muchtime 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. 9.5 MODEM 'CONSTELLATIONS' At this point we introduce the concept of modem constellation diagrams, because these assistin theexplanation of morecomplex 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
  7. MODEM 'CONSTELLATIONS' 183 Signal amplitude 0 90 180 270 360 ( a ) L - signal state constellation ( b ) Slgnal phase,commencing at 0 ' Signal phase ( c ) Signal phase,cammencing at 90' ( d ) The four signals line 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) showsasignalof 0" phase, in which thetimeperiod starts withthe 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 draweach of them, as shownin 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". Wearenowreadyto discusscomplicated a butcommonmodemmodulation technique known as quadrature amplitude modulation, or Q A M . Q A M is a technique usingacomplexhybrid of phase (or quadrature) as well as amplitudemodulation, hence the name. Figure 9.5 shows a simple eight-state form ofQAM in which each line signal state representsathree-bitsignal(values noughtto seven inbinarycan be representedwithonlythreebits). 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
  8. 184 PRINCIPLES DATA NETWORK AND PROTOCOLS l- iI Line signal Bit omblnation implitude Phase shift 000 low 0 ' l 00 1 high 0 ' 01 0 Iow 90' 01 1 high 90' 100 I ow 180' 101 high 180' 000.010 011. 111 .l00 001 110 101 . l 0 1 ' 110 low 270' (b) Typical line signal 11 1 high 270' ( a ) B i t combinations and line signal attributes High Amplitude 180' X 27' 0 t ( c ) Modem constellation Figure 9.5 Quadrature amplitudemodulation 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 individuallyhad 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 thesubject of modemconstellations,Figure9.6presents,without discussion, the constellation patternsof a coupleof very sophisticated modems,specified by ITU-T recommendationsV.22 bis and V.32 as a DCE/DCE interface. As in Figure9.5, 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 thecommon modem types and their uses. When reading the table, bear in mind that synchronous and asynchronous operation is to be discussed later in chapter, and that the 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.
  9. 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, phases) 28 12 p h a s e s ) Figure 9.6 More modemconstellations Table 9.1 Commonmodemtypes Modem type Synchronous (S) (ITU-T Modulation speed Bit or asynchronous Full or half Circuit type recommendation) type (bit/s) (A) operation required duplex ~ ~~~~~~ v.21 FSK up to 300 A Full 2w telephone line v.22 PSK 1200 S or A Full 2w telephone line V.22 bis QAM 2 400 S or A Full 2w telephone line V.23 FSK 60011 200 A Full 4w leaseline Half 2w telephone line V.26 2 400 S Full 4w leaseline V.26 bis 2 400/ 1 200 S Half 2w telephone line V.27 4 800 S Half 2w leaseline V.21 ter 4 800 S Half 2w leaseline V.29 9 600 S Full 4w leaseline V.32 up to 9 600 S or A Full 2w telephone line v.33 14 400 S or A Full 2w telephone line v.34 up to 33600 S or A Full 2w telephone line v.35 48 000 wideband Full groupband leaseline V. 36 AM 48 000 wideband Full groupband leaseline v.37 AM 12000 wideband Full groupband leaseline V.42 convert Error synchronous correcting to protocol asynchronous format V.42 bis up to 30 kbit/s in - Data association with compression V.32 modem technique
  10. 186 PRINCIPLES NETWORK DATA AND PROTOCOLS 9.6 COMPUTER-TO-NETWORK INTERFACES Returning nowto Figure 9. I , we can see that we have discussed in some detail the method of data conveyance overthe telecommunications line between one DCE and the other (or between one modem and the other) but what about the that connects computer interface a 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 sufficient instructions to has deliver the data correctly. also allows the It DCE to prepare the distant DTE confirm receipt of data if necessary. Physically, the and interface usually takes the form of a multiple pin connection (plug and socket)typically with 9,15,25 or pins arranged in a ‘D-formation’ 37 (so-called D-socket or sub-D-socket). Computer users will be familar with the type sockets shown in Figure 9.7. of 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 bitof information each. Thus during one time intervalall 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 insteadofone.Paralleldatatransmission is illustratedinFigure9.8(a),wherethe pattern ‘10101110’ 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 ‘101011 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 oneof 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) used on DCE for DCE/DTE used onDCE for DCE/DTE 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
  11. 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 ITU-T recommendationsdetailing the of most important physical layer DTE/DCE interfaces. These are those defined by ITU-T recommendations X.21 and X.21 bis as shown. Circuit voltage 1 f n 1 \ Computer Computer (integrated ’ ‘data bus’ Circuit) t Circuit number (a] P a r a l l er a n s m i s s i o n tl Direction o f transmission 0 1 1 1 0 1 0 1 DTE m Line ( b )e r i a r a n s m i s s i o n S tl Figure 9.8 ‘Parallel’ and ‘serial’ transmission
  12. 188 PRINCIPLES DATA NETWORK AND PROTOCOLS /&, procedure RS232 functional interface l I electrical interfa;ce Fm f 3 b f15V I I 25-ph &i Pn Figure 9.9 Common interfaces for DTE/DCE attachment The ITU-T recommendations listed in Figure 9.9 define for each of the interfaces: themechanicalnature of theplugs andsockets (i.e. theirphysicalstructure),the electricalcurrentsandvoltagesto be used,thefunctionalpurpose or meaning of on the various circuits which appear the various pinsof 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 physical layer interfaces. The first broad group are the of group sometimesloosely referred to as V.24 (RS-232) or X.21bis which were developed in conjunction with modems and are relativelylow speed devices (up to about 28 kbit/s and about 15 m cable length) allowing computer data tobe transferred to modems for carriage across analogue telephonelines. 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 (CSU), DSU(data service unit)) are V.24 for analogue leaselines and X.21 (or the subsets X.24 V. 11) for digital lines. In North AmericaRS-232 (V.24) is widely or used for analogue lines, but V.35 and V.36 predominate over X.21 for digital lines.
  13. SYNCHRONIZATION 189 You might wonder how you connect a V.35 DTE in the USA to an X.21 DTE in Europe. answer The is quitestraightforward. 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 (Chapter4), 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. 1 shows. Theoretically itis only necessary to sample the incomingdata at a l 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 detectionof 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 mayor 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 State ‘1’ State ‘0’ Figure 9.10 Typical data signal t t t t tttttttt Lsat)m p l e (aa e ( b l Early sample ( c ) High sample rate Figure 9.11 Effect of sample rate
  14. 190 PRINCIPLES NETWORK DATA AND PROTOCOLS Actualdatapattern : 1 0 1 0 1 0 1 0 1 0 I l r I I I l I I I I I I I l I I I I I I I I I I Signal t f f f f f t t t t t Sampling instant Interpreted recelved signal : 1 0 0 1 0 1 0 1 0 J-2 Error, caused by ‘ j i t t e r ’ 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. To 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 theexpected moments. In the medium term it is also necessary to ensure character or word synchronization, which prevents any confusion between the lastfew 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. 9.8 BIT SYNCHRONIZATION 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 Jm - , m (a) Non-return to -- zero ( N R 2 1 code 1 1 0 1 0 0 1 0 1 + Return to zero (b) ( R Z ) code 1 1 0 1 0 0 1 0 Figure 9.13 Methods of bit synchronization
  15. 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 Figure9.13(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 extra0 to 1 and 1 to 0 transitions, most noticeably within a string of consecutive ‘l’s. By so doing,itbettermaintainssynchronizationof clock speeds (or bitsynchronization) 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 dictateswhich type will be used. The synchronization codeused widely on IBM SDLC networks is called NRZI (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 DATATRANSFER Data conveyance a overtransmission 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 datacharacter(represented say by an eight-bit ‘byte’) is preceded by a few additional bits, which are sent to mark (ordelineate) 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 characterson an asynchronous transmissionsystemthe line is left ina 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 (value ‘0’). When a character bits (consistingofeightbits) is ready to besent,thetransmitterprecedestheeightbit 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 11010010isbeing sent. (Note: usually nowadays, only one stop bit used. This reduces the overall number of bits is 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 (thequiescent period) is filled by a string of 1S. The receiver can recognize the start of each character when sent by the presence of the start bit transition(fromstate ‘1’ tostate ‘0’). The followingeightbitsthenrepresentthe 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 andcheap.Asynchronous transmission is quite widely between used computer terminals andthecomputers 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.
  16. 192 PRINCIPLES DATA NETWORK AND PROTOCOLS Start stop bit bits State‘O‘ S t a t e ‘1’ Quiescent period 1 1 Figure 9.14 8- bit pattern 0 1 0 0 1 4 0 Asynchronous data transfer Quiescentperiod, or next character SY N User d a t a ( m a n y bytes) SYN SYN Figure 9.15 Synchronous data transfer As we can see fromFigure 9.14, out of eleven bitssentalongthe line, only eight represent useful information. In synchronous data transfer, the data areclocked 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 thatall the data bit patterns are of an equal length, the start ofeach is known to followimmediatelythepreviouscharacter. Theadvantage 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 sometime find themselves faced with the question of whetheramodem is ‘Hayes compatible’. By this, we ask whether it uses the Hayes command set, the procedure by
  17. FOR PROTOCOLS OF DATA 193 which the link is set up and the data transfer is controlled; if you like, the handshake and etiquetteof conversation. Itallows, for example, aPC to instructa modem to diala given telephonenumber and confirm when connection the 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. 9.11 PROTOCOLS FORTRANSFER OF DATA 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 themessage flow between the end devices and to provide for error correction of incomprehensible information. Unlike human beings engaged in conversation, machines can make sense at all of corrupted messages, and no 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 procedureare laid out in protocols. Now, sit down and make yourself comfortable. . . Once upon a time protocols were relatively unsophisticated like the simple computer- to-terminalnetworks which they supportedand they were containedwithinother computer application programmes.Thusthecomputer, besides itsmainprocessing 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- tionssoftware andhardware developed to such an extent asto be unwieldy and unmaintainable. Many computer items(particularly different manufacturers’equip- ments) were incompatible. Against this background the concept of layered protocols developed with theobjective 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 otherlayers. For example, a change in theroutingofa message (i.e.the topology of a network) could be carried out without affecting the functions used for correcting anycorrupteddata(orerrors)introducedonthe line between theend 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
  18. 194 PRINCIPLES DATA NETWORK AND PROTOCOLS systems interconnection (OSZ) model. The OSI model is not a set of protocols in itself, but itdoescarefully define thedivision of functionallayersto which all modern protocols are expected to conform. The protocols of each of the layers are defined in individual I S 0 standards and ITU-T recommendations. 9.12 THEOPENSYSTEMS INTERCONNECTIONMODEL The open systems interconnetion model, first standardized by I S 0 in 1983, classifies data transfer protocols in aseries of layers. It sets worldwide standards of design for all data telecommunicationprotocols,ensuringinterworkingcapabilityofequipments provided by different manufacturers. To understand the need for the model, let us start with an analogy, drawn from a simple exchange of ideas in the form of a dialogue between two people. The speaker has to convert his ideas into words; a translation may then be necessary into a foreign language which can be understood by the listener; the words are then converted into sound by nerve signals and appropriate muscular responses in the mouth and throat. The listener meanwhile is busy converting the sound back into the original idea. While this is going on, the speaker needs to make sure in one way or another that thelistener has received the information, and has understood it. If there is a breakdown in any of these activities,there can be nocertaintythatthe originalideahas been correctly conveyed between the two parties. Note that each function in our example is independent of every other function. It is not necessary to repeat the language translationif the receiver did not hearthe message, arequest (prompt)to replaya tape of thecorrectlytranslated message would be sufficient. The specialist translator could be getting on with the next job so long as the less-skilled tape operator was on hand. We thus have a layered series of functions. The idea starts at the top the talker’s stack of functions, andis converted by each function of in the stack, until at the bottom it turns up in a soundwave form.A reverse conversion stuck, used by the listener, re-converts the soundwaves back into the idea. Figure 9.16 shows our example. Each functionin the protocol stack of the speaker hasan exactly corresponding, or so- called peer function in the protocol stack the listener. The functions at the same of layer in the two stacks correspond to such extent that if we could conduct a direct an peer-to- peer interaction then we would actually be unaware of how the functions of the lower layers protocols had been undertaken. Let us, for example, replace layers 1 and 2 by using a telex machine instead. The speaker still needs to think up the idea, correct the grammar and see to the language translation, but now insteadof being aimed at mouth muscles and soundwaves, finger muscles and telex equipment do the rest, provided that the listeneralsohasa telex machine. We cannot, however, simply replace only the speaker’s layer 1 function (the mouth),if we do notcarry out simultaneouspeerprotocol changes on the listener’s side because an ear cannot pick up a telex message. The prin- ciple of layered protocols is that so long as the layers interact in apeer-to-peer manner, and so long as the interface between the function of one layer and its immediate higher and lower layers is unaffected, then itis unimportant how the function of that individual layer is carried out. This is the principle of the open systemsinterconnection (OSZ)
  19. THE OPEN SYSTEMS INTERCONNECTION MODEL 195 loyer 5 Ideo Talker Conversion t o Ideo Listener 1 8 L longuoge, grommor, syntax i s ready ond Language to ideo conversion Confirm receipt (e.g. smile) message (repeat i f necessary) 1 Message to mouth muscles I Message t o brain 1 Mouth b Sound L Eor I Figure 9.16 A layered protocol model for simple conversation model. The OS1 model sub-divides the function of data communication into a number of layered and peer-to-peer sub-functions, as shown in Figure 9.17. In all, seven layers are defined. Respectively, from layer seven to layer one these are called: the application layer, the presentation layer, the session layer, the transport layer, the network layer, the data link layer and the physical l q e r . Each layer of the OS1 model relies on the service of the layer beneath it. Thus the transport layer (layer 4) relies on the network service which is provided by the stack of layers 1-3 beneath it. Similarly the transport layer provides a transport service to the session layer, and so on. The functionsof the individual layers of the OS1 model are defined more fully in I S 0 standards (IS0 7498), and in ITU-T’s X.200 series of recommendations; in a nutshell they are as follows. 9.12.1 Application Layer(Layer 7) This is the layer that provides communication services to suit all types of data transfer between cooperating computers. It comprises a number of service elements (SEs), each suitedforaparticularpurpose orapplication.They may be combinedinvarious permutations to meet the needs of more complex applications. A wide range of application layer protocols will be defined over time, to accommodate all sorts of different computer equipment types, activities, controls other and applications.These will be defined in a modular fashion, the simplest common functions being termed application service elements (ASEs), which are sometimes grouped in specific functional combinations to form application entities (AEs). These are the functions coded as protocols.
  20. 196 PRINCIPLES NETWORK DATA AND PROTOCOLS OS1 layer number Peer - t o - peer Application Application protocol Presentation c-----+ Presentation S Session + - - - - - 4 Session Transport c------@Transport 3 Network Network Data link 1 Physical l c - - - - ) A c t u a lo m m u n i c a t i o n - - - - - c e- - -D I m a g i n a r yc o m m u n i c a t i o n( r e l y i n g on lower l a y e r s 1 Model. (Courtesy of CCZTT - derived Figure 9.17 The Open Systems Interconnection (0%) from Figures 12 and 13/X200) Although the application layer protocol will differ according to the particular situation, AE or ASE, the I S 0 standards at leastprovideastandardized notation (shorthand language)inwhichthe applicationlayerprotocols maybedefined and written to enable fairly rapid interpretation by experienced technicians and equipment designers. This notation is the abstract syntax notation I (ASN.1).It is laid out in ITU-T recommendations X.208 (old version) and X.680 version). Examples of application (new layer protocols covered by this book are the transaction capabilities of signalling system 7 (see Chapter 12) and the message handling system (Chapter 23). 9.12.2 PresentationLayer(Layer 6) As we found in Chapter 4, data may be coded in various forms such as binary, ASCII, ITU-T IA5, EBCDIC, faxencoding, multimedia signal format, etc. The task of the presentation layer is to negotiate a mutually agreeable technique for data encoding and
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