1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU GA42 (I 2/98) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits - General characteristics of the 4-wire chain of international circuits; i
2、 n tern a t i on ai transit Transmission characteristics of exchanges ITU-T Recommendation G.142 (Previously CCITT Recommendations) ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G. 1 OO-G. 199 G. 1 OO-G. 1
3、 O9 G. I 1 O-G. 1 1 9 General definitions General Recommendations on the transmission quality for an entire international telephone connection General characteristics of national systems forming part of international connections national extension circuits G. 120-G.129 General characteristics of the
4、 4-wire chain formed by the international circuits and G. 130-G. 139 G. 150-G. 159 circuits Apparatus associated with long-distance telephone circuits Transmission plan aspects of special circuits and connections using the international telephone connection network Protection and restoration of tran
5、smission systems Software tools for transmission systems INTERNA TIONAL ANALOGUE CARRIER SYSTEM GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER- TRANSMISSION SYSTEMS INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES GENERAL CHARACTERISTICS OF INTERNATIONA
6、L CARRIER TELEPHONE WITH METALLIC LINES SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY TESTING EQUIPMEN TS TRANSMISSION MEDIA CHA RA C TERIS TICS DIGITAL TRANSMISSION SYSTEMS TERMINAL EQUIPMENTS DIGITAL NETWORKS DIGITAL SECTIONS AND DI
7、GITAL LINE SYSTEM G. 160-G.169 G. 170-G. 179 G. 180-G. I 89 G.190-G.199 G.200-G.299 G. 300-G. 399 G.400-G.449 G.450-G.499 G.600-G.699 G.700-G.799 G.800-G.899 G .900-G . 999 For firther details, please refer to ITU-T List of Reconmendations. ITU-T RECOMMENDATION G.142 TRANSMISSION CHARACTERISTICS OF
8、EXCHANGES Summary This Recommendation was revised to remove text that referred to international analogue exchanges. The ITU-T no longer recommends the placement of new or continued operation of analogue international exchanges. Additionally, this Recommendation was written so as to recognize that bo
9、th Plesiochronous Digital Hierarchy/Synchronous Digital Hierarchy (PDWSDH) and digital ATM switching technology may be used in the PSTN. This Recommendation provides transmission performance planning guidance for digital exchanges. Soume ITU-T Recommendation G.142 was revised by ITU-T Study Group 12
10、 (1997-2000) and was approved under the WTSC Resolution No. 1 procedure on the 3rd of December 1998. Recommendation G.142 (12/98) 1 STD.1TU-T RECMN GmLLiZ-ENGL L996 .I YBbZ59L ObSb284 Ob3 m FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of
11、telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The IW-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The W
12、orld Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure la
13、id down in WTSC Resolution No. 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation the term recognized operating agency (ROA) includes any individual, company, corpora
14、tion or governmental organization that operates a public correspondence service. The terms Administration, ROA and public correspondence are defined in the Constitution of the ITU (Geneva, 1992). INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or implementat
15、ion of this Recommendation may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by IT members or others outside of the Recommendation development process. A
16、s of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly
17、urged to consult the TSB patent database. O ITU 1999 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. I Recommendation G.142 (12/98)
18、11 , - - - - _ STD-ITU-T RECMN G-IiVZ-ENGL 1998 a 4662593 Ob5bZ85 TTB I CONTENTS Page 1 2 3 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Scope General . References Digital exchanges . Digital processes . Effect on transmission . Transmission loss through a digital exchange Relative levels Echo and stability co
19、ntrol . 4.4.1 Transmission loss contribution . 4.4.2 Balance return loss contribution Local transmission I. Sidetone and input impedance Digital pads . Transmission delay . 1 1 1 2 2 4 5 7 8 8 . Recommendation 6.142 (12/98) 111 Recommendation G.142 TRANSMISSION CHARACTERISTICS OF EXCHANGES (Geneva,
20、1980; amended at Melbourne, 1988, revised in 1998) 1 Scope This Recommendation is concerned with the voice-frequency transmission considerations that should be taken into account in the design of digital exchanges and their incorporation into the network. The digital exchanges referred to include lo
21、cal exchanges and transit exchanges (national and intemational). The transmission considerations relate primarily to the properties which digital exchanges should possess to enable them to operate under different and changing network conditions with respect to the content of analogue, mixed analogue
22、/digital and all-digital plant. 2 General Detailed transmission characteristics for PDWSDH digital exchanges are contained in Recommendations Q.55 1 , 4.552, 4.553 and 4.554. While no detailed Recommendations have been specifically generated for switches which use ATM technology, in general, ATM swi
23、tches should comply with the requirements defined in the Q.550 series Recommendations. It is understood that due to the different mode of operation of ATM switches, as compared to their PDH/SDH counterparts, certain parameters like group delay may be different for an ATM switch that performs cell as
24、sembly when information from one DS-O is placed within a specific cell. 3 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated wer
25、e valid. All Recommendations and other references are subject to revision; all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recomm
26、endations is regularly published. - - - - ITU-T Recommendation G. 1 O1 (1 996), The transmission plan. ITU-T Recommendation G. 1 13 (1 996), Transmission impairments. ITU-T Recommendation G. 12 1 (1 993), Loudness Ratings (LRs) of national systems. ITU-T Recommendation G. 122 (1 993), Influence of n
27、ational systems on stability and talker echo in international connections. ITU-T Recommendation G. 126 (1 993), Listener echo in telephone networks. ITU-T Recommendation G. 176 (1997), Planning guidelines for the integration of ATM technology into networks supporting voiceband services. ITU-T Recomm
28、endation Q.55 1 (1 996), Transmission characteristics of digital exchanges. ITU-T Recommendation 4.552 (1 996), Transmission characteristics at 2-wire analogue interfaces of digital exchanges. - - - - Recommendation G.142 (12/98) 1 - ITU-T Recommendation 4.553 (1 996), Transmission characteristics a
29、t 4-wire analogue interfaces of digital exchanges. ITU-T Recommendation 4.554 (1 996), Transmission characteristics at digital interfaces of digital exchanges. ITU-T Supplement 3 1 to the G-series Recommendations (1 993), Principles of determining an impedance strategy for the local network. - - 4 D
30、igital exchanges 4.1 Digital PDH/SDH exchanges and digital ATM exchanges, to varying degrees, currently are required to include such digital processes as analogue-to-digital coders, digital-to-analogue decoders and digital recording processes, examples of which are companding law converters and digi
31、tal pads. The extent to which such digital processes might be included in a digital exchange is determined by the network environment in which the exchange is to operate (i.e. mixed analogue/digital or all-digital). While a number of signal processing functions such as echo control, speech compressi
32、on and PDH/SDH to ATM interworking may be performed by physically distinct network elements, this does not preclude the incorporation of these digital signal processing functions into digital switches. Digital processes such as those referred to above attract transmission penalties. These penalties
33、can be expressed in terms of “quantization distortion units (qdus) or units of equipment impairment factor (eif)“. The impact of these impairments on a connection can be evaluated using the “quantization factor method“ or the “equipment impairment factor method“, as discussed in Recommendation G. 1
34、13. Only the equipment impairment factor method which is discussed in clause 6/G. 1 13 is recommended for evaluating the impairments associated with transmission devices which use non-waveform coding techniques. A limit is placed on the permissible accumulation of units of transmission impairment in
35、 an international telephone connection. Details of the planning rule resulting from this limit and the penalties introduced by individual digital processes are given in 5.6/G. 1 13. In accordance with 5.6/G.113, it is recommended that no more than 14 units of transmission impairment be permitted to
36、accumulate in an international connection. Of these 14 units, a maximum of 5 units could be introduced by each national extension and a maximum of 4units by the international portion*. Since one 8-bit PCM codec pair (coder and decoder) introduces 1 unit of transmission impairment, it is clear that u
37、nintegrated PCM digital processes involving analogue/digital conversions (e.g. codecs), or digital processes involving the recoding of information (e.g. digital pads) should not be allowed to proliferate in an uncontrolled fashion. Figure 1 shows some of the transmission paths that might be establis
38、hed through a digital exchange and the “units of transmission impairment“ attributable to the digital processes in these paths. Figure 2 also shows some of the transmission paths that might be established through a digital exchange. However, Figure 2 is intended to identi examples of where digital s
39、ignal processing devices may be placed. It is recommended that the impairments caused by these signal processing devices be evaluated using the equipment impairment factor method presented in clause 6/G. 1 13. It is important to understand that the equipment impairment factor method allows the perfo
40、rmance evaluation to be performed if the impairments are specified in impairment units of eifs and or qdus. Digital processes - Effect on transmission It should be noted that 14 qdus equates to approximately 20 eif units. 2 Recommendation G.142 (12/98) Digital exchange 4 b + 4 Switching Transmission
41、 L 1/2* Il ,. I /2* II T1208970-98 Analogue transmission - Digital transmission - - - A/D coder or D/A decoder .+ Analogue pad Digital pad -1- -i- -1. 2WI4W terminating unit -lE B Balance network 0.7* R T L qd units of transmission impairment Receive (from the switch) pad at local exchange Transmit
42、(from the switch) pad at local exchange Pad (if required) for circuit loss adjustment _ Digital switch - - - AD coder or D/A decoder Analogue pads for control * * of stability and echo 2-wire/4-wire jHi terminating unit Balance network U Figure 3/G.142 - Configuration of digital local exchange on 2-
43、wire to 2-wire connections 4.6 Sidetone and input impedance Digital local exchanges can have a significant influence on the sidetone performance of telephone instruments, particularly those instruments on relatively short local lines. The reason for this can be seen in Figure 3 where the impedance p
44、resented by the exchange to the local line is a function of the input impedance of the line card and the characteristics of the singing and echo path within the exchange. For optimum sidetone performance on short local lines, the input impedance of the exchange line card should be close to the anti-
45、sidetone impedance of the telephone instrument. In the case where the telephone instrument is designed to give good sidetone performance on long local lines, this anti-sidetone impedance is likely to be close to the characteristic impedance of the 2-wire local cable. Recommendation 6.142 (12/98) 7 S
46、TD-ITU-T RECMN G-Li2-ENGL 1998 1111 48b259L Ob5b293 O34 This would lead to the digital local exchange also presenting an impedance close to that of the 2-wire local cable. On longer local lines, the exchange impedance will have less effect on the sidetone performance as the impedance presented to th
47、e telephone is masked by the local cable impedance. The final choice of exchange impedance needs to take into account a number of factors: - - local line network characteristics; - the objective being that the customer should not see a worsening in sidetone performance when connected to a digital ex
48、change. Impedances chosen by a number of Administrations are given in Recommendation Q.552, and it is clear that there is a considerable difference between the impedances which reflect the differences between the national networks. telephone set impedance and sensitivity characteristics; digital exc
49、hange current feeding arrangements, 4.7 Digital pads The use of a digital pad to produce the required transmission loss in a digital path attracts a transmission penalty. This penalty has to come out of the allowance of “units of transmission impairment“ allotted to the national and international portions of international connections - see 5.6/G.113. Additionally, since digital pads involve the use of digital recoding processes, the use of such pads in paths where bit integrity must be preserved is unattractive. This can be an important consideration where multi-purp
