1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3%.%2!,G0G0#(!2!#4%2)34)#3G0G0#/-/.G0G04/G0G0!,!.!,/5%G0G0#!22)%2G1342!.3-)33)/.G0G03934%-3/6%2!,G0G02%#/-%.$!4)/.3G0G02%,!4).4/G0G0#!22)%2G1342!.3-)33)/.G0G03934%-3
2、)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.221 was published in Fascicle III.2 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents
3、of the file are identical to the Blue Book version and copyright 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 re
4、served. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.2 - Rec. G.221 1Recommendation G.221Fascicle III.2 - Rec. G.221OVERALL RECOMMENDATIONS
5、 RELATING TO CARRIER-TRANSMISSION SYSTEMS(amended at Geneva, 1972 and 1980)1 Characteristics of complete circuitsThe characteristics of complete circuits, measured between audio-frequency terminals (overall loss in terminalservice and in transit service, frequency bands effectively transmitted and a
6、ttenuation distortion, variation of overall losswith time, phase distortion, stability, crosstalk, etc.) should meet the general conditions for 4-wire telephone circuitsindicated in Section 1 of the Series G Recommendations.2 Linear crosstalk2.1 Overall requirementsThe requirements as regards crosst
7、alk ratio between circuits in the case of telephony are the subjects ofRecommendation G.134 1 and the Recommendation cited in 2; for go-to-return crosstalk the Recommendation citedin 3 applies.As carrier transmission systems are also used for setting up sound-programme circuits, the relevantrequirem
8、ents given in the Series J Recommendations should be taken into consideration. Recommendation J.18 4 givesgeneral guidance on how the higher crosstalk ratios appropriate to sound-programme transmissions are achieved in atelephone network.In any case the near-end crosstalk ratio between the two direc
9、tions of transmission at all frequencies used forthe regulating and measuring pilots on carrier systems should be not less than 40 dB.2.2 Intelligible crosstalk caused by intermodulation with a signal which is a multiple of 4 kHzIntelligible crosstalk may arise between circuits by way of intermodula
10、tion with a signal which is a multiple of4 kHz, e.g. a line-regulating pilot. A design objective is that the intelligible crosstalk ratio in a single homogeneoussection of the appropriate hypothetical reference circuit should be not less than 74 dB.3 Noise transmitted between interconnected systemsA
11、 failure or malfunction in a chain of repeaters may lead to large values of noise in one or several signal bandsbeing transmitted by that chain. It is known that such high noise levels are generally caused by the operation ofparticular types of automatic line regulators. Given that such high noise l
12、evels may be transmitted to other chain links,and may overload those to which they are interconnected, it is desirable and recommended that care should be taken inthe future in order to avoid such troubles.Possible methods of dealing with this problem are described in Supplement No. 4 5.In respect o
13、f radio-relay links, it will be the concern of CCIR to enumerate suitable precautions.4 Single tone interferenceThe Recommendation cited in 6 indicates a limit for the single tone interference level in telephone circuits.Depending on the origin of such interferences, wide-band services and non-telep
14、hony services (e.g. soundprogrammecircuits, etc.) may also be affected. This should be considered when defining limits for transmission systems.2 Fascicle III.2 - Rec. G.221Practical experience shows that broadcasting transmitters are the main external source of single toneinterference. In order to
15、be usable under normal environmental working conditions, the carrier transmission equipmentshould be designed in such a way that it allows a certain electromagnetic field strength in its vicinity, caused bytransmitters. A figure of 0.5 to 0.7 V/m within a station should be tolerated by equipment whi
16、ch is installed as normallyspecified and working under normal conditions. Where higher field strengths are known to be expected, suitablescreening measures in the building may have to be adopted. Special attention should also be given to the stating cablingincluding power distribution and to the wir
17、ing of distribution racks to prevent interferences from entering the equipmentvia these points.Note - The Supplement No. 27 contains some information on possible measures to reduce effects frominterference and on measuring methods concerning interference.5 Total interference powerIn addition to the
18、above limitation of the single tone interference, it should be ascertained that the totalinterference power in each telephone channel within the band 300-3400 Hz, for each individual case of interference,should be lower than -65 dBm0.References1 CCITT Recommendation Linear crosstalk, Vol. III, Rec.
19、G.134.2 CCITT Recommendation General performance objectives applicable to all modern international circuits andnational extension circuits, Vol. III, Rec. G.151, 4.1.3 Ibid., 4.2.4 CCITT Recommendation Crosstalk in sound-programme circuits set up on carrier systems, Vol. III,Rec. J. 18.5 Certain met
20、hods of avoiding the transmission of excessive noise between interconnected systems, Green Book,Vo1. III-2, Supplement No. 4, ITU, Geneva, 1973.6 CCITT Recommendation General performance objectives applicable to all modern international circuits andnational extension circuits, Vol. III, Rec. G. 151, 8.
