Mạng và viễn thông P33

Chia sẻ: Hug Go Go | Ngày: | Loại File: PDF | Số trang:39

Thêm vào BST

Báo xấu

44
lượt xem 3
download

Mạng và viễn thông P33

Mô tả tài liệu

Download Vui lòng tải xuống để xem tài liệu đầy đủ

Practical Network Transmission Planning Transmissionmedia exist to convey information across networks. No matter what form the information takes (be voice, video,data, or some other form), the prime requirement that the it is information received atthedestinationshouldas closelyaspossiblematch that originally transmitted. The signal should be free from noise, echo, interference and distortion, and should be of sufficient strength (or volume) asto be clearly distinguishable by the receiver. The reliability of the service is also important. ...

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Mạng và viễn thông P33

Networks and Telecommunications; Design and Operations (Second Edition) Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 33 Practical Network Transmission Planning Transmissionmedia exist to convey information across networks. No matter what form the information takes (be voice, video,data, or some other form), the prime requirement that the it is information received atthedestinationshouldas closelyaspossiblematch that originally transmitted. The signal should be free from noise, echo, interference and distortion, and should be of sufficient strength (or volume) asto be clearly distinguishable by the receiver. The reliability of the service is also important. Meeting these objectives requires careful network transmission planning. This chapter covers two aspects of planning, first describing the electrical engineering design guidelines usually laid out in a formal network ‘transmission plan’, and thengoing on to discuss the general administrative and operational practicalities of ‘lining-up’ and operating a transmission network. Resource management is crucial to ensure that lineplant and circuits are available when needed, that radio bandwidth is available without interference, that cables have been laid and satellites put in orbit. 33.1 NETWORK TRANSMISSION PLAN A set of guidelines, laying down simple‘rules’ for planning andcommissioning new line systems or new circuits, is usually set out in a formal transmissionplan. These guidelines are intended to ensure that the electrical principles of telecommunications theory are adhered to. The transmission plan should include, for example, stringent rules on the use, positioning, and strength of amplifiers or regenerators to correct the effects of attenuation. The plan needs to give guidelines on minimization of noise and interference, on the use of echo control devices to eliminate echo, and on the use of equalizers to rectify frequency attenuation distortion. A good transmission plan is a guideline for network design, ensuring that circuits are electrically stable and perform toa standard acceptable to the majority their users. A typical targetis ‘to ensurethat 90% of people of consider the performance to be “fair”, or “better”’. Acceptable values of attenuation and distortion can beestablished by subjective tests, and thenthenetwork can be designed to accord with them. 593
594 PLANNING TRANSMISSION NETWORK PRACTICAL ITU-T’s recommendations on transmission planning include design guidance on the overall signal volume at all points through the network the control of signal loss and the circuit’s electrical stability the limits on acceptablesignal propagation times (excessive propagation times mani- fest themselves as a delay in transmission, a potential cause slow data throughput of and response times, or of unacceptable gaps and pauses in conversation) the limits on acceptable noise disturbance the control of sidetone and signal echo (both these effects manifest themselves to speakers, who hear their own words echoed back to them) the minimization of signal distortion, crosstalk and interference (for digital circuits) maximum allowed bit error rate (or bit error ratio, BER), jitter and quantisation distortion maximum line length (e.g. approximately 15 metres for V.24 interface, 100 metres for X.21, etc.) The recommendations lay down a standard reference system, against which national and international transmission networks may be designed or calibrated. These facilitate the correct placing of circuit conditioning equipment, and the establishment of cor- rectsignalvolumes andcontrolled levels of signal distortion at allpointsalonga connection. 33.2 SENDAND RECEIVEREFERENCE EQUIVALENTS When designing telephone or other voice transmission networks, the strength of the electrical signal induced by the microphone at the sending end and the final sound volume produced by the receiver is all-important. It varies accordingto the proximity of the speaker to the microphone and the listener to the ear piece, the total loss being composed of three basic parts; the loss incurred when inducing the electrical signal in the microphone, the electrical signal loss across the transmission network itself, and finally the sound induction loss in the receiver. The relationship is straightforward to understand, as Figure 33.1 illustrates, but the problem for the network designer lies in deciding just how much signal loss is acceptable in the main part of the network itself. With no overall loss, the listener will hear too loud a signal, equivalent to a talker at normal volume speaking directly into his ear. On the other hand too much loss will result in an inaudible signal. To ease the design dilemma, the system of reference equivalents was developed by CCITT (the forerunner to ITU-T). The send reference equivalent ( S R E ) of a telephone microphone is the signallossexpressedin dB (decibels) when theelectricalsignal volume is compared with the original speech volume. Similarly, the receive reference equivalent ( R R E ) of the earpiece is the signal loss when the final (heard) sound volume is compared with the electrical signal volume input the earpiece. Both SRE and RRE to
EQUIVALENTS RECEIVE SEND AND REFERENCE 595 Network equivalent l I 1 Sending loss --- I (send reference ' equivalent) Network loss L -- I ' Receiving loss (Receive reference I cl I I I l I Overall loss l- Figure 3 . 31 Send and receive referenceequivalents are shown on Figure 33.1. To measure the sending reference equivalent (SRE) of a handset, a special NOSFER equipment such as that shownin Figure 33.2 is used. This compares the equipment with a standarddevice, named after this system of calibration, nouveau systeme fondamental pour la determination des equivalents de reference (new fundamental system for determining reference equivalents). The NOSFER equipmentin Figure 33.2 consists of a high quality microphone with a device which maintains it at a fixed distance from the speaker's mouth. This equipment is connected to the listener's earpiece via a large attenuator, of strength A dB. To per- form the calibration, a person talks alternately into microphone a and microphone b, while the listener adjusts the variable attenuation B to a value at which the volumes appear to be the same. The difference between values B and A will then give a measure of the relative microphone efficiencies. The more that attenuation B must be reduced (i.e.thelowerthevalue of B ) the less efficient themicrophoneandthelargerthe numerical value of A - B, which is termed the send reference equivalent ( S R E ) . In other Standard equipment attenuation (large) Fixed Microphone rc Talker (alternates between Equipment Listener a and b) under test Variableattenuater Figure 33.2 Measuring send reference equivalent (SRE)
596 NETWORK PRACTICAL TRANSMISSION PLANNING Fixed (large) attenuation Standard equipment L Earpiece Microphone Talker Listener under test between Voriable attenuoter A and B 1 Figure 33.3 Measuringreceivereferenceequivalent (RRE) words, the higher the value SRE, then thelower the efficiency of the microphone and of the greater the loss of signal strength in it. Thus SRE is a measure of the signal loss in the sending device. Receive reference equivalents ( R R E s ) similarly are a measure of the signal loss in the receiving device, and they may be measured by an equivalent NOSFER equipment; but in this case the comparison must be made against a standard high quality receiver, as Figure 33.3 shows. 33.3 CONNECTION REFERENCE POINTSAND OVERALL REFERENCE EQUIVALENT For the design of the network itself, all telephone handsets are assumed have typical to nominal SRE and RRE values. The values chosen are usually quoted for the handset loss relative to an imaginary reference point on the customer’s line side of the telephone exchange, and they take account not only of typical handset SREs and RREs, butalso of the loss encountered on the local line. Typical values are 13 dB SRE and 2-3 dB RRE. In practice, customer lines have SRE and RRE of varying values, depending on the exact handset in use and the length of the line. However, choosing a nominal design value is important as aguide for handset design and also as a benchmark for the required performance of the network itself. In Figure 33.4 two reference points (a) and (b), have been marked, oneat each end of the connection, corresponding to the points relative to which SRE and RRE values apply. These points are marked on the diagram as0 dBr. The nomenclature dBr stands for decibels-relative, and it is used to indicate the received signal loudness at any point, measured relative to the signal strength at the reference point. Hence, at the reference point itself, where the relative strength is equal to the signal strength at the reference point, the value will be zero dBr. If the network introduces a loss of 3 dB in the transmission direction from A to B, and 3.5 dB in the transmission direction from B to A, then the received signal strengths at points (b) and (a), respectively, will be -3 dBr and -3.5 dBr (i.e. signals 3 dB and
CONNECTION POINTS REFERENCE AND OVERALL EQUIVALENT REFERENCE 597 (a1 Reference point I A end 1 0 dBr I I RRE (B1 Network t RRE(A1 . SRElBl 1 l I Reference point I I (B end) 0 dBr (b) Figure 33.4 Connection ‘referencepoint’ 3.5dB weaker than those sent from the opposite reference points). These values have been marked on the diagram in Figure 33.5. Now we can meaningfully discuss the end-to-end loss, called the overall reference equivalent (or ORE). Surprisingly, the ORE may not be precisely equal to the sum of the SRE, the RRE and the cross-network loss. This is because both the SRE and RRE are only subjective measurements made in isolation on the network’s general speech carrying performance, and the ORE is a more stringent measure of the actual end-to- end loss, usually measured using a single frequency tone of 800 Hz. Thus the formula ORE = SRE + network loss+ RRE does not apply. Initially, network planners thought the discrepancy was small enough to be ignored, of network design,and CCITT thereby greatly easing the task used to recommend that the network loss be adjusted to conform withan overall reference equivalent not exceeding ORE ( A B 1 I (a1 4 0 dBr -3 dBr I I SRE(A) I , I RRE (61 I CI I I - 3 .S dBr 0d 6 Ib l .p J ORE ( B A ) Figure 33.5 Overallreferenceequivalent
598 PLANNING TRANSMISSION NETWORK PRACTICAL 40dB. However,a number of networkoperatorsfound difficulty withthe system, reporting discrepancies of up to 5 dB, so in 1976 CCITT developed a new measure of performance called the loudness rating. Still using the NOSFER equipment asa funda- mental reference, and based on similar principles and reference points, the loudness rating method of network design was developed in such a way that loudness ratings (LRs) added to give a reliable and accurate algebraic sum. Send loudness ratings ( S L R s ) , receive ( R L R s ) and overall loudness ratings ( O L R ) were defined in a similar manner to the equivalent RES, but now the formula holds true OLR = SLR + network loss + RLR Themethod achieves greater reliability becauseit uses standardequipment, which although similar in principle to a NOSFER equipment, more closely responds to the frequencies of speech. The equipment is called an intermediate reference system (ZRS). First the IRS is calibrated against NOSFER, then the system under test is calibrated. The difference in the two attenuation values (needed for each system individually to give performance equal to NOSFER) is the loudness rating ( L R ) . Values of loudness rating differ from the reference equivalent by varying amounts, up to 3 dB. Loudness ratings ( L R ) are covered in recommendation P.76. The intermediate reference system (ZRS) used in their measurement is covered in Recommendation P.48, and the method- ology for determining LR is given in recommendation P.65. Following on from the successful introduction of its loudness rating method, CCITT in 1980 upgraded its original method of reference equivalents, applying a correction,fac- tor to them to enable the values tobe added algebraically. A simple mathematical form- ula converts referenceequivalents (RES) into correctedreferenceequivalents (CREs). Corrected reference equivalents are today’s standard method for designing networks against transmission loss. The following relationship applies. OCRE = SCRE + network loss RCRE + Corrected reference equivalent ( C R E ) values typically differ by a variable amount (up to 3dB) from corresponding reference equivalents (RES),but are usually a fixed amount (5 dB) greater than corresponding loudness ratings (LRs). ITU-Tnowadaysrecommendsthatthemaximum overallloudnessrating (OLR) should not exceed 29 dB and has set an objective optimum value of 10 dB. Such targets should be easily achievable with the excellent performance of modern digital networks. In practice the OLR or OCRE may differ slightly even from the planned value, but this will not matter if sufficient safety margin is allowed during planning, andprovided that the differential losses in the two directions of transmission are not too dissimilar. In Figure 33.5, for example, the ORES in the two directions are not quite the same. ITU-T recommends that the difference be limited to 8dB. 33.4 MEASURING NETWORK LOSS That the loss in decibels incurred by a signal traversing a network will depend on the frequency of that signal is a fact that we learned in Chapter 3. With that in mind, how
CORRECTING 599 can we meaningfully quote a value for network loss? The answer is that it is defined as the signal loss incurred by a standard tone of 1020 Hz frequency (formerly 800 Hz and l000Hz tones were also used). It is measured using a standard 1020Hz tone source which is set to generate a known absolute signal power at a refence point known as the transmission reference point ( T R P ) or OdBr point. Reference (dBr) values can then be measured at any other required points in the same transmit path, and compared with the nominal values laid down by the formal network transmission plan. 33.5 CORRECTING SIGNAL STRENGTH The transmissionplan (ITU-Trecommendations G.lO1,G.102,G.103, G . l l l and G.121) lays out a rigid framework for the expected signal strength at all points along aconnection.Any necessary adjustments in strengthare achieved by the use of amplz5ers and variable attenuators (also called pads). Weak signals are most common and are boosted by inserting amplifiers into the connection. The position of the ampli- fiers is critical. If insufficient amplification is used the signal strength becomes too weak, becomes inaudible, and is affected by noise interference. Subsequent amplifica- tion (if attempted) does not correct the situation, because it amplifies the noise as much as the original signal (in other words a given minimum signal-to-noise ( S j N ) ratio needs to be maintained). Ifover-amplification is used it is likely to causethe electrical circuit to saturate (i.e. overload), resulting in signal distortion and electrical instability (manifested as a whistling and loud feedback signal). Furthermore, over-amplification can lead to inter- ference in adjacentcircuits. Unduly strong signals are easily corrected either by reducing the amplification or by the introduction of attenuators. There are standard positions in the circuit, where amplifiers or attenuators may be used. The strengthnecessary is determined by comparing the actual signal strength with the pre-determined value appropriate for that particular reference point. As well as there being a reference point in the customer’s local exchange, reference points can also be defined at other points in the network, at least at each exchange. This allows each individual section of an overall transmission link to be designed and lined-up accord- ingly. The sub-sections then may be joined form to a high quality, end-to-end connection. If instead the circuitis lined up once (as asingle long section) there couldbe a counteracting effect between the performance of the various sub-sectionswhich would mask some of the impairment thatexists. We have, for example, already noted that it is not acceptable for us to allow the signal to fade toa strength equal to thatof the inter- fering noise before we start amplifying it. Hence the need for a segmented approach to transmissionplanning.Figure 33.6 illustratesthetransmissionplan for the North Americantelephonenetwork while it was analogue.(Theillustratedplanhas been superseded by the digital transmission plan, but it is still useful in illustrating the use of such a plan). Each link is marked with its maximum permitted loss. Any new circuit between any two exchanges in the network must be lined up to have an overall loss within the ranges shown on the plan. Thus the maximum end-to-end network loss is assured to be 19.5 dB. The Americans call this end-to-end loss between end offices the via net loss ( V N L ) .
600 TRANSMISSION NETWORK PRACTICAL PLANNING Class 1 Regional centre 0.5-is 0.5-1.5 dB class 3 Primary centre 0.5-1.5 dB Class L Toll centre C l a s s 5 End office (local exchange 1 Figure 33.6 The North American telephone network transmission plan It is best for longer links to be lined-up with loss values towards the upper end of the permitted range. This ensures greater electrical stability. Another important feature of the plan is that greater losses are permitted on the links at the bottom of the network 4 hierarchy (i.e. between Class and Class 5 exchanges). This is because the vast majority of linesare in this category, and considerable savings are possible if amplification can be avoided. Analogue and digital networks both need transmission plans, but they take different forms. In digital networks, the signal strength in decibels only an important consider- is ation in mixed networks at the pointsof analogue-to-digital signal conversion, because the regeneration of digital signals by amplification is quite unnecessary in pure digital networks. The digital transmission plan concerns itself instead with such factors as maximum permissible bit error ratios (BERs) and the extent of the quantization limit, which we shall describe more fully later in the chapter. In the meantime, Figure 33.7 shows the signallosstransmissionplans for the UK analogue and new UK digital networks. Figure 33.7(a) shows the connection of two normal telephones via two-wire analogue lines to their respective local exchanges, and then over four-wire digital transmission across both exchanges and the interconnecting link. At the originating exchange the speech signal is converted from a two-wire analogue form into a four-wire digital form by an analogue/digital conversion device, adjusted to produce a net loss of 1 dB. The signal is then switched through both exchanges in a digital form without further loss
CORRECTING 601 0 dBr -1 dBr - 7 dBr i 1 l v /\ \ / v /\ I I I I I I I I I S I Exchange Exchange 1 l 2 I a I l L /\ / \ & /\ I l - 7 dBr -1 dBr 0 dBr plant Digital ( Analogue) I (Analogue) T 2 to L wireandanaloguetodigital converter X Exchange motrix switch 2-wire line loss ' I I I 0 dBr 'I [ L 5 dB1 -3.5 dBr I I n V I I \/ /I' I - 7 dBr I JI IL5dBl ; 1 I. l L-wire L-wlre exchange exchange I I I I I I I 1.5 dB(Loss ocross 1.5 dB I , I I I I 2-wire exchange) n n I - 7 dB1 -3.5 dBr 0 dBr 2to L wireconverter X Exchange switch matrix Figure 33.7 (a) The UK digitalnetworktransmissionplan. (b) The UKanaloguenetwork transmission plan. untilitreachesthedigital-to-analogueconverter at theotherend. Unlikethe first converter, this one adds a further 6dB loss, giving an overall network performance of 7dB loss between reference points. The ORE can then be calculated by adding the relevant SRE and RRE values. This part of the transmission plan is very similar to the analogue plan (Figure 33.7(b)) which it replaces, except that although the total four- wire section loss is 7dB the recommended signal strengths at the various intermediate reference points in this part of the connection are different. Another difference is that the analogue plan allows a small number two-wire links to be used in the connection. of
602 NETWORK PRACTICAL TRANSMISSION PLANNING 33.6 THECONTROL OF SIDETONE Sidetone is thename given toan effect on two-wiresystems (e.g. basicanalogue telephones) where the speaker hears his own voice in his own earphone while he is speaking (we first introduced it in Chapter 2). Too little sidetone can make speakers think their telephone is dead, but too much leads them to lower their voices. ITU-T recommends sidetone reference equivalents of at least 17 dB. 33.7 THEPROBLEM OF ECHO By ensuring that the ITU-T recommended of 6 to 7 dB at least encountered between loss is the two reference points at either of the four-wire part a connection (be it digital end of or analogue), we not only safeguard the circuit against the of instability; also make effects we it unlikely that any signal echo is generated by the two-to-four-wire converter (the so- called hybridconverter). These echoes are caused by reflection of the speaker’s voice back 33.8 from the distant receiving end, and Figure shows a hybrid converter causing a signal echo of this kind. The problem of echo arises whenever a hybrid converter and itis used, results from the non-ideal electrical performance (i.e. the mismatch) of the device. No matter whether the four-wire line digital or analogue, the resultis an echo. is To understand the cause of the echo we have first to consider the composition the of hybriditself. It consists of bridge of four pairs of a wires, one pair corresponds to the two- wire circuit, two more pairs correspond to receiver and transmit pairs the four-wire the of circuit, and the final pairis a balance circuit, the function of which is explained below. The of wires are connectedto the bridge in such way as to create a separation the receive and a transmit signals on the two-wire circuit from or onto the corresponding receive and transmit pairsof the four-wire circuit. is easiest to understand the type hybrid which It of uses a pair of cross-coupled transformers as bridge. Thisis shown in Figure33.9. a Any signal generated at the telephone in Figure 33.9 appears on the transmit pair (but not on the receive pair), and any incoming signal on the receive pair appears on the telephone (but not on the transmit pair). It works as follows. Electrical signal output from the telephone produces equal magnetic fields around windings W1 and W2. Now the resistance in the balance circuitis set up to be equal to that of the telephone to induce equal fields around windings W3 and W4, but the cross- coupling gives them opposite polarity. The fields of windings W1 and W3 tend to act together and to induce an output winding W.5. Conversely, the fields of windings W2 in Original signal - Received signal I Transmi t pair Figure 33.8 Echo caused byahybridconverter
ONTROL ECHO INSTABILITY AND CIRCUIT 603 Transformer I == = Balance m- 4 Transformer 2 Receive Figure 33.9 A hybridtransformer and W4 cancel one another (due to the cross-coupling of W4), with the result that no output is induced in winding W6. This gives an output on the transmit pair as desired, but not on the receive pair. In the receive direction, the field around winding W6 induces fields in W2 and W4. This produces cancelling fields in windings W1and W3because of the cross-coupling of windings W3 andW4. An output signal is induced in the two-wire telephone circuit but not in the transmit pair, winding W5. Unfortunately, the balance resistance of practical networks is rarely matched to the resistance of the telephone. For one thing, thisis because the tolerance of workmanship in real networks is much greater. In addition, the use of exchanges in the two-wire part of the circuit (if relevant) means that it is impossible to match the balance resistance to the resistance of all the individual telephones to which the hybrid may be connected. The fields in the windings therefore do not always cancel out entirely as intended. So, for example, when receiving a signal via the receive pair and winding W6, the fields pro- duced in windings W1 and W3may not quite cancel, and a small electric current may be induced in winding W5. This manifests itself to the speaker as an echo. The strengthof the echo is usually denoted in terms of its decibel rating relative the incoming signal. to This is a value called balance returnloss, or sometimes, the echo returnloss. The more the efficient the hybrid, the greaterthe balance return loss (the isolation betweenreceive and transmit circuits). 33.8 ECHO CONTROL AND CIRCUITINSTABILITY A variety of problems can be caused by echo, the three most important of which are 0 electrical circuit instability (and possible feedback) 0 talker distraction 0 data corruption
604 TRANSMISSION NETWORK PRACTICAL PLANNING If the returned echo is nearly equal in volume to that of the original signal, and if a rebounding echo effect is taking place at both ends of the connection, then the volume of the signal can increase with each successive echo, leading to distortion and circuit overload. This is circuit instability, and as we already know the chance of it occurring is considerably reduced by adjusting the four-wire circuit to include more signal attenua- tion. The total round-loop loss in theUK digital network shown in Figure 33.7 is at least 14 dB, probably inflicting at least 30 dB attenuation on echoes even if the hybrid has only a modest isolating performance. Talker distraction (or data corruption)is another effect of echo, but if the time delay of the echo is not too long then distraction is unlikely, because all talkers hear their own voices anyway while they are talking. The echo delay timeis equal to the time taken for propagation over the transmission link and back again, and is thus related to the length of the line itself. The longer the line, the greater the delay. Should the one-way signal propagation time exceed around 8 ms, giving an echo delay of 15 ms or more, then corrective action is necessary to eliminate the echo which most telephone usersfind obstrusive. A one-way propagation time of 8 ms is inevitable on all long lines over 2500 km, so that undersea cables of this length and all satellite circuits usually require echo suppression. Further propagation delay can also be caused by certain types of switching and transmission equipment. Indeed a significant problem encountered with digital transmission media is that the time required for intermediate signal regeneration (detection and waveform reshaping) means that the overall speed of propagation is actually reduced to only 0.6 of the speed of light. This means that even quite short digital lines require echo suppression. Two methods of controlling echoes on long distance transmission links are common. These are termed echo suppression and echo cancellation. Echo cancellation is nowadays most common. An echo suppressor is a device inserted into the transmit path of a circuit. It acts to suppressretransmission of incoming receive path signals by insertinga very large attenuation into the transmit path whenever a signal is detected in the receive path. Figure 33.10 illustrates the principle. Actually, the device in Figure 33.10 is calleda halfechosuppressor as itacts to suppress only the transmit path. A full echo suppressor would suppress echoes in both transmit and receive paths. It is normal for a long connection to equipped with two half-echo suppressors, one be at each end, the actual position being specified by the formal transmission plan. Ideally half echo suppressors should be located as near to the source of the echo as possible (i.e. as near to the two-to-four-wireconversion point as possible), and works best when near the endsof the four-wire partof the connection. In practice it may not economic be to provide echo suppressors at all exchanges in the lowerlevels of the hierarchy, and so they are most commonlyprovided on the longlines which terminate at regional (class 1 of Figure 33.6) and internationalexchanges. Sophisticated inter-exchange signalling is used to control the use of half echo sup- pressors. Such signalling ensures that on tandem connections of long-haul links inter- mediate echo suppressors are ‘turned in the manner illustrated Figure 33.1 1. This off by ensures that a maximumof two half echo suppressors (one at each end of the four-wire part of the connection) are active at any one time. The amountof suppression (i.e. attenuation) required to reduce the subjective disturb- ance of echo depends on the number echo paths available, the echopath propagation of
ONTROL ECHO AND CIRCUIT INSTABILITY 605 I-----l Signal I Signal detector 'I I I , , - - g g I Echo ellmlnated ,I . Attenuatcr I ( S w i t c h e d on only signal the when detectordetects I IL----I I a n incoming signal Echo suppressor Figure 33.10 The action of an echo suppressor International Transit International exchange international exchange exchange Intermediate half -echo suppressors turnedoff National network B (no echo suppressors 1 -f- Transmit and receive circuit --t- ES Half -echo suppressor Figure 33.11 Controllingintermediatehalf-echosuppressors
606 PLANNING TRANSMISSION NETWORK PRACTICAL time, and on the tolerance of the telephone users (or data terminal devices). ITU-T recommends that echo suppression should exceed (1 5 + n ) dB where n is the number of links in the connection. Unfortunately echo suppressors cannot used on circuits carrying data, because the be switching time between attennuation-on and attennuation-off states too slow and can is itself cause lossor corruption of data. Most data modems designed for use on telephone circuits are therefore programmed to send an initiating 2100 Hz tone over the circuit,to disable the echo suppressors. Another form of echo control device, called an echo canceller, can be used on either voice or data circuits, and is the most common form of echo control device used in conjunctionwithdigitalcircuits. Like anechosuppressor,anecho canceller hasa signal detector unit in the receive path. However, instead of using it to switch on a large attenuator, itpredictsthe likely echo signal (digitally) and literally subtractsthis prediction from the returning ‘transmit’ signal, thereby largely ‘cancelling’ out the real echo signal. Other signals in the transmit path should be unaffected. Listeners rate the performanceofechocancellersto be betterthanthatofechosuppressors. This, coupled with the fact that they do not corrupt data signals, has made them standard equipment. 33.9 SIGNAL(OR ‘PROPAGATION’) DELAY An important consideration of the network transmission plan is the overall signal delay or propagation time. Excessive delay brings with it not only the risk of echo, but also a number of other impairments. In conversation, for example, long propagation times between talker and listener can lead to confusion. In the course a conversation, when of we have said what we want to say, we expect a fairly prompt response. If we are met with a silent pause, caused by a propagation delay, then we may well be tempted to speak again, to check that we have been heard (‘Are you still there?’). Sure as fate, as soon as we do that, the other party appears to start saying its piece, and everyone is talking at once. On videothe effect of signal delay is even more revealing. For example, on live satellite television broadcasts whoever is at the far end always gives the impression of pausing unduly before answering any question. Nothing can be done to reduce the delay incurred on a physical cable or satellite transmission link. Thus intercontinental telephone conversationsvia satellite are bound to experience a one-way propagation delay of about 1/4 second, giving a minimum pause between talking andresponse of l/2 second. Furthermore, the extremely rapid bit speeds and response times that computer and data circuitry are capable of, can be affected by line lengths of only a few centimetres or metres. Line lengths should therefore be mini- mized and circuitous routings avoided as far as possible. It is common for maximum physical line lengths to be quoted for data networks. Similarly, in telephone networks, rigid guidelines demand that double or treble satellite hops otherexcessive delay paths or 400 (i.e. those of ms one-waypropagation time or longer) are avoided whenever possible. Excessive delays can be kept in check by appropriate network routingalgorithms, as we saw in Chapter 28.
NOISE AND CROSSTALK 607 With the emergence of ATM and other ‘fast packet switching’ technology in some voice telephone networks (e.g. corporate telephone networks) the problem of speech propagation delay has been exacerbated by the time required to fill an ATM cell of 48 bytes (48j8000 = 0.6 ms) and by the delay caused by the exit buffer (sometimes up to 30ms). The exit buffer is intended to eliminate quality problems caused by cell delay variation ( C D V ) as we saw in Chapter 26, but leads to its own problems,not the least of which is the obligation to use echo cancellation devices to control echo. The use of speech compression techniques also leads to increased signal propagation delays (primarily due to the processing time needed to compress the signal, but maybe additionally dueto the increased time now needed fill a frameor ATMcell (at 32 kbit/s to an ATM takes 12 ms to fill)). Multiple occurrencesof compression and decompression cell are therefore to be avoided. This requires careful design of the network topology and planning of relevant echo cancellation. 33.10 NOISE AND CROSSTALK Noise and crosstalk areunwantedsignalsinducedontothetransmissionsystem by adjacent power lines, electrostatic interference, or other telecommunication lines. The only reliableway of controlling themis by careful initial planning design of the trans- and mission system and the route. Onesource of noise results from the induction of signals onto telecommunicationscableswhichpass too close to highpowerlines. Another source of noise is poorly soldered connections or component failures. Both of these are easily avoided. However,by far the most serious source noise in telecommunications of networks and the type most difficult to contain is that caused by electromechanical exchanges themselves. This type noise results from the electrical noise associated with of the electrical pulses needed to activate the exchange, and also from the mechanical ‘chatter’. Noise is minimized by ensuring that the signal strength is never allowed to fade to a volume level comparable with that of the surrounding noise. Thus a minimum signal- to-noise ( S j N ) ) ratio of signal strengths is maintained throughout the connection. This ensures that the real signal is still perceptible amongst all the background. If the signal becomes too weak in comparison with the noise; unfortunately amplificationis then of little value because it boosts signal and noise strength equally. It is difficult to remove noise without affecting the signal itself, but there is some scope for removing noise which lies outside the frequency spectrum the signal itself by simple filtering. This of can slightly improve the signal-to-noise ratio ( S j N ) . The noise caused by atmospheric interference and magnetic or electrostatic induction has a random nature and is heard as low level hum, hiss or crackle. When this type of noise is measured using a special type of filter, weighted to reflect the human audible range, then the noise strength can quoted in psophometric units. ITU-T recommends be that the strengthof psophometric noise should not exceed an electromotive force (e.m.f. ) at the receiving end of 1 millivolt (1 mV). Depending on the length the connection, the of transmission network designer may decide the maximum allowable noise disturbance per kilometre of line. Typically this value is around 2-3 picowatts (a very small unit of power) per kilometre. This might be written 2 pWOp/km, where pW forpicowatts, stands
608 TRANSMISSION NETWORK PRACTICAL PLANNING 0 indicates that thevalue is measured relative to the0 dBr reference point, and p denotes psophometric noise. Crosstalk is the interference caused by induction of telecommunications signals from adjacent transmission lines (see Chapter 3). The presence of crosstalk is usually an indication that the circuit has a fault which is causing it to perform outside its design range. Perhaps an amplifier is turned up too much or some other fault has caused an abnormally high volume signal in the adjacent transmission line, or perhaps there is a short circuit or an insulationbreakdowncaused by damp.The transmissionplan normally seeks to minimize crosstalk by ensuring that the transmit and receive signal levels at any point along the connection never differ by more than 20dB. 33.11 SIGNAL DISTORTION In Chapter 3 we discussed the use of equalizers to counteract the attenuation distortion of signals which is caused by differential attenuation of the component frequencies. Equalizers are generally located in transmissioncentres,alongside amplifiers. The equalizers which correct frequency distortion (also called attenuation distortion) make sure that the attenuation of component frequencies is less than a specified maximum. ITU-T recommendations for speech channels suggest a maximum differential attenua- tion of 9 dB, using appropriate equalization to ensure that the frequency response of the circuit is within the musk illustrated in Figure 33.12. However, frequency distortion equalizers are not the only type of equalizer. Another type is called a group delay equalizer. This acts by removing the distorting effects of group delay, which is an effect of signal phase distortion of the received signal caused by Attenuatlon - g 0.7 0- 7- 6- 5 - L.3 L- 3- 2.2 2- 1- (after equalization 1 0 300 LOO 600 g00 2LOO 3000 3400 -2.2 1//,////////////,///,//,/ Frequency IHz) Shaded region is the unallowed region, or ‘mask’. The frequency response line must wlthin mask lie the Figure 33.12 Frequency distortion: CCITT’s G132 ‘Mask’
DIGITAL PLAN TRANSMISSION FOR AND ‘DATA’ NETWORKS 609 slightly different propagation times of the component frequencies within the signal. Group delay is particularly harmful to data signals passing over FDM (frequency divi- sion multiplex) or radio carriers, and it can be correctedby a device which adds delay to those frequency components which are received first, to even up the delay incurred by all the frequency components. Both normal frequency attenuation equalizers and group delay equalizers are cali- brated when the circuit is initially lined up. A range of different frequencies of known phase and signal strength is sent along the transmission line, and the characteristics of the received signals are carefully measured. The equalizers are then adjusted accord- ingly and placed in the circuit. A quick re-test should reveal perfect circuit equalization. 33.12 TRANSMISSION PLAN FORDIGITAL AND ‘DATA’ NETWORKS Transmission planning of a digital network, as compared with its analogue equivalent, is relatively straightforward primarily because the technique of regeneration practically eliminates the problems of crosstalk, noise, attenuation and interference. However, a number of new factors need to be taken into account in a digital transmission plan; they are e the digital line bit error rate (or bit error ratio, B E R ) e thesynchronization of the network e the quantizationdistortion e (duringtheperiod of networktransition)thenumber of analogue-to-digital and digital-to-analogue conversions Let us take them in order. Regeneration of the digital bit pattern (asdescribed in Chapter 5) must be undertaken so frequently along the length of the lineensure thatmarks (1 S) and spaces (Os) are not to confused by the receiver. In addition, although less prone than analogue lines, it is still prudent to protect digital line far as a as possible from noise interference and crosstalk to prevent spurious bit errors. The frequencysuch error is measured in terms of the error of rate (the bit error rate or bit error ratio, BER). Typical acceptance values of BER range from around 1 bit error in 105 bits (BER = 10-5) to one error in 109bits, depending on the application. Some types modern transmission system (including fibre transmission of systems and modern digital radio systems) operate at very low bit error rates (BER = 10-l2). The error rate is usually checked when the digital line system is first established. All channels in the sameline system (e.g. each 64 kbit/s channel of a 2 Mbit/s line system) will experience the same bit error rate ( B E R ) . The synchronization of digital networks ensures that there is no build-up or loss of information in the line system. Without synchronization,if a transmitter sent data faster than the receiver was ready to receive it, theresult would be a build-up,and ultimately loss of information. Conversely, the receiver was expecting data to if arrive at a rate faster than the transmitter could send them, then imaginary ‘fill-in’ data would need to be created for
610 PLANNING TRANSMISSION NETWORK PRACTICAL the missing bits. Synchronization of digital networks is usually carried out in a hier- archical manner, with one exchange designated house themuster clock, providing syn- to chronization for otherexchanges. Figure 33.13 illustrates a typical three-tier synchroniza- of tion plan. At the top the hierarchy, asingle exchange has a master clock. In thesecond tier a number of main exchanges receive synchronization (i.e. a data transmitting and receiving rate) from the master clock exchange; additionally are locked together by they two-way synchronization links,keeping them all rigidly in step. Finally, at the bottom of the hierarchy the smaller exchanges merely receive synchronization sources from the higher level exchanges, but do not have two-way synchronization links. Any digital clocks which are located any of the exchanges of the network can be adjusted to run faster or slower to keep trackwith the synchronization clock. In case the main fails it is usual clock for one of the second tier exchanges to act asa standby master clock, ready take over to should the exchange with the master clock go off-air. Layer l rI ' Exchange with master clock' Layer 2 Layer 3 >PL m One-way synchronization Two-way (ink synchronization llnk --g U N e t w o r k node or exchange Figure 33.13 Hierarchicalsynchronizationplan
TRANSMISSION PLAN FOR DIGITAL AND ‘DATA’ NETWORKS 611 The thirdimportant element of a digital transmissionplan is the control of quantization distortion (also called quantizing distortion). As we learned in Chapter 5, quantization distortion arises because thequantum amplitude values which are available to representthe signal always differ slightly fromtheactual value. The relationship between the representative digital signal and the original is therefore less than perfect. In turn, there will be differences between the final signal and the original, manifested as slight (maybe even imperceptible) distortion. This type of quantization distortion occurs during the initial conversion of any analogue (e.g. speech) signal into its digital equivalent and cannot be recovered on re-conversion. Further quantization distortion can occur should certain other transmission equipments be installed on the digital transmission path. The use of such equipments may be unavoidable, but the transmissionplanshould clearly set out howmuchextradistortion is permissible. Either the equipments need to be designed to conform with these limits, or the quality of transmission will suffer. Examples are echo cancellers (discussed earlier in this chapter) and circuit multiplication devices (discussed in Chapter 38). Quantizationdistortion is measuredin quantizationdistortionunits(q.d.u.s). One q.d.u. is equivalent to a difference inamplitude between thedigitalsignal and the original analogue signal equalto onedigital quantum level (for explanation of quantum levels, return to Chapter 5). Thus if the final signal is reproduced with amplitudes varyingfrom that of theoriginalsignal by a whole quantum amplitude step,then 1 q.d.u. of distortion has been encountered. A digital transmission plan needs to lay out strict limits on the maximum quantization distortion that can be allowed in any link or part of thenetwork.CCITT 1984 recommendations (still valid today) suggested a maximum end-to-end quantisation distortion of 14 q.d.u.s, allocating 5 q.d.u.s for each nationalnetworkand a 4q.d.u. limit fortheinternationalconnection in between. Typical values of quantization distortion are given in the table of Table 33.1. They reflect the planning values given in ITU-T recommendation G. 1 13. From the table Table 33.1 it is clear that quantization distortion occurs as a result of of any form of signal processing, and is particularly sensitive to analogue/digitalcon- version processes and speech compression using either 32 kbit/s ADPCM or 16 kbit/s CELP algortihms (as we discuss more in Chapter 38). For this reason, connections of interleavedanalogue and digitalsections,as well asmultiple speech compression/ decompression stages should be avoided as far as possible. ITU-T states this require- ment by recommending the limitation the numberof unintegrated PCM of digital sections 7. to 3 or 4 and no more than Of course, as the network evolves and digital transmission becomes more widespread this problem will disappear. Table 33.1 Typical quantization distortion values Quantization Digital process distortion units A/D conversion by 8-bit PCM 1 A/D conversionPCM 7-bit by 3 A-law to Mu-law conversion 0.5 Digital attenuator 0.7 ADPCM (see Chapter 20) 3.5
612 PRACTICAL NETWORK TRANSMISSION PLANNING Digital data signals should not be recoded into an analogue form using a normal analogue/digital speech conversion device. On the other hand it is permissible to pass analogue encoded data over PCM lineplant, provided that the number of analogue/ digital conversions is limited as stated above. 33.13 INTERNATIONAL TRANSMISSION PLAN As always, ITU-T has some advice for international network transmission planning. This advice is to be found in its G-series recommendations. The principles described therein are exactly as discussed in this .chapter, although it is briefly worth explaining ITU-T’s concept of a virtual switching point V S P ) . This is a hypothetical reference ( point, like our other reference points. In particular it is the point at which a national network is assumed to be connected to the international network. The VSP concept allows ITU-T to lay out recommendations ensuring an acceptable transmission per- formance of the individual parts (national and international) of a connection. By so doing, the overall acceptability of the end-to-end network is assured without unneces- sary strain on the internal plans adopted any of the individual sub-sections. This is clearly important for international interconnection of networks, making it possible for dissimilarnetworks to worktogether.Figure33.14illustratestheconcept,andthe national and international network sub-components. Individual recommendations in ITU-T G-series may apply to one or more of the network sub-sections. By convention, the transmit VSP resides at the -3.5dBr point in analogue networks, and a nominal 0.5dB loss is allocated for each international transmission link. The signal reference value at a receiving international gateway in a direct link connection is thus 4.0 dBr. In Figure 33.14, however, the international connection comprises two links, so the receive level should therefore be set up to -4.5 dBr as shown. 2-wlre -1- 4- wire P T 4 I - 3 . 5 dBr - 4 . 5 dBr I I U v V V I n A VSP VSP I I v I l \/ n I I I I VSP VSP v \I V \I n A A A National network I 1 -,- International I network -L- j Natlonal network k T 1- 4 X - exchange VSP- virtual switching points Figure 33.14 National and international networks and VSPs

CÓ THỂ BẠN MUỐN DOWNLOAD