1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$)4!,G0G03%#4)/.3G0G0!.$G0G0$)4!,G0G0,).%G0G03934%-3$)4!,G0G0,).%G0G03934%-3G0G0“!3%$/.G0G04(%G0G0 G0+“)4 3G0G0()%2!2#(9/.G0G0#/!8)!,G0G00!)2G0G0#!“,%3)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)
2、NOTES1 ITU-T Recommendation G.954 was published in Fascicle III.5 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copy
3、right conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or ut
4、ilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.5 - Rec. G.954 1Recommendation G.954DIGITAL LINE SYSTEMS BASED ON THE 2048 kbit/sHIERARCHY ON COAXIAL PAIR CABLES(Malaga-Torremolinos, 1984; amen
5、ded at Melbourne, 1988)1 GeneralThis Recommendation covers digital line systems for the transmission of signals based on the 2048 kbit/shierarchy on coaxial pair cables and includes systems conveying the following bit rates:8 448 kbit/s34 368 kbit/s139 264 kbit/s4 139 264 kbit/sIn the case of 4 x 13
6、9264 kbit/s systems, a digital line muldex equipment combines the functions ofmultiplexing four digital signals at 139 264 kbit/s and of a line transmission equipment. Details of the digitalmultiplexing strategy are given in Annex B to this Recommendation.The requirements for overall performance and
7、 interfaces of the corresponding digital line section are given inRecommendation G.921.2 Transmission mediaThe systems can be operated on coaxial pairs, as defined in the series G.620 Recommendations, in accordancewith Table 1/G.954.TABLE 1/G.954Transmission mediaSystem (kbit/s) Cable Recommendation
8、8 448 G.621; G.62234 368 G.621; G.622; G.623139 264 G.622; G.6234 139 264 G.6233 Overall design features3.1 AvailabilityThe availability objective of the system should be derived taking into account the availability requirement forthe hypothetical reference digital section given in Recommendation G.
9、801.2 Fascicle III.5 - Rec. G.9543.2 ReliabilityMTBF values should be specified for the line system as a whole taking into account the requirementsconcerning availability.3.3 Repeater noise marginRepeater Noise Margin is defined in Annex A together with suggested measurement techniques. The NoiseMar
10、gin quantifies the performance of digital regenerators for coaxial pairs. This is a function of BER and repeaterspacing loss A0(at half the line system baud rate).At a BER = 10-7and over the loss range of the system A1 A0 A2, the Noise Margin should meet thefollowing specifications:Noise Margin (M)
11、B + C (A2- A0)It has not been possible to recommend specific values for parameters A1, A2, B and C.Note - The degrading effect of timing jitter on Noise Margin should be measured by superimposing appropriatejitter on the test signal.Examples of the values used by some Administrations are given below
12、:A1(dB) A2(dB) B (dB) C8 448 kbit/s systems 35 85 9 134 368 kbit/s systems 3456458482757.56120.70.51139 264 kbit/s systems 656084845.57.50.70.7 1Note - The values do not include any allowance for the effects of jitter.3.4 Error performanceThe design objective for the error ratio of the individual re
13、peater should take into account the networkperformance objectives given in Recommendation G.821.4 Specific design features4.1 Type of power feedingAlthough CCITT does not recommend the use of a specific remote power-feeding system for these coaxial linesystems, in practice only the constant current
14、d.c. feeding via the inner conductors of the two coaxial pairs of system isused.These coaxial cable systems may be subject to induced voltages and currents caused by lightning, power lines,railways, etc.Fascicle III.5 - Rec. G.954 3Precautions must be taken to protect the staff from any possible dan
15、ger arising from the normal operatingvoltages and remote power-feed currents as well as from the induced voltages and currents.Many national Administrations have issued detailed rules and regulations for the protection of persons. It isobligatory in most cases to meet these rules and regulations. In
16、 addition the CCITT Directives 1 give guidance on theseproblems.Precautions are also needed for the protection of the equipment against induced voltages and currents. Theequipment should therefore be designed in such a way that it passes the tests specified in Recommendation K.17 2.4.2 Nominal repea
17、ter spacingA specific repeater spacing is not recommended but in practice the nominal values indicated in Table 2 are usedby most Administrations:TABLE 2/G.954Nominal repeater spacingsNominal repeater spacing (km)System (kbit/s)Cable Recommendation a)G.621 G.622 G.6238 448 4.0 34 368 2.0 4.0(Note)13
18、9 264 2.04.5(Note)4 139 264 1.5a) G.621 refers to 0.7/2.9 mm coaxial pairs.G.622 refers to 1.2/4.4 mm coaxial pairs.G.623 refers to 2.6/9.5 mm coaxial pairs.Note - One Administration employs a nominal repeater spacing of 3 km.4.3 Maintenance strategy4.3.1 Type of supervision and fault locationIn-ser
19、vice monitoring or out-of-service fault location can be used. For bit rates equal to or above 139 264 kbit/sin-service monitoring is recommended.4 Fascicle III.5 - Rec. G.9544.3.2 Fault conditions and consequent actionsThe following fault conditions should be detected in addition to those specified
20、in Recommendation G.921 forthe relevant digital sections, and the associated consequent actions should be taken:a) failure of remote power feeding -a prompt maintenance alarm should be generated, if practicable;b) low error ratio threshold exceeded -this threshold is 110-5for systems at 8448 kbit/sa
21、nd 110-6for systems at higher bit rates;a deferred maintenance alarm should be generated to signify that performance is deteriorating.ANNEX A(to Recommendation G.954)Definition and measurement of repeater noise marginA.1 DefinitionThe noise margin mn:mn= SNR/SNRER(A-1)whereSNR = SNRth F(t, ER) (A-2)
22、The product SNRth F(t, ER) can be considered the actual signal-to-noise ratio SNR being the measure for theregenerator performance.SNRthis the theoretical signal-to-noise ratio determined by the system parameters such as output pulse,section loss, noise figure of the regenerator input amplifier etc.
23、F(t, ER) is the reduction factor due to an off-set from the optimum timing instant (including phase jitter) inconjunction with the pulse realized S(t), the intersymbol interference I(t) and any other disturbancewhich causes a corruption in the information signal (Ic).Note - The intersymbol interfere
24、nce and other disturbances are fluctuating processes with bounded distributions.The “mean“ reduction factor depends on ER, and, for a ternary signal, is given by:where S(0) is the realized pulse at t = 0 giving the maximum amplitude.SNRERis the signal-to-noise ratio required for an error ratio to ER
25、. For a ternary signal the relation between ERand SNRERis given by the known Gaussian distribution:ERSNRER= 43= 432(A - 4)PE e dxxpi12/A.2 Derived definitionsThe noise margin can be measured by applying an external disturbing signal. For that purpose more practicaldefinitions are derived.Fascicle II
26、I.5 - Rec. G.954 5A.2.1 SNRER(giving an error ratio ER ) can be achieved by injecting sufficient white noise into the input of theregenerator:whereNT= thermal noise that appears at the decision point during normal operation.NE= mean power of the external noise that appears at the decision point to i
27、nduce an error rate ER.Combining (A-2) and (A-5) results in the noise margin M:N0= power density of the external noise that is superimposed on the signalE(f) = transfer function of the regenerators equalizerk, T = Boltzmann constant and absolute temperatureF(f) = noise figure of the equalizer amplif
28、ier of the regeneratorA.2.2 By injecting a sine wave disturbing signal, a second definition for mncan be derived.This disturbance causes a decreasing F(t, ER), which can be defined by:6 Fascicle III.5 - Rec. G.954Sd= the magnitude of the disturbing signal at the input of the regeneratorfd= the frequ
29、ency of the disturbing signalac= a correction factor taking into account the effect of the disturbance on the peak detector of theautomatic equalizerR0= the real part of the characteristic impedance of the cable.A.3 MeasurementsMethod A is based on the definition directly related to the noise margin
30、 (A-6) and therefore, is the reference testmethod. Methods B and C are alternative test methods.Method A (Figure A-l/G.954)The values of NEand NTare measured directly at the decision point. The value of NTis measured in the absenceof both a signal and an externally applied noise. Under these conditi
31、ons the automatic gain control (AGC) of theequalizer must be externally controlled to a level appropriate to the corresponding cable attenuation. With the signalrestored, the level of the externally applied noise is adjusted to give the desired BER. The noise level (NT+ NE) is nowmeasured with the s
32、ignal removed and with the AGC set at the same value as in the measurement of NT.Method B (Figure A-2/G.954)This method realizes a measurement without the need to access the decision point. The applied noise at theinput, to cause a given BER, is measured directly. The corresponding value at the deci
33、sion point and also the thermalnoise (NT) are evaluated by means of the transfer function and the noise figure of the amplifier equalizer.Note - Both the transfer function and the noise figure of the amplifier equalizer need to be calculated andmeasured on a sample of repeaters before this method ca
34、n be applied to a particular repeater design.Method C (Figure A-2/G.954)This method is similar to the previous method (B) except that in this case the applied disturbance is a sine wavesignal. This applied signal at the input, to cause a given error ratio, is likewise measured directly.The correspon
35、ding disturbance at the decision point (Is) as well as the thermal noise voltage areevaluated by means of the transfer function, the noise figure of the equalizer and the correction factor ac, which have tobe determined.Fascicle III.5 - Rec. G.954 7Note 1 - It follows from (A-8) and (A-9):being an u
36、nknown factor, which has to be determined on the basis of measurements on a sample of prototyperegenerators before this method can be applied to a particular regenerator design.For this purpose, the noise margin of the prototype regenerators needs to be measured in accordance with the referencetest
37、method (A).Note 2 - This method allows the presence of an LBO-network at the regenerator input. In contrast to method B itis not necessary to insert a complementary filter in the injection path.Note 3 - To obtain the most accurate measurement the disturbing frequency should be around the Nyquistfreq
38、uency.8 Fascicle III.5 - Rec. G.954ANNEX B(to Recommendation G.954)Digital multiplexing strategy for4 139 264 kbit/s systemsB.1 GeneralThe digital multiplexing strategy is based on the use of positive justification and combines four 139 264 kbit/stributaries into one composite signal.B.2 Bit rateThe
39、 nominal bit rate should be 564 992 kbit/s. The tolerance on that rate should be 15 parts per million(15 ppm).B.3 Frame structureTable B-l/G.954 gives:- the tributary bit rate and the number of tributaries,- the number of bits per frame,- the bit numbering scheme,- the bit assignment,- the bunched f
40、rame alignment signal.Note - Possible alternative frame structures with the characteristics indicated in Appendix II are left for furtherstudy.B.4 Loss and recovery of frame alignmentLoss of frame alignment should be assumed to have taken place when four consecutive frame alignment signalshave been
41、incorrectly received in their predicted positions.Fascicle III.5 - Rec. G.954 9When frame alignment is assumed to be lost, the frame alignment device should decide that such alignment haseffectively been recovered when it detects the presence of three consecutive frame alignment signals.The frame al
42、ignment device, having detected the appearance of a single correct frame alignment signal, shouldbegin a new search for the frame alignment signal when it detects the absence of the frame alignment signal in one of thetwo following frames.Note - As it is not strictly necessary to specify the detaile
43、d frame alignment strategy, any suitable framealignment strategy may be used provided the performance achieved is at least as efficient in all respects as that obtainedby the above frame alignment strategy.TABLE B-1/G.954564 992 kbit/s multiplexing frame structureTributary bit rate (kbit/s) 139 264N
44、umber of tributaries 4Frame structure Bit numberFrame alignment signal (binary content under study)bits from tributariesSet I1 to 1213 to 384Justification service bits Cjn(n = 1 to 5) (see Note)Bits from tributariesSets II to VI1 to 45 to 384Remote alarm indication, spare for national useBits from t
45、ributaries available for justificationBits from tributariesSet VII1 to 45 to 89 to 384Frame lengthBits per tributaryMaximum justification rate per tributaryNominal justification ratio2688 bits663 bits210 190 bit/s0.4390Note - Cjnindicates the nthjustification service bit of the jthtributary.B.5 Mult
46、iplexing methodCyclic bit interleaving in the tributary numbering order and positive justification is recommended. Thejustification control signal should be distributed and use the Cjnbits (n = 1, 2, 3, 4, 5), see Table B-1/G.954. Positivejustification should be indicated by the signal 11111, no jus
47、tification by the signal 00000. Majority decision isrecommended.Table B-1/G.954 gives the maximum justification rate per tributary and the nominal justification ratio.B.6 JitterB.6.1 Jitter transfer characteristics (under study).B.6.2 Tributary output jitter (under study).10 Fascicle III.5 - Rec. G.
48、954B.7 Service digitsThe first four bits in Set VII of the pulse frame are available for service functions. The first of these bits is usedto indicate a prompt alarm condition, see Table C-1/G.954.Note - A possible solution for scrambler and frame alignment signal is given in Appendix I.APPENDIX I(t
49、o Annex B of Recommendation G.954)A possible solution for scrambler and framealignment signals for a digital line systemat 4 139 264 kbit/sI.1 Reset scramblerIt is proposed to use a “reset scrambler“, i.e. one which is reset at the start of each frame. Advantages of such ascrambler 3 as compared to a free-running or “self-synchronizing“ scrambler, are:- no error multiplication, and- no necessity to provide additional measures to avoid periodic output signals.If it is accepted that with an all 1 or all 0 input signal (e.g. with AIS on all fo
