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.951 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.951 1Recommendation G.951DIGITAL LINE SYSTEMS BASED ON THE 1544 KBIT/S HIERARCHYON 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 1544 kbit/shierarchy on symmetric pair cables and includes systems operating at the following bit rates:1544 kbit/s,3152 kbit/s,6312 kbit/s.2 Transmission mediumThe system
6、can be operated on symmetrical pair cables of various wire diameters and cable constructionsincluding those given in Recommendations G.611, G.612, and G.613.3 Protection against interference from external sourcesThe digital line system can be disturbed by interference from telephone circuits carried
7、 within the same cableand by a switch when repeaters are installed in switching centres. Examples of possible ways of reducing the effect ofthis type of interference is the reduction of repeater section length near switching centres, segregation of pairs, the use ofparticular line codes, etc.4 Overa
8、ll design features4.1 AvailabilityThe 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 w
9、hole taking into account the requirementsconcerning 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 c
10、rosstalk interference. Theyare functions of BER, line system line code, cable characteristics, environmental conditions, and repeater spacing loss A0(at half the line system baud rate).At a BER = 10-xand over a loss range Al A0 A2, crosstalk-noise figures should meet the followingspecifications:2 Fa
11、scicle III.5 - Rec. G.951a) NEXT-Noise 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.4.4 Error PerformanceThe design objective for the error ratio of the individual repeater should take into account the networkp
12、erformance 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 circuits of t
13、he 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 remote power-
14、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 theseproblems
15、.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 fillThe specific repeater spacing cannot be recom
16、mended, but general considerations concerning system planningare contained in Annex B to this Recommendation.5.3 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 conditions and consequent actionsThe fault c
17、onditions and consequent actions in this section should be complementary to those recommended fordigital line sections.Fascicle III.5 - Rec. G.951 3ANNEX A(to Recommendation G.951)Definition and measurement of repeaters crosstalk noise figuresA.1 Definitiona) NEXT-Noise Figure RNRN = IN - N0IN= mean
18、 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 NEXT interference voltage at decision point which procedures specified BER, anddepends on paramete
19、rs 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 point.E(f) = regenerator equalizer frequency transfer function.P(f) = power spect
20、ral density (single sided) of line system line code.f0= half line system baud rate.Quantities in square brackets are in dB, i.e.X = 10 log10| X |.b) FEXT-Noise Figure RFRF = IF - N0IF= mean square far-end crosstalk (FEXT) voltage produced by a single interfering regenerator thatwould appear at the d
21、ecision 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 parameters which affect the decision process and reflects impairments arising fromintersymbol interference and offsets fr
22、om 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 of cable.4 Fascicle III.5 - Rec. G.951A.2 MeasurementMethod a) directly relates to the definition of crosstalk-noise figure a
23、nd 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 Figure can be measured using the configuration shown inFigure A 1/G.951 with the Function Switch in t
24、he 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 attenuatoruntil the desired BER = 10-xis achieved. The value of the attenuator, R dB, is then the NEXT-Noise Fig
25、ure or FEXT-Noise Figure for the desired A0and BER.Method b)The NEXT-Noise Figure RN can be measured using “input S/N ratio“ test sets by employing the test set in a“manual mode“ and performing external measurements with a selective filter (see Figure A-2/G.951). The measurementconsists of:i) Set ar
26、tificial line to 0 dB loss 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 obtained, switchoff test signal, and using selective filter, measure noise power P dBm.iii) Then RN = S0 - P for desired A
27、0and 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.Fascicle III.5 - Rec. G.951 5Method c)The NEXT-Noise Figure RN can be measured using “input S/N ratio“ test sets in “manual mode“ w
28、ith theinsertion of an additional variable attenuator between the test signal and the artificial line, as shown in Figure A-3/G.951.The measurement procedure is as follows:i) set the artificial line to 0 dB loss and the additional variable attenuator to A dB loss;ii) regulate the variable gain ampli
29、fier 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 to 0 dB loss;iv) adjust the variable attenuator until the desired BER = 10-x is reached. Th
30、e attenuation value of theattenuator is N dB;v) calculate RN = N + A - WNin which PR(f) = spectral power density (single sided) of line code.It would be better to obtain WNby measurement. Of course, the value of WNcan also be calculated according toPR(f) of AMI or HDB3in a certain frequency range, f
31、or example, WN= -3.59 dB in the range from 0 to 10 240 kHz.6 Fascicle III.5 - Rec. G.951ANNEX B(to Recommendation G.951)Guidance notes for the satisfactory achievement of theerror performance objectivesB.1 To comply with the Network Performance Objectives (NPO) it is necessary to take into account m
32、anyinterrelated factors. Figure B-1/G.951 illustrates diagrammatically the interrelationship between all the factors thatimpact on this matter. The basis upon which digital line system installation planning guidelines are formulated isdependent on the circumstances of each Administration. For exampl
33、e, 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, therefore, make a judgement as to the significance of each effectin their network and for
34、mulate cable utilization guidelines which satisfy the digital line section error performancerequirements.Fascicle III.5 - Rec. G.951 7The following notes highlight a number of important considerations concerning the formulation of systeminstallation planning guidelines.Note 1 - In the process of est
35、ablishing 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 on the average power spectral density of the total crosstalk interference, provides a usef
36、ul 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 also based on an assumption of random dataon the disturbing systems and therefore cannot b
37、e 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 subject to other sources of interference which aredifficult to quantify and which may induce er
38、rors. 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 CCITT and as operational experience is gained it might be possible to introduce further t
39、eststhat 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 one of the following approaches:i) In circumstances where Administrations are able to ju
40、dge 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 Administrations are not able to judge the significance of the network effects, cablefill c
41、alculations should be based on the equipment design objective.8 Fascicle III.5 - Rec. G.951Note 4 - The use of a reduced line symbol rate code provides a more favourable crosstalk environment, and thisfeature will impact on cable fill calculations.Note 5 - When changing from a plesiochronous to a sy
42、nchronous 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 for practical systems. It is believed that there are more cases with increasedmargin than r
43、educed 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 identical repetitive sequences does not exceed thelevels occurring with random data.Referen
44、ces1 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-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.
