Mạng và viễn thông P3

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Long-haul . Communication None of the circuits that we have so far discussed are suitable as they stand for long haul communication. To get us anywhere with long haulwe need to address ourselves to the following inescapably pertinent topics:

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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) 3 Long-haul . Communication None of the circuits that we have so far discussed are suitable as they stand for long haul communication. To get us anywhere with long haulwe need to address ourselves to the following inescapably pertinent topics: 0 attenuation (loss of signal strength over distance) 0 line loading (a way of reducing attenuation on medium length links) 0 amplification (how to boost signals on long haul links) 0 equalization (how to correct tonal distortion) 0 multiplexing (how to increase the number of ‘circuits’ that may be obtained from one physical cable) In thischapter we discusspredominantlyhowtheselineissues affect analoguetransmission systems and how they may be countered. The effects on digital transmission are discussed in later chapters. 3.1 ATTENUATION AND REPEATERS Sound waves diminish thefurther they travel and electrical signals become weaker as they pass along electromagnetic transmission lines. With electrical signals the attenuation (as this type loss in signal strength called) is caused by the various electrical properties of is of the itself. These properties are known asresistance, the capacitance, the leakance line the and the inductance. The attenuation becomes more severe as theline gets longer. On very long haul links thereceived signals become weak as to be so imperceptible, and something needs to be done about it. Usually the attenuation in analogue transmission lines is countered by devices called repeaters, which are located at intervals along the line, their function being to restore the signalto its original wave shape and strength. 29
  2. 30 LONG-HAUL Signal attenuation occurs in simple wireline systems, in radio and in optical fibre systems. 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 fade with distance travelled according to a negative exponential function, the rate of decay of 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 the attenuationconstantforanyparticular signal frequency onan electrical wire line transmission system is given by the following formula: cli + + + = J ( i { J [ ( R 2 47r2f2L2)(G2 47r2f2C2)] (RG - 47r2f2LC)}) The larger the value of (, the greater the attenuation, the exact value depending on the following line characteristics: R : the electrical resistance per kilometre of the line, in ohms G: the electrical leakance per kilometre of the line, in mhos L: the inductance per kilometre of the line, in henries C: the capacitance per kilometre of the line, in farads f : the frequency of the particular component of the signal. The resistance of the line causes direct power loss impeding the onwardpassage of the by signal. The leakance is the power lost by conduction through the insulation of the line. The inductance and Capacitance are more complex and current-impeding phenomena caused by the magnetic effects of alternating electric currents. It is important to realize that because the amount of attenuation depends on the frequency 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 be disproportionately attenuated, leaving the low frequency (bass tones) to dominate. This leads to distortion, also called frequency attenuation distortion or simply attenuation distortion. - Transmitted signol omplitude Ta 1\ c = Amplltude at any pomt = Ta X e-& U 3 Dosronced Figure 3.1 The effect of distance on attenuation. cr = attenuation constant
  3. LINE LOADING 31 3.2 LINE LOADING As the inductance and capacitance workagainstone another, asimplemeans of minimizing the effect of unwanted attenuation distortion and is to increasethe inductance of the line to counteract some or all of the line capacitance. Taking a closer look at the equation given in the last section, find that attenuation we is zero when both resistance and leakance are zero, but will have a real value when either resistance or leakance is non-zero. The lesson is that both the resistance and the leakance of the line should be designed to be as low as is practically and economically possible. This is done by using large gauge (ordiameter) wire andgoodquality insulating sheath. A second conclusion from the formulafor the attenuation constant that its value is is minimized when the inductance has a value given by the expression L = CR/G henrieslkm This, in effect, reduces the electrical properties of the line to a simple resistance, mini- mizing both the attenuation and the distortion simultaneously. In practice, the attenua- tion and distortion of a line can be reduced artificially by increasing its inductance L ideally in a continuous manner along the line’s length. The technique is called line loading. It can be achieved by winding iron tape or someothermagneticmaterial directly around the conductor, but it is cheaper and easier to provide a lumped loading coil at intervals (say 1-2 km) along theline. The attenuationcharacteristics of unloaded and loaded lines are shown in Figure 3.2. Unloaded line / - . a Lump loaded line Continuously loaded line I - I Speech band I Signalfrequency Figure 3.2 Attenuation characteristics of loaded line
  4. 32 LONG-HAUL The use of line loading has the disadvantage that it acts as a high frequency filter, tending to suppress the high frequencies. It is therefore important when lump loading is used, tomakesurethatthewanted speech band frequencies suffer only minimal attenuation. If the steep part of lump loaded line curve (marked by an asterisk in Figure 3.2) were to occur in the middle of the speech band, then the higher frequencies intheconversationwouldbedisproportionatelyattenuated,resultingin heavy and unacceptable distortion of the signal at the receiving end. 3.3 AMPLIFICATION Although lineloading reduces speech-band attenuation, there is still a loss of signal which accumulates with distance, and at some stage it becomes necessary to boost the signal strength. This is done by the use of an electrical amplifier. The reader may well ask what precise distance line loading is good for. There is, alas, no simple answer as it depends on the gauge of the wire and on the transmission bandwidth required by the user. 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 transmission line, radio system or other transmission medium. Repeaters consist of amplifiers and other equipment, the purpose of which is to boost the basic signal strength. Normallya repeater comprisestwo amplifiers, oneforeachdirection of signal transmission. A splitting device is also required to separate transmit and receive signals. This is so that each signal can befed to a relevant transmit or receive amplifier, as Figure 3.3 illustrates.The splitting device is called a hybrid or hybridtransformer. Essentially it converts a two-way communication over two wires into two one-way, two-wire connections, and it 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 single two-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 r--- - _ _ _ _ - - - Repeater - - -l I Amplifier I Figure 3.3 A simple telephone repeater system
  5. AMPLIFICATION 33 four-wire communication is called for. Figure 3.3 shows a line between two telephones with a simple telephonerepeater in the line. Eachrepeaterconsists of twohybrid transformers and two amplifiers (one for each signal direction). The amplijication introduced at eachrepeaterhas to be carefully controlledto overcomethe effects of attenuation,without adversely affecting what is called the stability of the circuit, and without interfering with other circuits in the same cable. Repeaters too far apart or with too little amplification would allow the signal current to fade to such an extent as to be subsumed in the electrical noise present on the line. Conversely, repeaters that are too close together or have too much amplification, can lead 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, causingfeedback and even moreamplification.Thisin turn leads to even greater feedback, and so on and on, until the signal is so strong that it reaches the maximum power that the circuit can carry. The signal is now distorted beyond cure and all the listener hears is a very loud singing noise. For the causes of this distressing situation let us look at the simple circuit of Figure 3.4. The diagram of Figure 3.4 shows a poorly engineered circuit which is electrically unstable. At first sight, the diagram is identical to Figure 3.3. The only difference is that various signal attenuation values (indicated as negative) and amplification values (indicated as positive) have been marked using the standard unit of measurement, the decibel (dB). The problem is that the net gain around the loop is greater than the net loss. Let us look more closely. The hybrid transformer H1 receives the incoming signal from telephone Q and transmits it to telephone P, separating this signal from the one that will be transmitted on the outgoing pair of wires towards Q. Both signals suffer a 3 dB attenuation during this ‘line-splitting’ process. Adding the attenuation of 1 dB which is suffered on the local access line by the outgoing signal coming from telephone P, the total attenuation of the signal by the time it reaches the output of hybrid H1 is therefore 4 dB.The signal is further attenuated by 5 dB as a result of line loss. Thus the input to amplifier A1 is 9 dB below the strength of the original signal. Amplifier A1 is set to more than make up for attenuation by boosting the signal by 13 dB, so that at this + 1 dB 3 Line loss Arnplifler + l 3 dB Figure 3.4 An unstablecircuit
  6. 34 LONG-HAUL its output the signal is actually 4dB louder than it was at the outset. However, by the time the second hybrid loss (in H2)and Q’s access lineattenuation have been taken into account, 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 neither of thehybridtransformerscanactuallycarryouttheirline-splittingfunctionto perfection; a certain amount of the signal received by hybrid H2 from the output of amplifier 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 receive and transmit channels. However, in our example, the hybrid has either been poorly installed or become faulty, and the unwanted retransmittedsignal (originated by P but returned by hybrid H2) is only 7 dB weaker at hybrid Hs’s point of output than it was at the output from Al, and is then attenuated by 5 dB andamplified 13 dB before finding its way back to hybrid H l , where it goes through another undesired retransmission, albeit at a cost of 7 dB in signal strength. The strength of this signal, which has now entirely ‘lapped’ the four-wire sectionof the circuit, is 2 dBless than that of the original signal emanating from telephone P. However, on its first ‘lap’ it had a strength 4dB lower than the original. In other words, the re-circulated signal is actually louder than it was on thefirst lap!What is more, if it goesaround again it will gain 2dB in strength for each lap, quickly getting louder and louder and out of control. This phenomenon is called 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 fromthenon-idealperformance of thehybrid.Inpracticeit is impossible to exactly balance the hybrid’s resistance with that of the end telephone handset. One way to correct circuit instability is to change the hybrids for more efficient (and probablymore expensive) ones. solution This requirescarefulbalancing of each telephone handset and corresponding hybrid, and is not possible if the customer lines and the hybrids are on opposite sides of the switch matrix (as would be the case for a two-wire customer local line connected via a local exchange to a four-wire trunk or junction). A cheaper and quicker alternative to the instabilityproblem is simply to reduce the amplificationinthefeedbackloop.This is done simply by adding an attenuating device.Indeedmostvariableamplifiers are in fact fixed gainamplifiers (around 30 dB) followed immediately be variable attenuators or pads. For example, in the case illustrated in Figure 3.4 a reduction in the gainboth amplifiers A1 and A2 to of 12 dB will mean that the feedback signal is exactly equal in amplitude to the original. In this state the circuit may just be stable, but it is normal to design circuits with a much greater margin of stability, typically at least 10dB. For the Figure 3.4 example this would restrict amplifier gain to no more than 7 dB. Under these conditions the volume of the signalheardintelephone Q will be 6dB quieterthanthattransmitted by telephone P, but this is unlikely to trouble the listener. Crosstalk is the name given toanoverheard signal onanadjacentcircuit.It is broughtabout by electromagneticinduction of an over-amplifiedsignalfrom one circuit onto its neighbour, and Figure3.5 gives a simplified diagram of how it happens. Becausethe transit amplifier on the circuitfromtelephone Ato telephone B in Figure 3.5 is over-amplifying the signal, it is creating a strong electromagnetic field around the circuit and the same signal is induced into the circuit from P to Q. As a result the user of telephone Q annoyingly overhears the user of telephone A as well as
  7. CIRCUITS TWO- AND FOUR-WIRE 35 Repeaters Figure 3.5 Crosstalk. Q hears A the conversation from P. It is resolvedeither by turning down the amplifier or by increasing the separation of the circuits. If neither of these solutions is possible then a third, more expensive, option is available, involving the use of specially screened or transverse-screened cable. In such cable a foil screen wrapped around the individual pairs of wires makes it relatively immune to electromagnetic interference. 3.4 TWO- AND FOUR-WIRE CIRCUITS The diagram in Figure 3.3 illustrates a single repeater, used for boosting signals on a two-wire line system. If the wire is a long one, a number of individual repeaters may be required. Figure 3.6 shows an example of a long line in which three amplifiers have been deployed. Such a system may work quite well but it has a number of drawbacks, the most important of which is the difficulty inmaintainingcircuitstabilityandacceptable received signal volume simultaneously; this difficulty arises from the interaction of the variousrepeaters.Aneconomicconsiderationisthehighcost of themanyhybrid transformersthat need to beprovided. All butthefirstandlasthybrids could be dispensed with if the circuit were wired instead as a four-wire system along the total length of its repeatered section, as shown in Figure 3.7. This arrangement reduces the problem of achieving circuit stability when a large number repeaters are needed, and of Repeater 1 Repeater 2 Repeater 3 2 -wire 2 -wire I ine I ine Figure 3.6 A repeatered 2-wirecircuit
  8. 36 LONG-HAUL Repeater 1 Repeater 2 Repeater 3 L-wire L- wire line Line Figure 3.7 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 requiring only 1-2 repeaters (i.e. junction circuits), can however make do with two-wire systems. 3.5 EQUALIZATION We have mentioned the need for equalization on long haul circuits, to minimize the signal distortion. Speech and data signals comprise a complex mixture of pure single frequency components, each of which is affected differently by transmission lines. The result of different attenuation of the various frequencies is tonal degradation of the received signal; at worst, the entirehigh or low-frequency range couldbe lost. Figure 3.8 shows the relative amplitudes of individual signal frequencies of a distorted and an undistorted signal. Amp1 i t u d e (signal strength) I- -r- / c - \ listortedsignal I / I p- Speechband - 4 I I D Signal frequency Figure 3.8 Amplitude spectrum of distorted and undistorted signals (attenuation distortion)
  9. FREQUENCY DIVISION MULTIPLEXING (FDM) 37 In Figure3.8 the amplitude of the frequencies in the undistorted (received) signal is the same across the entire speech bandwidth, but the high and low frequency signals (high and low notes) have been disproportionately attenuated (lost) in the distorted signal. The effect is known as attenuation distortion or frequency attenuation distortion. To counteract this effect, equalizers are used, which are circuits designed to amplify orattenuate differentfrequencies by different amounts.Theaim is to ‘flatten’ the frequency response diagram to bring it in line with the undistorted frequency response diagram. In our example above, the equalizer would need to amplify the low and high frequencies more than it would amplify the intermediate frequencies. Equalizers are normallyincluded in repeaters, so that the effects of distortion can be corrected all the way along the in the same way that amplifiers counteract the effect line, of attenuation. 3.6 FREQUENCY DIVISION MULTIPLEXING (FDM) When a large number of individual communication channels are required between two points a long distance apart, providing alargenumberofindividualphysical wire circuits, one for each channel, can a very expensive business. For this reason, whatis be known as multiplexingwasdeveloped as a way of making better use of lineplant. Multiplexing allowsmany transmission channels share the same to physical pair of wires or other transmission medium. It requires sophisticated and expensive transmission line- terminating equipment ( L T E ) ,but has the potential for overall savingin cost because the number of wire pairs required between the end points can be reduced. Table 3.1 Frequency division multiplex (FDM) hierarchy Consists Bandwidth of of name Channel 24 telegraph 4 kHz 0-4 kHz 1 (1 telephone channel) subchannels 120 Hz spacing Group 12 channels 48 kHz 60-108 kHz 12 Supergroup 5 groups 240 kHz 3 12-552 kHz 60 Basic hypergroup 15 supergroups 3.7 MHz (3.6 MHz used) 312-4082 kHz 900 (also called a ‘super (3 mastergroups) (240 kHz per supergroup (4 MHz line) mastergroup’) with 8 kHz spacing normally between each) Basic hypergroup 16 supergroups 4 MHz 60-4028 kHz 960 (alternative) Mastergroup 5 supergroups 1.2 MHz 3 12-1 548 kHz 300 Hypergroup 9 mastergroups 12 MHz 312-12336kHz 2 700 (12 MHz) Hypergroup 36 mastergroups 60 MHz 4404-59 580 kHz 10 800 (60 MHz)
  10. 38 LONG-HAUL The method of multiplexing used in analogue networks is called frequency division multiplex ( F D M ) . FDM calls for a single, high grade, four-wire transmission line (or equivalent), and both pairs must capable of supporting avery large bandwidth. Some be FDM cables have a bandwidth as high as 12 MHz (million cycles per second), or even 60 MHz. This compareswith the modest 3. l kHz (thousandcycles per second) required for asingle telephone channel. The large bandwidththe key to the technique, as it sub- is divides readily into a much larger number of individual small bandwidth channels. The lowest constituent bandwidth that makes up an FDMsystem is a single channel bandwidth of 4 kHz. This comprises the 3. l kHz needed for a normal speech channel, togetherwithsomesparebandwidth to createseparationbetweenchannelsonthe system as a whole. Various other standard bandwidths are then integral multiples of asinglechannel.Table3.1illustratesthis and gives thenames of these standard bandwidths. The overall bandwidth of the FDM transmission line is equalto one of the standard bandwidths (e.g. supergroup, group)named in Table 3.1, and is then broken down into a number of sub-bandwidths, called tributaries in the manner shown in Figure 3.9. The equipment which performs this segregation bandwidth is called of translating equipment. Thus supergrouptranslatingequipment ( S T E ) subdividesasupergroup into its five component groups, and a channel translating equipment ( C T E ) subdivides a group into twelve individual channels. Not all the available bandwidth needs broken be downinto individual 4kHz channels. If required, some of it can be used directly for large bandwidth applications suchasconcertgrademusicortelevisiontransmission.InFigure 3.9, two 48 kHz circuits are derived the from supergroup, togetherwith 36 individual telephone . , - One. L wire ( supergroupFDM line 1 12 individual circuits ( & -wire 1 +Transmit 12 individual circuits El CTE H ST E 12 individual circuits 2 X 18 k H z bandwith, { Receive &-wirelines Figure 3.9 Breaking-up bandwidth in FDM
  11. DIVISION FREQUENCY (FDM) 39 channels. The same principle can be applied at other levels in the hierarchy. Thus, for example, 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 used directly for bandwidth applications of 240 kHz. Before multiplexing,the audio signals which areto bemultiplexed-up are first converted to four-wire transmission, if they are not so already. The signals on each transmit pair are then accurately filtered so that stray signals outside the allocated bandwidth are suppressed. In fact, a telephone channel is filtered to be only 3.1 kHz in bandwidth (of the available 4 kHz). The remaining 0.9 kHz separation prevents speech interference between adjacent channels. Groups are filtered to 48 kHz, supergroups to 240 kHz, etc. Each filtered signal is then modulated by a carrier frequency, which has the effect offrequency shifting the original signal into another part of the frequency spectrum. For example,atelephonechannelstarting out in thebandwidthrange 300-3400 Hz might end up in the range 4600-7700 Hz. Another could be shifted to the range 8300-11 400Hz, and so on. Each component channel of an FDM group is frequency shifted by the CTE to a different bandwidth slot within the 48 kHz available;so that in total 12 individual channels may be carried. Likewise, in a supergroup, five already made-up groups of 48 kHz bandwidth are slotted in to the 240 kHz bandwidth by the STE. The frequency shift is achieved by modulation of the component bandwidths with different carrier signal frequencies. The frequency of the carrier signal which is used to modulate the original signal (or baseband) will be equal to the value of the frequency shift required. Each carrier signal must be produced by the translating equipment. Themodulation of a signal inthefrequency band 300-3400Hz using acarrier frequency of 8000Hz produces a signal of bandwidth from 4600Hz (8000-3400) to 1 1 Hz (8000 + 3400). The original frequency spectrum of 400 300-3400 Hz is reproduced in two mirror image forms,called sidebands. One sideband is in the range 4600-7700 Hz and the other is in the range 8300-1 1 400 Hz. Both sidebands are shown in Figure3.10. As all the information is duplicated in both sidebands, only one of the sidebands needs to be transmitted. For economy in the electrical power needed to be transmitted to line, it is normalfor FDM systems tooperate ina singlesideband ( S S B ) and Amditude Carrier frequency t I Original signal spectrum [ baseband) 300 Hz 3600 HZ 6600 Hz 7700 Hz 8300 Hz llLO0 Hz Signal Lower Baseband sideband sideband Figure 3.10 Frequency shifting by carrier modulation
  12. 40 LONG-HAUL suppressed carrier mode. The original signal is reconstructed at the receiving end by modulating (mixing) a locally generated frequency. (Note: single sideband operation may also be used in radio systems, but the carrier not suppressed in this case because is it is often inconvenient 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 FDM system is modulated with a different carrier frequency, the lower sideband is extracted for conveyance. It may seem ineffic- ient not to double up the use of carrier frequencies, adopting alternating upper and lower sidebands of different channels, but by always using the lower sidebandwe obtain a better overall structure, allowing easier extraction of singlechannelsfromhigher order FDM systems. The carrier frequencies needed to produce a standard group are thus 64 kHz, 68 kHz, 72 kHz, . . . ,108 kHz and the overall structure is as shown in Figure 3.1 1. ... 1L 1 2 1 1 1 0 9 8 7 6 5 0 3 2 7 Audio Channels ! I I Baseband 0 - h kHZ Channel modulating frequency 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 ( lower sideband used 1 1211 1 0 9 8 7 6 5 h 3 2 1 Basicgroup structure kHz 60 108 kHz (when frequency 120 k H z 1 2 3 h 5 6 7 8 9 1 0 1 1 1 2 12 kHz kHz 60 Figure 3.11 The structure of an FDM group
  13. CROSSTALK AND ATTENUATION ON FDM SYSTEMS 41 Supergroups hypergroups and may be modulated in a similar fashion, using appropriate carrier frequencies and single sideband operation. 3.7 CROSSTALK AND ATTENUATION ON FDMSYSTEMS Therecan be as muchcrosstalkandattenuation in FDM systems as in the single channel or audio circuits which we discussed in the first part of the chapter. They require just as muchif not more planning, as FDM systems are generally more sensitive and complex.
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