1、 INTERNATIONAL TELECOMMUNICATION UNION CCITT Series GTHE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE Supplement 29(11/1988) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS OBJECTIVE FOR THE MIXED ANALOGUE/ DIGITAL CHAIN OF 4-WIRE CIRCUITS Reedition of CCITT G
2、Series Recommendation Supplement 29 published in the Blue Book, Fascicle III.1 (1988) NOTES 1 CCITT G-series Recommendation Supplement 29 was published in Fascicle III.1 of the Blue Book. This file is an extract from the Blue Book. While the presentation and layout of the text might be slightly diff
3、erent from the Blue Book version, the contents 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 a telecommunication administration and a recogni
4、zed operating agency. ITU 1988, 2007 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Fascicle III.1 Supplement 29 1 Supplement No. 29 OBJECTIVE FOR THE MIXED ANALOGUE/DIGITAL CHAIN OF 4-WIRE CIRCUITS Draft Reco
5、mmendation G.136 (This Supplement is proposed for further study during the present study period with the aim to convert the supplement into a Recommendation.) 1 General In the period of transition from a fully analogue to a fully digital network, there will be, on international and national networks
6、, mixed type chain of 4-wire telephone circuits (see Recommendation G.101, 4.2), some sections of which can be made with analogue or digital transmission systems. Considering the fact that the transition period may last for a fairly prolonged time, and also considering the need for guaranteeing a ce
7、rtain quality of transmission on mixed chain of circuits, the CCITT recommends observance of some principles for the composition of mixed chain of circuits as set forth below and some objectives for their parameters. The main principle in the standardization of mixed circuits lies in the retaining o
8、f the standards adopted for the FDM circuits. This would have resulted in retaining the transmission quality over the 4-wire chain formed by the international circuits and national extension circuits. For some parameters this can be achieved, but as far as some other parameters are concerned due to
9、analogue/digital conversions and errors in digital sections there are some considerable differences in standards and measuring methods. Objectives for some mixed circuit parameters are contained in a number of G-, Q-, and M-series Recommendations. However, these objectives do not take due account of
10、 the addition laws for distortions based on the multitude of mixed circuit structures and specific features of the measuring methods involved. Considering the importance of retaining the transmission quality during the transition period and attaching great importance to the standardization of mixed
11、analogue/digital circuits the multitudinous types of which emerge while using various kinds of analogue-to-digital conversions, CCITT thinks it worth while to have a specific Recommendation on objectives for mixed analogue/digital circuits and 4-wire chains including both analogue and digital circui
12、ts. The present Recommendation related to mixed 4-wire chain of circuits and the analogue/digital mixed connections dealt with in this Recommendation are those with analogue telephone sets at both ends. It is based on the existing Recommendations for FDM channel equipment G.232, for PCM channel equi
13、pment G.712, for analogue switching centres Q.45, Q.45 bis, for digital switching centres Q.551 to Q.554, and takes account of other existing Recommendations of G- and M-series. Later on in accordance with the study results of Question 26/XII the present Recommendation will have to be supplimented b
14、y objectives for mixed chain of circuits formed with the help of various methods of analogue-to-digital conversion such as transmultiplexers (Recommendations G.793, G.794), modems (Recommendations G.941, V.37), transcoders (Recommendation G.761), group codecs (Recommendation G.795), DCME, as well as
15、 connections with a digital telephone at one end and an analogue telephone at the other end. 2 Structure of a mixed analogue/digital voice frequence chain of 4-wire circuit The parameters of a mixed 4-wire chain are essentially dependent on the number of analogue sections and on the number of analog
16、ue/digital conversions in the chain. According to Recommendation G.103 the total number of 4-wire circuits in a 4-wire chain of the maximum length is 12 in exceptional cases (Table 2/G.101) so that it may be assumed that the number of circuits will not exceed 12. The worst cases in terms of distorti
17、ons occur when: all switching centres are digital, and the circuit sections from and to the centres are set up on analogue transmission systems. The number of analogue/digital conversions is then 11, the number of analogue sections is 12; 2 Fascicle III.1 Supplement 29 all switching centres are anal
18、ogue, and the circuit sections from and to the centres are set up on digital systems. The number of analogue/digital conversions is 12 in this case, the number of digital sections is 12. Such cases are very rare. More representative is considered to be a case where the number of analogue/digital con
19、versions makes one half of the maximum number (Recommendation G.103, Annex B), that is 6, and digital islands are available. The structure of such a 4-wire chain is presented in Figure 1. The number of analogue sections is 6, the number of digital sections is also 6. Other structures of mixed 4-wire
20、 chain come into the picture when connection of the sections is realized without a switching equipment. These structures are considered in Recommendation M.562 ( 3.2). The worst case for a circuit of 12 sections without switching centres occurs when digital and analogue sections alternate (see Figur
21、e 2), the number of analogue-digital conversions being equal to 6, the number of digital sections to 6, and the number of analogue sections also to 6. FIGURE 1 FIGURE 2 Thus, the examination of various structures of mixed analogue/digital voice-frequency chain of circuits shows that for a 4-wire cha
22、in of maximum length having 12 sections, it is advisable to establish objectives of distortions based on 6 analogue/digital conversions, 6 analogue and 6 digital sections. Intermediate variants for combinations of analogue, digital sections and analogue-to-digital conversions will be: It should be b
23、orne in mind that the chains may most frequently consist of less than 12 sections. The contribution of switching centres to distortion is negligible, if they do not contain analogue/digital conversions. 3 Objectives for parameters of mixed analogue/digital circuits 3.1 The nominal value of the input
24、/output impedance of the analogue and digital sections and of a switching equipment should be 600 ohms. 3.2 Return loss of the input/output impedance referred to the nominal value of the analogue and digital sections and of a switching equipment should preferably be not less than 20 dB in the 300-34
25、00 Hz band. Note For a switching centre and channel FDM equipment, the value of 15 dB is permissible in the 300-600 Hz band (see Recommendation Q.45, 6.3 and Recommendation G.232, 7). Fascicle III.1 Rec. Supplement 29 3 3.3 Unbalance loss in respect to earth The existing Recommendations for switchin
26、g centres (Q.45, Q.553) and channel FDM equipment (G.712) standardize the unbalance loss in respect to the earth in different ways. There are differences in the measuring methods as well. The Recommendation for the FDM-channel equipment (G.232), does not specify this parameter. The question of stand
27、ardization and methods of measuring this parameter for mixed circuits channels is under study. Pending the establishment of unified objectives and measuring methods, Recommendation K.10 on the unbalance loss of communication equipment should be referred to in general guidelines in the case of mixed
28、chain of 4-wire circuits. 3.4 Nominal relative level The nominal relative level on the transmit side of each section (analogue and digital) is -14 (-16) dBr. The nominal relative level on the receive side of each section (analogue and digital) is +4 (+7) dBr (see Recommendations G.232, 11, G.712, 14
29、, Q.45, 3 and Q.553 2.2) The nominal relative level at the virtual analogue switching point is sending: 3.5 dBr receiving: 4.0 dBr for analogue 3.5 dBr for digital (See Recommendation G.101, 5.2.) The nominal relative value in a mixed circuit is defined for a frequency which is not a subharmonic of
30、the sampling frequency. The recommended tentative value for the frequency is 1020 Hz. 3.5 Variations of transmission loss with time The standard deviation of the transmission loss should not exceed 1 dB. The difference between mean and nominal value of the transmission loss should not exceed 0.5 dB.
31、 Note The indicated values are defined in Recommendation G.151, 3 for a fully analogue circuit under the condition that the channels are part of a single group equipped with automatic regulation. For mixed chains the stability conditions improve on the one hand because of the existence of digital se
32、ctions which have a higher stability than analogue ones; but on the other hand in the mixed circuits there is no possibility of a transit automatic regulation of analogue sections, which deteriorates the overall stability. That is why the indicated values should be considered as tentative and are to
33、 be confirmed. 3.6 Attenuation/frequency distortion Attenuation/frequency distortion for the whole 4-wire chain should not exceed the values given in Figure 1/G.132. For mixed chains (without consideration of switching centre distortions) the accumulation law of attenuation/frequency distortions is
34、expressed by the following formula: (1) with n1: number of analogue sections; n2: number of analogue/digital conversions; FDMa : average value (determined component) of attenuation/frequency distortions of the analogue sections; FDM: r.m.s. deviation of attenuation/frequency distortions of analogue
35、sections; aPCM: attenuation/frequency characteristics of analogue/digital equipment; K = 1, 2 or 3: factor defining the probability of maximum/minimum value of attenuation/frequency distortions. K is usually taken as equal to 3. The justification of the choice for K = 3 depending on a given probabil
36、ity can be found in 1, 2. 4 Fascicle III.1 Supplement 29 “K” is usually taken as equal to 3. The justification of the choice for K = 3 depending on a given probability can be found in 1, 2. Note 1 Attenuation/frequency characteristics of analogue/digital equipment of the same type are similar. That
37、is why, if in a mixed/chain of circuits analogue/digital equipment of the same type is used, in the sum formula (1) can be replaced by a product n2aPCM. Note 2 The analogue-digital equipment distortion limits recommended in Recommendation G.712 ( 1, Figure 1) and the FDM-channel equipment distortion
38、 limits recommended in Recommendation G.232 ( 1, Figure 1) meet the limits indicated in Recommendation G.132 for mixed circuits in which the number of sections does not exceed 4. When composing mixed chains with a greater number of sections, it is advisable to utilize modern channel equipment the at
39、tenuation/frequency distortions of which are considerably lower than those indicated in Recommendations G.232 and G.712. Note 3 Attenuation/frequency distortions are measured relative to the reference frequency of 1020 (1000) Hz. Note 4 See Recommendation Q.45 ( 3.4 and Q.553) to take account of the
40、 switching equipment distortions. 3.7 Group delay distortions Group delay distortions should not exceed the values indicated in Recommendation G.133 for the 4-wire chain. The law of imposition of group delay distortions is expressed by the following formula: where n1the number of analogue sections,
41、n2the number of analogue/digital conversions. Note 1 If, in a mixed chain, analogue/digital equipment of the same type is used, then the sum is substituted by a product n2. PCM. Note 2 It is expected that the group delay distortion in mixed chains will be less than that of a fully analogue link for
42、any combination of analogue and digital sections. But nevertheless the characteristics of distortion (symmetry) can change considerably. This should be taken into account when transmitting data on mixed circuits containing group delay equalizers. Note 3 Group delay distortions are measured with refe
43、rence to a frequency situated at the lower band end of the analogue channel, i.e. 190-200 Hz. Note 4 Switching centre distortions are negligible and can be ignored. Fascicle III.1 Rec. Supplement 29 5 3.8 Intelligible crosstalk Near-end and far-end signal-to-intelligible crosstalk ratios between cir
44、cuits and between send and receive directions should satisfy Recommendation G.151 ( 4). Note 1 It is expected that the values indicated in Recommendation G.151, will be maintained and even better for mixed chains for any combination of analogue and digital sections, due to higher values achieved in
45、the analogue/digital conversion equipment. Note 2 Measurement of the signal-to-crosstalk ratio between circuits can be performed without feeding an auxiliary signal into a channel affected by crosstalk (unlike that provided for in the note to point 11 of Recommendation G.712). This can be explained
46、by the fact that in a mixed circuit, as a rule, and in an analogue circuit noise will be present at the input of analogue/digital converters in a mixed chain. 3.9 Non-linear distortions The existing Recommendations for analogue circuits (M.1020, 2.11), for switching equipment (Q.45, 6.1) and Recomme
47、ndation G.712 for analogue/digital equipment contain different specifications for non-linear distortions, the methods of their measurement differ too. The Recommendations for digital centres (Q.551 to Q.554) do not contain specifications for non-linear distortions. At present it is not possible to r
48、ecommend permissible values of non-linear distortions and a method for measuring mixed chains of circuits. This question needs to be studied. 3.10 Noise (total distortions) The notion of noise in mixed chains of circuits due to analogue-to-digital conversions producing quantization distortions which
49、 accompany the signal has lost its initial meaning and therefore instead of the term “noise” applicable to mixed chain of circuits the term “total distortions” is used very often. This is stipulated by the fact that the measurement of quantization distortions (Recommendation Q.132) includes part of non-linear distortions and single-frequency interferences. From this view point the total distortions in mixed chains include analogue section noise which depends on the length of the sections in case of terrestrial tr
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