1、ITU-T *RECMN*G-122 93 m 4862571 0589493 453 m INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU G.122 (O 3/93) TRANSMISSION SYSTEMS AND MEDIA GENERAL CHARACTERISTICS OF NATIONAL SYSTEMS FORMING PART OF INTERNATIONAL CONNECTIONS INFLUENCE OF NATIONAL SYSTEMS
2、ON STABILITY AND TALKER ECHO IN INTERNATIONAL CONNECTIONS ITU-T Recommendation G.122 (Previously “CC ITT Recommendation”) FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying techn
3、ical, 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 the topics for study by the ITU-T Study Groups which
4、, in their turn, produce Recommendations on these topics. ITU-T Recommendation G.122 was revised 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 reform process within the International Telecommunication Union (ITU), th
5、e CCITT ceased to exist as of 28 February 1993. In its place, the ITU 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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of th
6、is 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 will contain the proper terminology related to the new ITU structure. 2 t
7、elecommunication 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 may be reproduced or utilized in any form or by any means, electronic or m
8、echanical, including photocopying and microfilm, without permission in writing from the ITU. CONTENTS 1 Introduction 2 Loss (a-b) to avoid instability during set.up. clear-down and changes in a connection 3 4 Unweighted loss (a-b) on established connections Echo loss (a-b) on established connections
9、 Annex A - Measurement of stability loss (a-b) and echo loss (a-b) . Annex B - Explanation of terms associated with the path a-f-b . B.l Return loss B.2 Balance return loss B.3 Transmission loss of the path a-r-b . B.4 B.5 B.6 Stability and echo losses . Overall loudness rating of the echo path (tal
10、ker echo loudness rating, TELR) Summary of useful terms References . Recommendation G.122 (03193) Page 1 2 3 3 4 6 6 6 7 9 10 11 11 1 ITU-T *RECMN*G*122 93 48b259L 0589497 OT9 Recommendation G.122 INFLUENCE OF NATIONAL SYSTEMS ON STABILITY AND TALKER ECHO IN INTERNATIONAL CONNECTIONS (Geneva, 1964;
11、amended at Mar del Plata, 1968; Geneva, 1972,1976 and 1980; Malaga-Torremolinos, 1984; Melbourne. 1988 and Helsinki, 1993) 1 Introduction The information provided in this Recommendation applies to all national systems. Representations of a national system which extend up to the virtual analogue swit
12、ching points are shown in Figure 1. 3.5 dBr -1- / / / / / A - /b V v / / I ,r V v v v - 4 dBr (for anaiogue international circuit) 1 tt Niitkrul system pry %wire -3.5 dBr (for digital internatiid circuit) NOTE -a, b are the virtual analogue switching points of the international circuit. FIGURE VG.12
13、2 National system representation and virtuai analogue switching point definition The transmission loss introduced between a and b by the national system, referred to as the loss (a-b), is important from three points of view: a) it contributes to the margin that the intemational connection has agains
14、t oscillation during the setting-up and clearing-down of the connection. A minimum loss over the band 0-4 kHz is the characteristic value; b) it contributes to the margin of stability during a communication. Again, a minimum loss in the band 0-4 kHz is the characteristic value, but in this case the
15、subscribers apparatus (telephone, modem, etc.) are assumed to be connected and in the operating condition; c) it contributes to the control of echoes and, in respect of the subjective effect of talker echo, a weighted sum of the loss (a-b) over the band 300-3400 Hz is the characteristic value. Recom
16、mendation G.122 (03/93) 1 In addition, echoes circulating in any of the 4-wire loops in the national system or in the international 4-wire chain,give rise to listener echo, which can affect voice-band analogue data transmission. The requirements stated in this Recommendation represent network perfor
17、mance objectives. 2 Loss (a-b) to avoid instability during Set-up, clear-down and changes in a connection 2.1 Instability should be avoided during all normal conditions of Set-up, clear-down and other changes in the composition (e.g. call-transfer) of a complete connection. To ensure adequate stabil
18、ity of intemational connections, the distribution (taken over many actual calls) of the loss (u-b) during the worst situation should be such that the risk of a loss (a-b) of O dB or less does not exceed 6 in loo0 calls when using the calculation method applied in 2.2. This requirement should be obse
19、rved at any frequency in the band O to 4 kHz. NOTES 1 The signalling and switching systems have an influence on the loss (a-b) under Set-up and clear-down conditions. For example, in some systems 4-wire registers control the Set-up and do not establish the 4-wire path until the answer signal is succ
20、essfully received. In others, circuits are released immediately if busy conditions are encountered. In these circumstances the risk of oscillation does not anse. 2 Recommendation Q.32 gives information on methods of securing an adequate loss (a-b) of an incoming national system before the called-sub
21、scriber answers (Le. while ringing tone is transmitted) or if busy or number unobtainable conditions are encountered. 3 If there are no such arrangements as described in Notes 1 or 2 above then, in general, it would be safe to assume that there is no balance return loss provided by the called local
22、telephone circuit (if 2-wire). In this case the necessary loss (a-b) must be provided by the transmission losses in the national system. The stability of international telephone connections at frequencies outside the band of effectively transmitted frequencies (Le. below 300 Hz and above 3400 Hz) is
23、 governed by the following transmission losses at the frequencies of interest: the balance return loss at the terminating units; the transmission losses of the terminating units; the transmission losses of the 4-wire circuits. Conditions which only last for a few tens of milliseconds can be left out
24、 of consideration because in such a short time 4 - - - 5 oscillations cannot build up to a significant level. 2.2 the national network: 1) The limit recommended in 2.1 may be met, for instance, by imposing the following simultaneous conditions on The sum of the nominal transmission losses in both di
25、rections of transmission a-b and r-b measured between the 2-wire input of the terminating set t, and one or other of the virtual switching points on the international circuit, u or b should not be less than (4 + n) dB, where n is the number of analogue or mixed analogue-digital 4-wire circuits in th
26、e national chain. The stability balance return loss at the terminating set t, should have a value not less than 2 dB for the terminal conditions encountered during normal operation. 3) The standard deviation of variations of transmission loss of a circuit should not exceed 1 dB 2) (see 3/G.151). In
27、a calculation to verify if these values are acceptable, it may be assumed that (see l): - - there is no significant difference between nominal and mean value of the transmission losses of circuits; variations of losses for both directions of transmission of the same circuit are fully correlated; - d
28、istributions are Gaussian. For the loss (a-b), it then results: Mean value: 2+4+n = 6+ndB Standard deviation: fi dB With n = 4 the mean value becomes 10 dB and the standard deviation 4 dB, resulting in a probability for values lower than O dB of 6. lW3. 2 Recommendation G.122 (03193) NOTE - There is
29、 no need for the two quantities 0-1 and t-b to be equal, so that differential gain can be used in the national network. This practice may be needed to meet the requirements of UG.121, but it implies that the transmission loss in terminal service of the 4-wire chain plus the terminating sets may be d
30、ifferent according to the direction of transmission. The choice of the nominal value of the transmission loss r-b should in all cases be made with an eye to YG.121 dealing with the minimum sending reference equivalent to be imposed in each national chain, to avoid any risk of overloading in the inte
31、rnational network. 3 Unweighted loss (a-b) on established connections 3.1 The objective is that the risk of the loss (a-b) reaching low values at any frequency in the range 0-4 kHz should be as small as practicable. This requires restrictions on the distribution of values of stability loss (a-b) for
32、 the population of actual international calls established over the national system. Such a distribution can be characterized by a mean value and a standard deviation. The objective will be obtained by a national system sharing a mean value of at least (10 + n) dB together with a standard deviation n
33、ot larger than J6.25.4n dB in the band 0-4 kHz; where n is the number of analogue or mixed analogue-digital 4-wire circuits in the national chain. Other distributions are acceptable as well, as long as they yield equivalent or better results calculated according to the convention of i. NOTES i See N
34、ote in 2.2. 2 In the more conventional case of a of Figure 1, the loss (a-b) is calculated as the sum of circuit losses, terminating loss and stability balance return loss. In fact the loss (a-b) at a given frequency is the sum of the circuit losses at that frequency plus the balance return loss at
35、the same frequency. For planning purposes, it may be assumed that the stability loss is equal to or greater than the sum of the stability balance return loss plus the sum of the circuit losses at the reference frequency. This follows from the observation that the least circuit loss typically occurs
36、in the vicinity of the reference frequency. 3 Wholly digital circuits may be assumed to have a transmission loss with mean value and standard deviation equal to zero. Voice coders in circuits or in exchanges are expected to offer smaller variations in transmission loss than camer circuits. For the v
37、ariations in transmission loss of a coder-decoder combination, standard deviations in the order of 0.4 dB have been reported. 4 The subscribers apparatus (telephone, modem, etc.) in the local telephone circuit are assumed to be “off hook” or equivalent, and thus providing balance return loss. 5 In p
38、ractice, the distribution of stability balance return loss is distinctly skew, most of the standard deviations being provided by values above the mean. It could be unduly restrictive to assume a normal distribution. 6 The CCITT manual cited in 2 describes some of the methods proposed, and in some ca
39、ses successfully applied, by Administrations to improve balance return losses. 3.2 The distribution of stability loss (a-b) recommended in 3.1 above could, for example, be attained if, in addition to meeting the conditions of 2.2, the mean value of the stability balance return loss at the terminatin
40、g set is not less than 6 dB and the standard deviation not larger than 2.5 dB. 4 Echo loss (u-b) on established connections 4.1 In order to minimize the effects of echo on international connections it is recommended that the distribution of echo loss (a-b) for the population of actual international
41、calls established over the national system should have a mean value of not less than 15 + n dB with a standard deviation not exceeding ,/%, where n is the number of analogue or mixed analogue-digital 4-wire circuits in the national chain. Other distributions are acceptable as well, as long as they y
42、ield equivalent or better results calculated according to the convention of Supplement No. 2 Recommendation G. 13 1, NOTES 1 Echo suppressors and cancellers according to Recommendations G.164 and (3.165, typically require 6 dB of signal loss (a-b) for the actual signai converging the canceller or be
43、ing controlled by the suppressor. This signal loss (a-b) is the ratio of incident to reflected signal power on the return path. The value of signal loss (a-b) will depend both upon the loss (a-b) frequency response and the signal spectrum. Therefore, it is desirable from a performance point of view
44、that the stability loss (a-b) during an established connection should be at least 6 dB, since this will ensure proper operation for any signal (frequency spectrum) in the band 0-4 kHz. Recommendation G.122 (03193) 3 ITU-T *RECMN*G=L22 93 4862591 0589500 413 However it may not be practical to always
45、achieve this level of performance, especially at the higher frequencies characteristic of voice-band data signals. For speech, typically the speech signai loss (a-b) will be at least 6 dB if the echo loss is 6 dB. However, for voice-band data signals a higher echo loss is required to ensure a data s
46、ignal loss (a-b) of 6 dB. For some data signais an echo loss of at least 10 dB is required. It should be noted that some modems operating half-duplex on satellite circuits equipped with echo cancellers may require proper operation of the canceller to prevent long delay echoes that exceed the receive
47、r squelch period from causing data transmission problems. 2 See Note 2 in 3.1. In a similar manner for planning purposes it can be assumed that the echo loss is equal to or greater than the sum of the echo balance return loss and the circuit losses at the reference frequency. 3 Recommendation G.131
48、provides guidance on the application of echo control devices. 4.2 negative slope of 3 dB/octave starting at 300 Hz, extending to 3400 Hz. as follows: The echo loss (a-b) is derived from the integrai of the power transfer characteristic Acf) weighted by a 3*A (f) Echo loss Le = 3.85 - 10 log10 1 7.1
49、dB L 300 J where -Lab (f) A(f) = 10 O with L - unwanted signals that may give rise to errors, e.g. hum, circuit noise, carrier leak signals, are prevented from entering the receiver. Appropriate arrangements (not shown) are needed for automatic or manual access to the 4-wire switches at the international centre and also to ensure that due account is taken of the transmission levels at the actual switching points. As far as the stability measurement is concerned, if a sweep oscillator is used, attention must be paid to the risks of engendering false operat
