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/.G0G039-%42)#G0G00!)2G0G0#!“,%3)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK
2、)NOTES1 ITU-T Recommendation G.952 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 cop
3、yright 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 u
4、tilized 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.952 1Recommendation G.952DIGITAL LINE SYSTEMS BASED ON THE 2048 kbit/sHIERARCHY ON SYMMETRIC PAIR CABLES(Malaga-Torremolinos, 1984; a
5、mended at Melbourne, 1988)1 GeneralThis Recommendation covers digital line systems for the transmission of signals based on the 2048 kbit/shierarchy on symmetric pair cables and includes systems operating at the following bit rates:2 048 kbit/s8 448 kbit/s34 368 kbit/sThe requirement for overall per
6、formance and interfaces of the corresponding digital line sections are given inRecommendation G.921.2 Transmission mediumThe system can be operated on symmetrical pair cables of various wire diameters and cable constructionsincluding those given in Recommendations G.611, 612 and 613.Note - 34 368 kb
7、it/s systems should be operated on high performance cables and may require one cable for eachdirection of transmission.3 Protection against interference from external sourcesThe digital line system can be disturbed by interference from telephone circuits carried within the same cableand by a switch
8、when repeaters are installed in switching centres. Examples of possible ways of reducing the effect ofthis type of interference are the reduction of repeater section length near switching centres, segregation of pairs, the useof particular line codes, etc.4 Overall design features4.1 AvailabilityThe
9、 availability objective of the system should be derived taking into account the availability requirement forthe hypothetical reference digital section as given in draft Recommendation G.801.4.2 ReliabilityMTBF values should be specified for the line system as a whole taking into account the requirem
10、entsconcerning availability.4.3 Repeater crosstalk-noise figuresRepeater crosstalk-noise figures are defined in Annex A, together with suggested measurement techniques.Crosstalk-noise figures quantify the performance of digital regenerators which are subject to crosstalk interference. Theyare functi
11、ons of BER, line system line code, cable characteristics, environmental conditions, and repeater spacing loss A0(at half the line system baud rate).2 Fascicle III.5 - Rec. G.952At a BER = 10-xand over a loss range Al A0 A2, crosstalk-noise figures should meet the followingspecifications:a) NEXT-Nois
12、e Figure RN CA0+ D*b) FEXT-Noise Figure RF E*.* It has not been possible to recommend specific values for parameters x, A1, A2, C, D, and E.Examples of the values used by some Administrations for 2 Mbit/s systems are given below:Example xA1A2CDETest methodiiiiii67751074040381.11.01.014.7191817.5-abb
13、Note 1 - In example ii, a filter with a centre frequency of 1020 kHz and a bandwidth of 3.1 kHz is employed.Note 2 - The values do not include any allowance for the effects of jitter.4.4 Error performanceThe design objective for the error ratio of the individual repeater should take into account the
14、 networkperformance objectives given in Recommendation G.821.5 Specific design features5.1 Type of power feedingAlthough CCITT does not recommend the use of a specific remote power-feeding system for this symmetricalline system, in practice only the constant current d.c. feeding via the phantom circ
15、uits of the two symmetrical pairs of asystem is used.This symmetrical cable system may be subject to induced voltages and currents caused by lightning, powerlines, railways, etc.Precautions must be taken to protect the staff from any possible danger arising from the normal operatingvoltages and remo
16、te 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 addition the CCITT Directives 1 give guidance on thes
17、eproblems.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 2.5.2 Repeater spacing and cable fillA specific repeater spacing cannot b
18、e recommended, but general considerations concerning system planning arecontained in Annex B to this Recommendation.Fascicle III.5 - Rec. G.952 35.3 Maintenance strategy5.3.1 Type of supervision and fault locationIn-service monitoring or out-of-service fault location can be used.5.3.2 Fault conditio
19、ns and consequent actionsThe following fault conditions should be detected in addition to those specified 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
20、, if practicable.b) low error ratio threshold exceeded -this threshold is 1 . 10-5for systems at 2048 and 8448 kbit/sand 1 . 10-6for systems at higher bit rates;a deferred maintenance alarm should be generated to signify that performance is deteriorating.ANNEX A(to Recommendation G.952)Definition an
21、d measurement of repeaters crosstalk-noise figuresA.1 Definitiona) NEXT-Noise Figure RNIN= mean square near-end crosstalk (NEXT) voltage produced by a single interfering regeneratorthat would appear at the decision point if the NEXT loss were 0 dB at half the line system baudrate.N0= mean square NEX
22、T interference voltage at decision point which procedures specified BER, anddepends on parameters which affect the decision process and reflects impairments arising fromintersymbol interference and offsets from the optimum position of the decision threshold levelsand sampling instants at the regener
23、ator decision point.E( f ) = regenerator equalizer frequency transfer function.P( f ) = power spectral density (single sided) of line system line code.f0= half line system baud rate.and quantities in square brackets are in dB, i.e.X = 10 log10| X |.4 Fascicle III.5 - Rec. G.952b) FEXT-Noise Figure R
24、FIF= mean square far-end crosstalk (FEXT) voltage produced by a single interfering regenerator thatwould appear at the decision point if the FEXT loss were 0 dB at half the line system baud rate.N0= mean square FEXT interference voltage at decision point which produces specified BER, anddepends on p
25、arameters which affect the decision process and reflects impairments arising fromintersymbol interference and offsets from the optimum position of the decision threshold levelsand sampling instants at the regenerator decision points.E( f ), P( f ), f0as in a), andG( f ) = frequency transfer function
26、 of cable.A.2 MeasurementMethod a) directly relates to the definition of crosstalk-noise figure and is therefore the reference measuringmethod. Methods b) and c) are the possible practical alternatives. Method c) avoids the use of a selective filter.Method a)The NEXT-Noise Figure and FEXT-Noise Figu
27、re can be measured using the configuration shown inFigure A-1/G.952, with the Function Switch in the N and F position, respectively. The measurement consists of equatingthe r.m.s. voltages at A and A1, setting the artificial line to the desired loss A0, and then adjusting the variable attenuatorunti
28、l the desired BER = 10-xis achieved. The value of the attenuator, RdB, is then the NEXT-Noise Figure or FEXT-Noise Figure for the desired A0and BER.Fascicle III.5 - Rec. G.952 5Method b)The NEXT-Noise Figure RN can be measured using “input S/N ratio“ test sets by employing the test set in a“manual m
29、ode“ and performing external measurements with a selective filter, see Figure A-2/G.952. The measurementconsists of:i) set artificial line to 0 dB and using selective measure test signal power S0 dBm.ii) Set artificial line to desired loss A0, adjust variable attenuator until desired BER = 10-xis ob
30、tained, switchoff test signal, and using selective filter, measure noise power P dBm.iii) Then RN = S0 - P for desired A0and BER.Note - The degrading effect of clock jitter on NEXT-Noise Figure and FEXT-Noise Figure should be measuredby superimposing appropriate jitter on the test signal.Method c)Th
31、e NEXT-Noise Figure RN can be measured using “input S/N ratio“ test sets in “manual mode“ with theinsertion of an additional variable attenuator between the test signal and the artificial line, as shown in Figure A-3/G.952.The measurement procedure is as follows:i) set the artificial line to 0 dB lo
32、ss and the additional variable attenuator to A dB loss;ii) regulate the variable gain amplifier until the power level of the variable attenuator input is equal toQ1 - A dB, the power level of the artificial line output;iii) set the artificial line to A dB loss and the additional variable attenuator
33、to 0 dB loss;iv) adjust the variable attenuator until the desired BER = 10-xis reached. The attenuation value of theattenuator is N dB;v) calculate RN = N + A - WNin which PR(f) = spectral power density (single sided) of line code.6 Fascicle III.5 - Rec. G.952In would be better to obtain WNby measur
34、ement. Of course, the value of WNcan also be calculated accordingto PR(f) of AMI or HDB3in a certain frequency range, for example, WN= -3.59 dB in the range from 0 to 10 240 kHz.ANNEX B(to Recommendation G.952)Guidance notes for the satisfactory achievement of theerror performance objectivesB.1 To c
35、omply with the Network Performance Objectives (NPO) it is necessary to take into account manyinterrelated factors. Figure B-1/G.952 illustrates diagrammatically the interrelationship between all the factors thatimpact on this matter. The basis upon which digital line system installation planning gui
36、delines are formulated isdependent on the circumstances of each Administration. For example, some Administrations may have cables withfavourable characteristics, whilst at the same time the network may experience serious levels of unquantifiableinterference (network effects). An Administration must,
37、 therefore, make a judgment as to the significance of each effectin their network and formulate cable utilization guidelines which satisfy the digital line section error performancerequirements.Fascicle III.5 - Rec. G.952 7The following notes highlight a number of important considerations concerning
38、 the formulation of systeminstallation planning guidelines.Note 1 - In the process of establishing cable utilization guidelines the crosstalk-noise figure is the onlyparameter describing the intrinsic quality of the regenerator under crosstalk interference conditions. This parameter,which is based o
39、n the average power spectral density of the total crosstalk interference, provides a useful approximationto the systems immunity to crosstalk from plesiochronous data streams, and is the correct measure for synchronous datastreams provided the phases of the disturbing systems are randomized. It is a
40、lso based on an assumption of random dataon the disturbing systems and therefore cannot be applied to the case of repetitive data patterns. However the use ofscramblers effectively makes almost all data patterns appear to be random 3.Note 2 - In an operational environment, regenerators may be subjec
41、t to other sources of interference which aredifficult to quantify and which may induce errors. In some instances specific interference mechanisms have beenquantified and appropriate limits and testing procedures are reflected in national specifications. These aspects arecurrently under study within
42、CCITT and as operational experience is gained it might be possible to introduce further teststhat accommodate these other interference mechanisms.Note 3 - Maximum cable utilization should be based on complying with the network performance objective. Tosatisfy this objective Administrations may adopt
43、 one of the following approaches:i) In circumstances where Administrations are able to judge the significance of the “network effects“ cablefill calculations should be based on an objective determined by discounting “network effects“ from thenetwork performance objective.ii) In circumstances where A
44、dministrations are not able to judge the significance of the network effects, cablefill calculations should be based on the equipment design objective.8 Fascicle III.5 - Rec. G.952Note 4 - The use of a reduced line symbol rate code provides a more favourable crosstalk environment, and thisfeature wi
45、ll impact on cable fill calculations.Note 5 - When changing from a plesiochronous to a synchronous network operation, some cable crosstalkcouplings and relative phasings of the system clocks lead to increases in system margins whilst others lead to reducedsystem margins by up to a maximum of 3 dB fo
46、r practical systems. It is believed that there are more cases with increasedmargin than reduced margin and that there is therefore no need to introduce any extra margin when changing fromplesiochronous to synchronous operations 3.Scramblers may be used to ensure that the interference from several id
47、entical repetitive sequences does notexceed the levels occurring with random data.References1 CCITT Manual Directives concerning the protection of telecommunication lines against harmful effects fromelectric power and electrified railway lines, ITU, Geneva, 1988.2 CCITT Recommendation Tests on power
48、-fed repeaters using solid state devices in order to check thearrangements for protection from external interference, Vol. IX, Rec. K.17.3 SMITH, B.M. and POTTER, P. G. June 1986 - Design Criteria for Crosstalk Interference between DigitalSignals in Multipair Cable, IEEE Trans. Commun., Vol. COM-34, No. 6.
