1、Superseded by a more recent version INTERNATIONAL TELECOMMUNICATION UNION ITU-T G.171TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (11/88) TRANSMISSION SYSTEMS AND MEDIA TRANSMISSION PLAN ASPECTS OF SPECIAL CIRCUITS AND CONNECTIONS USING THE INTERNATIONAL TELEPHONE CONNECTION NETWORK TRANSMISSION
2、PLAN ASPECTS OF PRIVATELY OPERATED NETWORKS ITU-T Recommendation G.171 Superseded by a more recent version (Extract from the Blue Book) Superseded by a more recent version NOTES 1 ITU-T Recommendation G.171 was published in Fascicle III.1 of the Blue Book. This file is an extract from the Blue Book.
3、 While the presentation and layout of the text might be slightly different 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 concisen
4、ess to indicate both a telecommunication administration and a recognized operating agency. Ge3 ITU 1988, 1993 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
5、 in writing from the ITU. Superseded by a more recent version Fascicle III.1 Rec. G.171 Superseded by a more recent version 1 Recommendation G.171 Fascicle III.1 Rec. G.171 Superseded by a more recent version TRANSMISSION PLAN ASPECTS OF PRIVATELY OPERATED NETWORKS (Geneva, 1980; amended at Malaga-T
6、orremolinos, 1984 and Melbourne, 1988) 1 General This Recommendation primarily concerns privately switched networks for telephony. In certain circumstances these networks may be suitable for the transmission of analogue encoded data signals but no special arrangements have been made to ensure satisf
7、actory performance in this respect. Although digital facilities on a portion of a circuit or digital switches may be employed, 1-9 of this Recommendation mainly covers analogue interconnection of circuits and switches. 10 and 11 cover some aspects of all digital connections. It should be noted that
8、not all Administrations provide such a facility. Others permit interconnection between private telephone networks and the public telephone network. In this latter case assurance cannot always be given that transmission performance conforming to CCITT standards will be obtained. In a similar manner t
9、he interconnection of multiple private networks may result in connections with degraded transmission performance. It is not intended that this Recommendation should prevent the making of bilateral agreements for special network configurations. In such circumstances it is suggested that the network p
10、lans given here be used as a guide to permissible alternative arrangements. The transmission plan described in this Recommendation is similar to that of the switched public network and therefore it is desirable that several other Recommendations such as G.151 be complied with where possible and appr
11、opriate. In this respect, it is noted that some requirements in Recommendation G.151 are more stringent than those contained in this Recommendation (e.g. attenuation distortion), and some impairments which are more important for voice-band data are covered in G.151 but are not included in this Recom
12、mendation. A major consideration in the private plan is that typically, a PBX functions both in the role of a local exchange and a tandem centre and therefore it is necessary to use a technique such as pad switching to achieve the appropriate connection loss. The network configurations discussed in
13、this Recommendation may also be implemented by replacing some or all of the PBXs with switching capability dedicated to a private user that is located on the premises of the telephone Administration rather than on the customers premises. Recommendation M.1030 provides information on the maintenance
14、of international leased circuits forming part of private switched networks. Recommendation Q.8 describes signalling systems to be used for international leased circuits. 2 Network configurations 2.1 Preferred 4-wire network configurations The preferred network configurations are shown in Figure 1/G.
15、171 and Figure 2/G.171. Four-wire PBXs are used in conjunction with low loss 4-wire circuits. The loss plans shown are for illustration and are based on the national plans discussed in Recommendation G.121. For convenience the later figures will only use the variable loss plan for illustration. It s
16、hould be noted that the fixed loss plan without modification, (Figure 2/G.171) is only suitable when the national system is limited in size at most to 1000 to 1500 km. Superseded by a more recent version 2 Fascicle III.1 Rec. G.171 Superseded by a more recent version d01FIGURE 1/G.171.D01 d02FIGURE
17、2/G.171.D02 Superseded by a more recent version Fascicle III.1 Rec. G.171 Superseded by a more recent version 3 At each PBX a switchable pad or equivalent is used in such a manner that the pad is “out” of the circuit when the PBX switch is in the tandem mode but is “in” the circuit at an originating
18、/terminating PBX. This allows a flexible configuration of PBXs while maintaining control on echo loss and overall loudness rating. The PBX terminating the international chain is referred to as the International PBX (IPBX). Conceptually the virtual analogue switching points are located at the IPBX. I
19、t should be noted that typically short PBX subscriber lines may need more loss in the connections to meet the Recommendations on send and receive LR at the virtual analogue switching points. This will of course depend on the send and receive LR of the telephone and subscriber line. It may also be ne
20、cessary to add loss on intra-PBX calls. 2.2 Allowed network configuration using 2-wire circuits The configuration shown in Figure 3/G.171 allows for the use of 2-wire circuits. This is not desirable and should be avoided. If used, 2-wire circuits should only be deployed between an originating/termin
21、ating PBX and the first tandem PBX. A 2-wire circuit may be all 2-wire or consist of a mix of 2-wire and 4-wire segments. The use of 2-wire circuits may require special loss control at the connecting tandem PBX. If the stability/echo requirements of 5 and 6 cannot be met otherwise, it will be necess
22、ary to switch the pad or equivalent loss into the tandem connection to the 2-wire circuit. This would require special translation and control at the tandem PBX to identify 2-wire trunks not consistent with the stability/echo requirements. If this is not possible the added loss is required on all tan
23、dem connections, causing a degradation in overall loudness rating. 2.3 Balanced 2-wire tandem PBXs As shown in Figure 3/G.171, 2-wire PBXs may be used in the tandem mode if the collection of interconnected 4-wire interfaces meet balance requirements as shown in Note 2. With a mean echo loss of 27 dB
24、 and a standard deviation of 3 dB, the effects of echo at the PBX are negligible with respect to the principal echo at the originating/terminating PBX or at the tandem PBX connected to a 2-wire circuit. Recommendation G.131 refers to these balance values in reference to tandem 2-wire switches. It is
25、 provisionally recommended, that at most three 2-wire PBXs be contained in a single national extension. This would correspond to a 2-wire terminating/originating PBX with two additional balanced 2-wire tandem PBXs. As shown in Figure 4/G.171, the IPBX may be 2-wire. The virtual analogue switching po
26、ints are adjacent to the 2-wire/4-wire terminating unit on the 4-wire side. If the PBX is used for tandem switching it must be balanced and pad switching or equivalent should be employed as previously described. 2.4 Network constraints To control loss, distortion, noise and delay a maximum of seven
27、circuits in tandem is recommended from originating to terminating PBX. Allocation of the number of circuits between national and international chains should remain flexible and should be done on an individual network basis subject to the seven circuit maximum. There should, however, be a maximum of
28、five tandem circuits in a connection in any single national extension. Superseded by a more recent version 4 Fascicle III.1 Rec. G.171 Superseded by a more recent version d03FIGURE 3/G.171.D03 d04FIGURE 4/G.171.D04 Superseded by a more recent version Fascicle III.1 Rec. G.171 Superseded by a more re
29、cent version 5 Recommendation G.114 on mean one-way propagation time should be observed. In particular, at most one satellite circuit should be present in a connection. If it is not possible to adhere to this constraint assurance cannot be given that transmission conforming to CCITT standards will b
30、e obtained. The arrangements shown in the Figures 1/G.171 to 4/G.171, are suggested methods of meeting the Recommendations on stability, echo and CRE (LR) as given in 5, 6 and 7. Other approaches achieving the same performance are acceptable. 3 Nominal transmission loss of international circuits 3.1
31、 Four-wire circuits Recommendation G.111 is applicable to this type of circuit and therefore the normal transmission loss at the reference frequency between the virtual analogue switching points will be 0.5 dB for circuits employing analogue transmission. An indication of the locations of the virtua
32、l analogue switching points is also given in Recommenda-tion G.111 and conceptually these will be at the private exchange on which the circuit terminates. Four-wire circuits do not contain 2-wire circuit sections. 3.2 Two-wire presented circuits This nomenclature is intended to cover circuits which
33、are not available with a 4-wire interface (e.g., circuits between 2-wire switching nodes). For the purposes of this Recommendation the location of the virtual analogue switching points for this type of circuit can be considered as being adjacent to the 2-wire/4-wire terminating unit (4-wire side). I
34、t can then be treated in the same way as a 4-wire circuit. (See Figure 4/G.171.) Note 1 The real loss of the circuit between actual switching points at the reference frequency cannot be exactly specified without prior knowledge of the switching levels. Note 2 Differences between the two directions o
35、f transmission in the real loss of the circuit may occur. The annexes to Recommendation G.121 examine this effect in some detail. Note 3 A circuit is defined as the complete transmission path between the switch points of the two private exchanges concerned. Note 4 Actual transmission loss will diffe
36、r from the nominal values and will vary with time. For all circuits, variations with time of the overall loss at the reference frequency (including daily and seasonal variations but excluding amplitude hits) should be as small as possible but should not exceed 4 dB. 4 Nominal transmission loss of na
37、tional circuits 4.1 Four-wire circuits The nominal loss at the reference frequency should be 0.5 dB between actual switching points. This includes 4-wire circuits terminated on balanced 2-wire PBXs. The loss of the circuit between the actual and virtual analogue switching points of the IPBX depends
38、upon the PBX transmission level used in the national plan. 4.2 Two-wire circuits Two-wire circuits may contain mixed 2-wire/4-wire segments. The nominal loss at the reference frequency should not exceed 7 dB, and should preferably be lower, for example 4 dB. Note 1 Certain national arrangements in l
39、arge countries may employ a nominal loss in excess of 0.5 dB on 4-wire circuits or may employ a distance dependent loss in order to improve talker echo performance without use of echo control devices. This approach is acceptable if the Recommendations on LR of 7 are satisfied. Superseded by a more r
40、ecent version 6 Fascicle III.1 Rec. G.171 Superseded by a more recent version Note 2 Since leased circuits may contain circuit sections routed in local unloaded distribution cable pairs, care will be needed to ensure that there is an adequate stability bearing in mind the relative gain introduced by
41、 unloaded cable pairs. Note 3 Loss variation should be controlled as described for international circuits. 5 Stability 5.1 National 2-wire circuits/2-wire presented circuits Two-wire presented circuits are 4-wire circuits terminated on 2-wire PBXs. Provisionally the nominal loss around any 4-wire lo
42、op should not be less than 6 dB at any frequency in the band 0 to 4 kHz, for all the terminal conditions encountered in normal operation (e.g. including the idle state and the set-up phase of the connection). 5.2 Terminating systems for international circuits National terminating systems which inter
43、face with international circuits should comply with the stability requirements of Recommendation G.122. In the case of 2-wire presented international circuits, the virtual analogue switching points can be considered as being adjacent to the 2-wire/4-wire terminating unit (4-wire side). (See Fig-ure
44、4/G.171.) During the set-up and clear-down of a call the loss between virtual analogue switching points (a-b) must satisfy that of Recommendation G.122, 1. The signalling system has an influence on the loss under set-up conditions as explained in Recommenda-tion G.122. If the requirement cannot be m
45、et with the configurations described herein, it will be necessary to increase either the switched or fixed losses. During an established communication, the suggested configurations of Figures 1/G.171, 2/G.171 and 3/G.171 provide for compliance with Recommendation G.122 as follows. Assuming that the
46、PBX subscriber lines have a distribution of stability balance return loss equivalent or superior to that of public subscriber lines and that the distribution has a mean value of 6 dB and a standard deviation of 6.25 dB, then the distribution of stability of loss (a-b) is consistent with the recommen
47、ded distribution of Recommendation G.122, 1 using the same assumptions as contained in that Recommendation. Note In order to obtain the recommended value of stability on 2-wire presented low-loss (e.g. 3 dB) circuits, it will be necessary for the 2-wire/4-wire terminating units to be located at the
48、private exchanges. This may not be necessary on circuits with a higher nominal loss. The CCITT manual cited in 1 gives guidance on this topic. 6 Echo 6.1 Terminating systems for international circuits National terminating systems which interface with international circuits should comply with the ech
49、o loss (a-b) requirements of Recommendation G.122, 2 and the requirements of Recommendation G.131, 2 for the control of echo. During an established communication, the suggested configuration of Figures 1/G.171, 2/G.171 and 3/G.171 provide for compliance with Recommendation G.122, 2 as follows. Assuming that PBX subscriber lines have a distribution of echo balance return loss equivalent or superior to that of public subscriber lines and that the distribution has a mean value of 11 dB with a standard deviation of
