long-haul communication

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3Long-haul.Communication

None of the circuits that we have so far discussed are suitable as they stand or ong haulcommunication.Togetusanywhere with long haulwe need to address ourselves to the followinginescapably pertinent topics:0 attenuation (loss of signal strength over distance)0 line loading (a way of reducing attenuation on medium ength links)0 amplification (how to boost signals on long haul inks)0 equalization (how to correct tonal distortion)0 multiplexing (how to increase the number of circuits that may be obtained fromone physical cable)In thischapter we discusspredominantlyhow hese ine ssues affect analoguetransmissionsystems and how they may be countered. The effects on digital transmission are discussed nlater chapters.

3.1 ATTENUATION AND REPEATERSSound waves diminish the urther they travel andlectrical signals become weaker as theypass along electromagnetic transmission lines. With electrical signals the attenuation(as this typef loss in signal strengths called) is caused by the various electrical propertieof the ine itself. These properties are known as theesistance, thecapacitance,theleakanceand the inductance. The attenuation becomes more severe as the ine gets longer. Onerylong haul links theeceived signals becomeoweak as to bemperceptible, and somethingneeds to be done about t. Usually the attenuation in analogue transmission lines iscountered by devices calledrepeaters, which are located at ntervals along the ine, theirfunction being to restore the signal o its original wave shape and strength.

29

Networks and Telecommunications: Design and Operation, Second Edition.Martin P. Clark

Copyright 1991, 1997 John Wiley & Sons LtdISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)


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30 LONG-HAULSignal attenuation occurs n simple wireline systems, in radio and in optical fibresystems. The effect of attenuation on a line follows the function shown in Figure 3.1,where the signal amplitude (the technical term for signal strength) can be seen to fadewith distance travelled according to a negative exponential function, the rate of decayof the exponential function along the length of the line being governed by the attenua-

tion constant, alpha (a).Complex mathematics, which we will not go into here, reveal that the value of theattenuationconstant oranyparticular signal frequency onan electrical wire linetransmission system is given by the following formula:cl i = J ( i {J [ (R2+47r2f2L2)(G2+47r2f2C2)]+(RG -47r2f2LC)})

The larger the value of (, the greater the attenuation, the exact value depending on thefollowing line characteristics:R: the electrical resistance per kilometre of the line, in ohmsG: the electrical leakance per kilometre of the line, in mhosL: the inductance per kilometre of the line, in henriesC: the capacitance per kilometre of the line, in faradsf : thefrequency of the particular component of the signal.Theresistance of the line causes direct power lossy impeding the onwardpassage of thesignal. The leakance is the power lost by conduction through the insulation of the line.The inductance and Capacitance are more complex and current-impeding phenomenacaused by the magnetic effects of alternating electric currents.It is important to realize that because the amount of attenuation depends on thefrequency of the signal, then the attenuation may differ for different frequency compo-nents of the signal. For example, it is common for high frequencies (treble tones) to bedisproportionately attenuated, leaving the low frequency (bass tones) to dominate. Thisleads to distortion, also called frequency attenuation distortion or simply attenuationdistortion.

Amplltude at any pomt = Ta X e-&U3

Dosronced

1\- Transmitted signol omplitude Ta

Figure 3.1 The effectof distanceon attenuation. cr =attenuation constant


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L I N E LOADING 31

3.2 LINE LOADINGA s the inductance and capacitance workagainstone another, asimplemeans ofminimizing he effect of unwantedattenuationanddistortion is to increase heinductance of the line to counteract some or all of the line capacitance.Taking a closer look at the equation given in the last section,we find that attenuationis zero when both resistance and leakance are zero, but will have a real value wheneither resistance or leakance is non-zero. The lesson is that both the resistance and theleakance of the line should be designed to be as low as is practically and economicallypossible. This is done by using large gauge (ordiameter) wire andgoodqualityinsulating sheath.A second conclusion from the formula or the attenuation constants that its value isminimized when the inductance has a value given by the expression

L =CR/G henrieslkmThis, in effect, reduces the electrical properties of the line to a simple resistance, mini-mizing both the attenuation and the distortion imultaneously. In practice, theattenua-tion and distortion of a line can be reduced artificially by increasing ts nductanceL ideally in a continuous manner along the lines length. The technique is called lineloading. It can be achieved by winding iron tape or someothermagneticmaterialdirectly around the conductor, but it is cheaper and easier to provide a lumped loadingcoil at intervals (say 1-2 km) along the ine. The attenuationcharacteristics of unloadedand loaded lines are shown in Figure 3.2.

Unloaded line/a. Lump loaded l ine Cont i nuous l y

l o a d e d line- p e e c ha n dI I IS i g n a l r e q u e n c y

Figure 3.2 Attenuation characteristics of loaded line


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32 LONG-HAULThe use of line oading has the disadvantage that it acts as a high frequency filter,tending to suppress the high frequencies. It is therefore important when lump loading isused, tomakesure hat hewanted speech band frequencies suffer only minimalattenuation. If the steep part of lump oaded line curve (marked by an asterisk inFigure 3.2)were to occur in the middle of the speech band, then the higher frequencies

intheconversationwouldbedisproportionatelyattenuated,resulting n heavy andunacceptable distortion of the signal at the receiving end.

3.3 A M PLIF ICA TIONAlthough line oading reduces speech-band attenuation, there is still a loss of signalwhich accumulates with distance, and at some stage it becomes necessary to boost thesignal strength. This is done by the use of an electrical amplifier. The reader may wellask what precise distance line loading is good for. There s, alas, no simple answer as i tdepends on the gauge of the wire and on the transmission bandwidth required by theuser. In general, the higher the bandwidth, the shorter the length limit of loaded lines(10-15 km is a practical limit).Devices called repeaters are spaced equally along the length of a long transmissionline, radio system or other transmission medium. Repeaters consist of amplifiers andother equipment, the purpose of which is to boost the basic signal strength.Normallya repeater comprises wo amplifiers, one oreachdirection of signaltransmission. A splitting device is also required to separate transmit and receive signals.This is so that each signal can be ed to a relevant transmit or receive amplifier, asFigure 3.3 illustrates.The splitting device is called a hybrid or hybrid ransformer.Essentially t converts a two-way communication over two wires into two one-way,two-wire connections, and t is then usually referred to as a four-wire communication(one direction of signal transmission on each pair of a two-pair set). So,while a singletwo-wire line is adequate for two-way telephone communication over a short distance,as soon as the distance is great enough to require amplification then a conversion to

Repeaterr - - - _ _ _ _ - - - - - - lI A m p l i f i e r I

Figure3.3 A simple telephone repeater system


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A MP L I F I C A T I ON 33four-wire communication is called for. Figure 3.3 showsa line between two telephoneswith a simple telephonerepeater in the line. Eachrepeaterconsists of twohybridtransformers and two amplifiers (one for each signal direction).The amplijication introduced at each epeaterhas to be carefully controlled oovercome he effects of attenuation,without adversely affecting what is called thestability of the circuit, and without interfering with other circuits n the same cable.Repeaters too far apart orwith too little amplification would allow the signal current tofade to such an extent as to be subsumed n the electrical noise present on the line.Conversely, repeaters that are too close together or have too much amplification, canlead to circuit instability, and to yet another problem known as crosstalk.A circuit is said to be unstable when the signal that it is carrying is over-amplified,causing eedback and even moreamplification.This n turn leads to even greaterfeedback, and so on and on, until the signal is so strong that it reaches the maximumpower that the circuit can carry. The signal is now distorted beyond cure and all thelistener hears is a very loud singing noise. For the causes of this distressing situation letus look at the simple circuit of Figure 3.4.The diagram of Figure 3.4 shows a poorly engineered circuit which is electricallyunstable. At first sight, the diagram is identical to Figure 3.3. The only difference is thatvarious signal attenuation values (indicated as negative) and amplification values(indicated as positive) have been marked using the standard unit of measurement, thedecibel (dB). The problem is that the net gain around the loop is greater than the netloss. Let us look more closely. The hybrid transformer H1 receives the incoming signalfrom telephone Q and transmits it to telephone P, separating this signal from the onethat will be transmitted on the outgoing pair of wires towards Q.Both signals suffer a3 dB attenuation during this line-splitting process. Adding the attenuation of 1dBwhich is suffered on the local access line by the outgoing signal coming from telephoneP, the total attenuation of the signal by the time it reaches the output of hybrid H1 istherefore 4 dB. The signal is further attenuated by 5dB as a result of lineloss. Thus theinput to amplifier A1 is 9dB below the strength of the original signal. Amplifier A1 isset to more than make up forhis attenuation by boosting the signal by 13 dB, sothat at

+13dBLine oss Arnplifler

+l3dBFigure3.4 An unstablecircuit


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34 LONG-HAULits output the signal is actually 4dB louder than it was at the outset. However, by thetime the second hybrid loss (in H2)andQsaccess line attenuation have been taken intoaccount, the signal is back to its original volume. This might suggest a happy ending,but unfortunately the circuit is unstable. Its instability arises from the fact that neitherof thehybrid ransformerscanactuallycarryout heir ine-splitting unction operfection; a certain amount of the signal received by hybrid H2 from the output ofamplifier A1 is finding its way back onto the return circuit (Q-to-P).A well-designed and installed hybrid would give at least 30 dB separation of receiveand transmit channels. However, in our example, the hybrid has either been poorlyinstalled or become faulty, and the unwanted retransmittedsignal (originated by P butreturned by hybrid H2) is only 7dB weaker at hybridHsspoint of output than t was atthe output from Al, and s then attenuated by 5 dB andamplified 13 dB before findingits way back to hybrid Hl , where it goes through another undesired retransmission,albeit at a cost of 7dB in signal strength. The strength of this signal, which has nowentirely lapped the four-wire sectionof the circuit, is 2 dB ess than that of the originalsignal emanating from telephone P. However, on ts first lap t had a strength 4dBlower than the original. In other words, thee-circulated signal is actually louder than itwas on the irst lap!What is more, if it goes around again t will gain 2dB in strength foreach ap, quickly getting ouder and ouder and out of control. This phenomenon iscalled instability. The primary cause is the feedback path available across both hybrids,which is allowing incoming signals to be retransmitted (or fed back) on their output.The path results from henon-idealperformance of thehybrid. npractice t isimpossible to exactly balance the hybrids resistance with that of the end telephonehandset.One way to correct circuit instability is to change the hybrids for more efficient (andprobablymore expensive)ones.Thissolution equirescarefulbalancing of eachtelephone handset and corresponding hybrid, and is not possible if the customer linesand the hybrids are on opposite sides of the switch matrix (as would be the case for atwo-wire customer ocal ine connected via a ocal exchange to a four-wire trunk orjunction). A cheaper and quicker alternative to the instability problem is simply toreduce the amplification n he eedback oop.This is done simply by adding anattenuating device. ndeedmostvariableamplifiers are in act fixed gainamplifiers(around 30 dB) followed immediately be variable attenuators orpads. For example, inthe case illustrated in Figure 3.4 a reduction in the gainf both amplifiers A1 and A2 to12 dB will mean that the feedback signal is exactly equal in amplitude to the original. Inthis state the circuit may just be stable, but it is normal to design circuits with a muchgreater margin of stability, typically at least 10dB. For the Figure 3.4 example thiswould restrict amplifier gain to no more than7dB. Under these conditions the volumeof the signalheard n elephone Q will be 6dB quieter han hat ransmitted bytelephone P, but this is unlikely to trouble the listener.Crosstalk is the name given toanoverheard signal onanadjacentcircuit. t isbroughtabout by electromagnetic nduction of an over-amplifiedsignal rom onecircuit onto its neighbour, and Figure3.5 gives a simplified diagram of how it happens.Because he transit amplifier onthe circuit rom elephone Ato telephone B inFigure 3.5 is over-amplifying the signal, it is creating a strong electromagnetic fieldaround the circuit and the same signal s nduced into the circuit from P to Q. As aresult the user of telephoneQ annoyingly overhears the user of telephone A as well as


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TWO- AND FOUR-WIREIRCUITS 35Repeaters

Figure 3.5 Crosstalk. Q hears Athe conversation rom P. It is resolvedeither by turning down the amplifier or byincreasing the separation of the circuits. If neither of these solutions is possible then athird, more expensive, option s available, nvolving the use of specially screened ortransverse-screened cable. In such cable a foil screen wrapped around the individualpairs of wires makes it relatively immune to electromagnetic interference.

3.4 TWO- A ND FOUR-WIRE CIRCUITSThe diagram in Figure 3.3 illustrates a single repeater, used for boosting signals on atwo-wire line system. If the wire is a long one, a number of individual repeaters maybe required. Figure 3.6 shows an example of a long line in which three amplifiers havebeen deployed.Such a system may work quite well but t has a number of drawbacks, the mostimportant of which is the difficulty inmaintainingcircuitstabilityandacceptablereceived signal volume simultaneously; this difficulty arises from the interaction of thevariousrepeaters.Aneconomicconsideration sthehighcost of themanyhybridtransformersthat need to beprovided. All butthe irstand asthybrids could bedispensed with if the circuit were wired instead as a four-wire system along the totallength of its repeatered section, as shown in Figure 3.7. This arrangement reduces theproblem of achieving circuit stability when a large number f repeaters are needed, and

Repeater 1 Repeater 2 Repeater 3

2 -w i r e 2 - w i r eI ine I ne

Figure3.6 A repeatered 2-wirecircuit


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36 LONG-HAULR e p e a t e r 1 Repeater 2 Repeater 3

L - w i r el i n e L- wi reLine

Figure3.7 A repeatered4-wirecircuit

it eases the maintenance burden. This is why most amplified longhaul (i.e. trunk)circuits are set up on four-wire transmission lines. Shorter circuits, typically requiringonly 1-2 repeaters (i.e. junction circuits), can however make do with two-wire systems.

3.5 EQUALIZATIONWe have mentioned the need for equalization on long haul circuits, to minimize thesignal distortion. Speech and data signals comprise a complex mixture of pure singlefrequency components, each of which is affected differently by transmission lines. Theresult of different attenuation of the various frequencies is tonal degradation of thereceived signal; at worst, the entirehigh or low-frequency range could be lost. Figure3.8shows the relative amplitudes of ndividual signal frequencies of a distorted and anundistorted signal.

Amp1 i t u d e( s i g n a l s t r e n g t h )

I - - r - c -/ \ l i s t o r t e d i g n a lI /

Ip S p e e c h b a n d -4I I D S i g n a l f r e q u e n c y

Figure3.8 Amplitude spectrumof distorted and undistorted signals (attenuation distortion)


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FREQUENCY DIVISION MULTIPLEXING (FDM) 37In Figure3.8 the amplitude of the frequencies in the undistortedreceived) signal is thesame across the entire speech bandwidth, but the high and low frequency signals (highand low notes) have been disproportionately attenuated (lost) in the distorted signal.The effect is known asattenuation distortion or requency attenuation distortion.To counteract this effect, equalizers are used, which are circuits designed to amplify

orattenuate different requencies by different amounts.Theaim is to flatten thefrequency response diagram to bring it in line with the undistorted frequency responsediagram. In our example above, the equalizer would need to amplify the low and highfrequencies more than it would amplify the intermediate frequencies.Equalizers are normally ncluded in repeaters, so that the effects of distortion can becorrected all the way along theine, in the same way that amplifiers counteract the effectof attenuation.

3.6 FREQUENCY DIVISION MULTIPLEXING (FDM)When a large number of individual communication channels are required between twopoints a ong distance apart, providing a argenumberof ndividualphysical wirecircuits, one for each channel, cane a very expensive business. For this reason, what sknown as multiplexingwasdeveloped as a way of making better use of ineplant.Multiplexing allowsmany transmission channels o share the same hysical pair of wiresor other transmission medium. Itequires sophisticated and expensivetransmission line-terminating equipment(LTE),but has the potential forverall saving n cost because thenumber of wire pairs required between the end points can be reduced.

Table 3.1 Frequency division multiplex (FDM) hierarchyBandwidth ofname

Channel(1 telephone channel)GroupSupergroupBasic hypergroup(also called a supermastergroup)Basic hypergroup(alternative)MastergroupHypergroup(12MHz)Hypergroup(60MHz)

24 telegraphsubchannels120Hz spacing12channels5 groups15 supergroups(3 mastergroups)16supergroups5 supergroups9mastergroups36mastergroups

4 kHz

48 kHz240 kHz3.7MHz (3.6MHz used)(240kHz per supergroupwith8kHz spacingnormally between each)

4MHz1.2MHz12MHz60MHz

0-4 kHz

60-108 kHz312-552 kHz(4MHz line)312-4082 kHz

60-4028 kH z312-1548 kHz312-12336kHz

4404-59 580kHz

1

1260900

9603002700

10800


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38 LONG-HAULThe method of multiplexing used in analogue networks is calledfrequency divisionmultiplex ( F D M ) .FDM calls for a single, high grade, four-wire transmission ine (orequivalent), and both pairs muste capableof supporting avery large bandwidth. SomeFDM cables have a bandwidth ashigh as 12MHz (million cycles per second), or even60MHz. This compareswith the modest 3. kHz (thousandcycles per second) required

for asingle telephone channel. The large bandwidths the key to the technique, as it sub-divides readily into a much larger number of individual small bandwidth channels.The lowest constituent bandwidth that makes up an FDM ystem is a single channelbandwidthof 4kHz. This comprises the 3. kHz needed for a normal speech channel,togetherwithsomesparebandwidth to createseparationbetweenchannelsonthesystem as a whole. Various other standard bandwidths are then integral multiples ofasinglechannel.Table3.1 llustrates his and gives thenames of these standardbandwidths.The overall bandwidth of theFDM transmission line is equal o one of the standardbandwidths (e.g.supergroup, group)named in Table3.1,and is then broken down into anumber of sub-bandwidths, called tributaries in the manner shown in Figure 3.9. Theequipment which performs this segregationf bandwidth is calledranslating equipment.Thus supergroup ranslatingequipment ( S T E ) subdividesasupergroup into its fivecomponentgroups, and achannel translating equipment ( C T E )subdivides a group intotwelve individual channels.Not all the availablebandwidthneedsbebroken down nto individual 4kHzchannels. If required, some of it can be used directly for large bandwidth applicationssuchasconcertgrademusicortelevisiontransmission. nFigure 3.9, two 48 kHzcircuits are derivedromhe upergroup,ogetherwith 36 individualelephone

12 indiv idualc i rcui ts( & -w i re 1

12 i nd i v i dua lc i rcui ts

.E C TEH

12 i nd i v i dua lc i r c u i t s

2 X 1 8 k H z {b a n d w i t h ,&-wire ines

ST E

One. L - wi re( supergroup FDM line 1+ransmi t

Receive

F igure3.9 Breaking-up bandwidth i n FDM


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FREQUENCY 39channels. The same principle can be applied at other levels in the hierarchy. Thus, forexample, all the supergroups of a hypergroup could be broken down into their compo-nent groups using HTE and STE.Alternatively, some of the supergroups could be useddirectly for bandwidth applications of 240 kHz.Before multiplexing, he audio signals which are o bemultiplexed-up are firstconverted to four-wire transmission, if they are not so already. The signals on eachtransmit pair are then accurately filtered so that stray signals outside the allocatedbandwidth are suppressed. In fact, a telephone channel is filtered to be only 3.1kHz inbandwidth (of the available 4 kHz). The remaining 0.9kHz separation prevents speechinterference between adjacent channels. Groups are filtered to 48 kHz, supergroups to240 kHz, etc. Each filtered signal is then modulated by a carrier frequency, which hasthe effect offrequency shifting the original signal into another part of the frequencyspectrum. For example,a elephonechannelstarting out in thebandwidth ange300-3400 Hz might end up in the range 4600-7700 Hz. Another could be shifted to therange 8300-11 400Hz, and so on.Each component channel of an FDM group is frequency shifted by the CTE to adifferent bandwidth slot within the 48 kHz available;so that in total 12 individualchannels may be carried. Likewise, in a supergroup, five already made-up groups of48 kHz bandwidth are slotted in to the 240 kHz bandwidth by the STE.The frequency shift is achieved by modulation of the component bandwidths withdifferent carrier signal frequencies. The frequency of the carrier signal which is used tomodulate the original signal (or baseband) will be equal to the value of the frequencyshift required. Each carrier signal must be produced by the translating equipment.Themodulation of a signal in he requency band 300-3400Hz using acarrierfrequency of 8000Hz produces a signal of bandwidth from 4600Hz (8000-3400) to1 1 00 Hz (8000+3400). The original frequency spectrum of 00-3400 Hz is reproducedin two mirror image forms,called sidebands. One sideband is in the range 4600-7700 Hzand the other s in the range8300-1 1400H z. Both sidebands are shown in Figure3.10.A s all the nformation is duplicated in both sidebands, only one of the sidebandsneeds to be transmitted. For economy in the electrical power needed to be transmittedto line, it is normal or FDM systems tooperate ina singlesideband (SSB)and

A m d i t u d e C a r r i e rt frequencyIOriginal ignalspectrum [ b a s e b a n d )

300 Hz 3600 HZ 6600 Hz 7700 Hz 8300 Hz l l LO0 Hz Si gna lBasebandower sidebandideband

Figure3.10 Frequency shifting by carrier modulation


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40 LONG-HAULsuppressed carrier mode. The original signal is reconstructed at the receiving end bymodulating (mixing) a ocally generated requency. (Note: single sideband operationmay also be used in radio systems, but the carriers not suppressed in this case becauseit is often nconvenient to make a carrier signal generator available at the receiver.When the carrier is not suppressed a much simpler and cheaper detector can be used.)

Each baseband signal to be included in an FDMsystem is modulated with a differentcarrier frequency, the lower sideband is extracted for conveyance. It may seem neffic-ient not to double up the use of carrier frequencies, adopting alternating upper andlower sidebands of different channels, but by always using the lower sidebandwe obtaina better overall structure, allowing easier extraction of single channels rom higherorder FDM systems. The carrier frequencies needed to produce a standard group arethus 64kHz, 68kHz, 72kHz, . . . ,108kHz and the overall structure is as shown inFigure 3.11.

...LA u d i o

C h a n n e l s1 2 1 1 1 0 9 8 7 6 5 0 3 2 7

B a s e b a n d

C h a n n e l !o d u l a t i n g

f r e q u e n c y 6 6 , 6 8 , 7 2 , 7 6 , 8 0 , 8 h , 8 8 , 9 2 , 9 6 , 1 0 0 , 1 0 h ,108 kHz

I I0 - h kHZ

( l o w e rsideband used 1

1 2 1 1 1 0 9 8 7 6 5 h 3 2 1Bas i cgroups t r u c t u r e 6 0H z 108 k H z

f r e q u e n c yw h e n 1 2 0 k H z

1 2 3 h 5 6 7 8 9 1 0 1 1 1 2

12 kHz 60H zFigure3.11 The structureof an FDM group


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CROSSTALK AND ATTENUATION ON FDM SYSTEMS 41Supergroupsandhypergroupsmay be modulated in a imilarashion,usingappropriate carrier frequencies and single sideband operation.

3.7 CROSSTALK AND ATTENUATION ON FDMSYSTEMSTherecan be as muchcrosstalkandattenuation in FDM systems as in the singlechannel or audio circuits which we discussed in the first part of the chapter. Theyrequire just as much f not more planning, asFDM systems are generally more sensitiveand complex.

long-haul communication

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