1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STAN DARD IZAT I O N SECTOR OF ITU Supplement 31 (Series G) (03/93) TRANSMISSION SYSTEMS AND MEDIA GENERAL CHARACTERISTICS OF NATIONAL SYSTEMS FORMING PART OF INTERNATIONAL CONNECTIONS PRINCIPLES OF DETERMINING AN IMPEDANCE STRATEGY FOR T
2、HE LOCAL NETWORK Supplement 31 to ITU-T Series G Recommendations (Previously “CCITT Recommendations”) STD-ITU-T RECMN SERIES G SUPP 31-ENGL 1993 111111 4862591 0671930 719 FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication
3、Union. The ITU-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 World Telecommunication Standardization Conference (WTSC), which meets every four years, established th
4、e topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. Supplement 31 to ITU-T Series G Recommendations was prepared by the ITU-T Study Group XII (1988-1993) and was approved by the WTSC (Helsinki, March 1 - 12,1993). NOTES 1 As a consequence of a
5、reform process within the International Telecommunication Union (ITU), the CCITT ceased to exist as of 28 February 1993. In its place, the IT Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replaced b
6、y the Radiocommunication Sector. In order not to delay publication of this Recommendation, no change has been made in the text to references containing the acronyms “CCITT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation w
7、ill contain the proper terminology related to the new ITU structure. 2 telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a O ITU 1994 All rights reserved. No part of this publication ma
8、y 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. STD-LTU-T RECMN SERIES G SUPP 31-ENGL 1793 W 4662571 Ob717LL 655 CONTENTS 1 2 3 3.1 3.2 4 4.1 4.2 4.3 4.4 4.5 4.6 5 5.1 5.2 5.3 5.4 Int
9、roduction Structure . Relevant impedances Principal impedances Customer and network equipment. . Influence of impedances on principal transmission parameters . General 3 Sidetone ratings (STMR and LSTR) Talker echo . 4 Stability 4 Listener echo Loss frequency distortion . Scope 5 Digital local trans
10、mission . Outline approach to determining values for key impedances . Analogue 2-wire cabling Analogue/digital intenvorking situations Page 1 1 1 1 2 3 3 supplement 3 1 PRINCIPLES OF DETERMINING AN IMPEDANCE STRATEGY FOR THE LOCAL NETWORK (Helsinki, 1993) (referred to in Series G Recommendations) 1
11、Introduction This Supplement gives the background and guidance required to support a consistent approach to impedance planning in the local network. It is clear that impedance issues are having an increasingly important influence on end-to-end performance as they can affect a significant number of t
12、ransmission parameters. There is a need to bring together the important impedance issues relating to the local part of the overall connection. Previous contributions to CCITT have indicated that it is highly unlikely that a single set of impedances will be suitable for all networks hence this Supple
13、ment does not set out to derive such a unified set of values. The aim is to identie the overall issues and provide sufficient guidance that the transmission planner can develop the optimum solution for his local situation whilst still meeting the CCITT objectives for overall performance. For the pur
14、poses of this Supplement ?local network? is taken to include all the customer equipment and network equipment required to provide the transmission path between the acoustic reference point at the customer?s telephone instrument and the first 4-wire digital point in the national and international 4-w
15、ire chain of circuits. This, therefore, includes all the analogue portions of the connection where impedance can be an issue. 2 Structure The Supplement is divided into three main clauses: - impedances (e.g. input and balance impedances); - rating, echo and stability. The clause quantifies the contr
16、ibution of the specific impedances to these parameters and identifies any existing Recommendation covering specific performance objectives; - network situation or to reappraise an existing situation. an outline of the relevant network components (e.g. digital local exchange) and the associated impor
17、tant a guide to the influence of the impedances on the principal transmission planning parameters such as loudness an outline of a recommended procedure enabling development of a consistent set of impedances for a new local 3 Relevant impedances 3.1 Customer and network equipment There are clearly a
18、 very large number of combinations of customer and network equipment that could be used to access the national 4-wire chain of circuits. Customer equipment can include fixed or cordless telephones (analogue or digital), analogue or digital PAJ3Xs and the interconnecting leased lines. In addition, it
19、 is important that non-speech terminal equipment such as voice-band data modems is taken into account as echo can affect the performance of these devices. The network equipment will include the local loop (analogue or digital) and analogue or digital exchanges with their interconnecting junctions. W
20、hilst it is difficult to illustrate every combination of equipment it is possible to indicate the combinations most likely to influence the impedance arguments. Figure 1 shows typical customer and network equipment, including possible interconnection arrangements for access to the 4-wire chain for c
21、ommonly occurring equipment. As an example it shows a 2-wire telephone connected to the national 4-wire chain via 2 PABXs, a 4-wire local line, an analogue local exchange and a PCM multiplexor. Figure 1 also identifies the 2-wire to 4-wire conversion units (also referred to as hybrids in the followi
22、ng text) which influence the impedance planning. T CT DT DCT HIT, H9 PC HI H2 H3 H4 H5 H6, H7 H8 H9 I I I I l I I 2-wire telephone connected to PABX or local loop Cordless digital telephone connected to PABXor local loop Digital telephone connected to PABX OT digital local loop Cordless telephone co
23、nnected to PABX or digital local loop 2-wire to 4-wire converting units Primary centre PCM mux terminating 4-wire chain 2-wire interface on Digital Local Exchange (DLE) Local line transmission mux (exchange end) Remote mux 2-wire interface for DT and DCT 2-wire interface for T and CT PABX 2-wire ext
24、ension interface PABX 2-wire local loop interface TI 203250-91 1401 FIGURE 1 Local network interconnections 3.2 Principai impedances From Figure 1 the principal impedances are as follows: - - the input and balancing impedances of the various 2-wire to 4-wire conversion units denoted as H1 to H9; the
25、 impedances associated with the 2-wire cable sections (e.g. 2-wire local loop); the “impedance transparency” of the 2-wire switches (e.g. analogue local exchange); the impedance of 1 port customer terminal equipment (e.g. telephone instruments or data modems). - - The above impedances all need to be
26、 taken into account when considering the principal transmission planning parameters. 28 - - - STD.1TU-T RECMN SERIES G SUPP 31-ENGL 1773 48b259L Ob71914 3b4 4 Influence of impedances on principal transmission parameters 4.1 General This subclause considers each impedance (or group of impedances) in
27、turn and identifies its contribution to the principal transmission parameters. The parameters covered are: - sidetone ratings (STMR, LSTR); - taker echo; - stability loss; - listener echo; - loss frequency distortion. The aim is for the subclause to provide: - - a reference to identie the effect of
28、any one impedance on overall performance; the basic data to enable clause 5 to propose a logical approach to impedance planning. 4.2 Sidetone ratings (STMR and LSTR) Deffitions and objectives for STMR and LSTR are found in Annex AG. 11 1. The assumption that STMR applies for round-trip delays of up
29、to 3 ms will be used here; for longer delays the taker echo approach will be used. The contribution of impedance matching to achieving the objectives can best be illustrated by the expressions below taken from Annex AG.111. STMR = SLR(set) + RLR(set) + A,-1 where A, is a function of the telephone in
30、put and sidetone balancing impedances and the impedance presented to the telephone set. From Figure 1 the key impedances influencing the value of A, and hence the sidetone performance are: - - - - - balance impedances of telephone hybrids H5, H6 and H7; balance impedance of PBX hybrid H9; balance im
31、pedance of local transmission mm; input impedance of hybrids H1, H2, H4 and H8; image impedance of 2-wire cable sections. From this list it may be concluded that: a) values of sidetone and should be considered together; b) presented to the telephone set should be considered together; cl listed in b)
32、. balance impedances of hybrids part of the sidetone paths (H3, H5, H6, H7 and H9) will have similar effects on input impedances of hybrids H1, H2, H4 and H8, i.e. those having a significant influence on the impedance optimum performance would be achieved if the impedances listed in a) had the simil
33、ar values to the impedances It will be noted that there are both upper and lower limits for sidetone, not very far apart. To respect these limits for all lengths of line presents a problem for an impedance strategy, which is different according to whether regulated or non-regulated telephones are us
34、ed. For non-regulated telephone sets, SLR and RLR remain constant in equation (i), so that Am (and hence the impedance presented to the telephone set) must remain substantially constant, irrespective of line length. For regulated sets, SLR and RLR both increase for sort lines, so that A, must corres
35、pondingly fall off, i.e. the exchange impedance can, and should, differ quite considerably om the image impedance of the line. 4.3 Talker echo The factors governing talker echo and its effect on customer opinion are discussed in Supplement 2G.131 - Talker echo on International Connections. This Supp
36、lement shows that the Overall Loudness Rating of the talker-echo path for connections with 2-wire points is strongly influenced by the echo balance retuni loss at the listeners end. Echo balance return loss is a weighted average of the individual balance return losses at fiequencies in the range 300
37、-3400 Hz. (See Recommendation G. 122). Echo balance return loss is a function of the various impedances of the connection which need to be considered in any echo strategy. Annex B/G. 122 shows that for the case where the national extension includes a 2-wire section echo loss is a hction of the balan
38、ce impedances of the hybrid terminating the 4-wire national and international chain and the impedance presented to that hybrid, i.e. the combined effect of the local network impedances. From Figure 1 the key impedances influencing talker-echo performance are: - the balance impedance of hybrids that
39、may terminate the 4-wire chain of circuits; - the input impedance of hybrids connected to the 2-wire connection sections; - the image impedance of 2-wire cable sections. From this list it may be concluded that: a) sidetone path and should be considered together; balance impedances of hybrids “facing
40、” the echo path will have similar effects on the echo balance return loss b) input impedances of relevant hybrids connected to the 2-wire section should be considered together; cl listed in b). optimm performance would be achieved if the impedances listed in a) had similar values to the impedances 4
41、.4 Stability 4.4.1 Call phases The stability of connections involving 2-wire and 4-wire sections is of importance both during the call Set-up (and clear-down) phase and the conversation phase. In both cases the value of interest is the lowest loss of the path a-t-b in the 0-4 kHz frequency range as
42、shown in Figure UG.122. As outlined in Recommendation G.122 the balance return loss of the relevant hybrids are a key factor in determining the lowest loss of the path a-t-b; this is normally called the stability balance return loss - the lowest value in the 0-4 kHz frequency range. However, the ter
43、minating conditions for the 2 phases are not equal hence they need to be treated separately as outlined in Recommendation G. 122. 4.4.2 Stability balance return loss of an established connection A comparison of 4.3 and 4.4 will show that echo balance return loss (EBRL) and stabiliy balance return lo
44、ss (SBRL) of an established connection are both calculated from the same data, i.e. balance return losses at frequencies in the range 0-4 Hz. The difference being that EBRL is a weighted average of balance return losses in the range 300-3400 Hz and SBRL is the lowest value of balance return loss in
45、the range 0-4000 Hz. The impedances that are significant in determining talker echo (and EBRL) are also the key impedances for determining SBRL of an established connection, hence the conclusions in 4.3 apply. STDOITU-T RECMN SERIES G SUPP 3L-ENGL 1993 48b259L Ob7L9Lb 137 a 4.4.3 Stability balance r
46、eturn loss during Set-up and clear-down For the Set-up and clear-down condition, customer terminal equipment and network equipment does not necessarily present the same impedances as under conversational conditions. Where these impedances are not known it is common practice to assume worst case term
47、inating impedances, e.g. open circuit and short circuit, and evaluate SBIU. Under these conditions the input impedance of the relevant hybrids are also important as the mismatch factor can be a significant contributor to the balance return loss. However, it should be recognized that SBRL is only one
48、 of the factors uiriuencing stability and should not be treated in isolation. For example, it may be that a digital exchange will not connect both directions of transmission before the called subscriber answer signal is received, hence there will be no path for oscillations to build up. These aspect
49、s are discussed in Recommendation G.122. 4.5 Listener echo Listener echo can be experienced by telephony customers where it results in a “hollow” quality which may be found disturbing. This can be encountered when 4-wire digital exchanges (including PBXs) are set up with a low transmission loss to interwork with existing 2-wire cabling. Listener echo for telephony customers is determined by the balance return losses of the relevant hybrids in a similar manner to talker echo. The same impedances therefore determine the contribution to both talker echo and listener echo
