1、ITU-T RECMN*G.L2L 93 4862593 0585558 670 D INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU G.121 (03193) TRANSMISSION SYSTEMS AND MEDIA GENERAL CHARACTERISTICS OF NATIONAL SYSTEMS FORMING PART OF INTERNATIONAL CONNECTIONS LOUDNESS RATINGS (LRs) OF NATIONAL
2、 SYSTEMS ITU-T Recommendation G.121 (Previously “CCITT Recommendation“) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*G=L23 73 M 4862573 0585557 507 FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a pe
3、rmanent organ of the International Telecom- munication 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 Conferen
4、ce (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recommendation G.121 was revised by the ITU-T Study Group XII (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993)
5、. NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITU), the 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
6、and the IFRB have been replaced by the Radiocommunication Sector.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
7、editions of this Recommendation will contain the proper terminology related to the new 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. N
8、o 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. COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Se
9、rvicesITU-T RECMN*G-L2L 93 H 4862591 05855b0 229 CONTENTS Nominal LRs of the national systems . 1.1 Definition of nominal LRs of the national systems 1.2 Traffic-weighted mean values of the distribution of send and receive loudness ratings, SLRs and RLRs . Maximum Send and Receive Loudness Ratings.
10、SLR and RLR . 2.1 Values for each direction of transmission . 2.2 Difference in transmission loss between the two directions of transmission in national systems Minimum SLR Determination of nominal Loudness Ratings . Sidetone 5.1 General 5.2 Talkers sidetone STMR . 5.3 Listeners sidetone LSTR . Inco
11、rporation of PCM digital processes in national extensions . 6.1 Effect on national transmission plans . 6.2 Transmission loss considerations Annex A . Evaluation of the nominal differences of loss between the two directions of transmission Annex B . Transmission considerations for a 4-wire loop inse
12、rted in a 2-wire circuit B.l General B.2 Attenuation . B.3 Impedance . B.4 Sidetone considerations Annex C - Examples of values of R and T pads adopted by some administrations . Recommendation C.121 (0-3) Page 1 1 1 2 2 2 3 3 3 3 4 5 5 5 5 7 8 8 10 10 11 11 i COPYRIGHT International Telecommunicatio
13、ns Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*G=121 93 m 4862591 058.5561 165 m PREAMBLE Clauses 1 to 5 of this Recommendation apply in general to all analogue, mixed analogue/digital and all-digital international telephone connections. However, where recommenda
14、tions are made on specific aspects in clause 6 for mixed analogue/digital or alldigital connections, clause 6 will govern. All sending and receiving LRs in this Recommendation are “nominal values” as explained in clause 4 and are referred to the corresponding Virtual International Connecting Points
15、(VICPs) of an intemational circuit at the International Switching Centre unless otherwise stated. The definition of the Virtual International Connecting Points (VICPs) of intemational circuits can be found in Recommendation 2.1UG.101. Additional information is given in Supplement No. 31 (to this Rec
16、ommendation): “Principles of determining an impedance strategy for the local networks”. The values given below in terms of LR should be used by Administrations to verify that their national systems meet the general objectives resulting from Recommendation G. 1 1 1. Administrations employing CREs sho
17、uld preferably translate the LRs of this Recommendation into their national CREs by the methods given in Annex UG.111 or, as a second choice, apply the values given in Volume III of the Red Book (Geneva, 1985). NOTES 1 2 The main terms used in this Recommendation are defined and/or explained in Anne
18、x NG. 1 1 1. For many telephone sets using carbon microphones, the SLR and STMR values can only be determined with limited accuracy. Recommendation 6.121 (0-3) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECflN*G-LZL 93 = 48b2
19、591 0585562 OTL m Recommendation 6.121 LOUDNESS RATINGS (LRs) OF NATIONAL SYSTEMS (amended at Helsinki, 1993) 1 Nominal LRs of the national systems 1.1 Send and Receive Loudness Ratings, SLRs and RLRs respectively, may in principie be determined at any interface in the telephone network. In this Rec
20、ommendation and in Recommendation G. 1 1 1. the SLRs and RLRs of the national systems are referred to the Virtual International Connecting Points (VICPs) of the first international circuit, as defined in Recommenda- tion 2.lUG.101. Definition of nominal LRs of the national systems The Virtual Intern
21、ational Connecting Points are points in the intemational circuit at the International Switching Centre (ISC). The relative levels at the VICP are: - Sending: OdBr; - Receiving: O dBr for digital circuits; -0.5 dBr for analogue and mixed analoguddigital circuits. NOTES 1 These levels do not necessari
22、ly apply to national circuits, which are governed by the national transmission plan. In determining these levels, Recommendation 2.11G.131 may have been applied, or 2-wire switching may still be in use; see Figure 6/G.101. Earlier the “virtual analogue switching points (VASP)” having the relative le
23、vels -3.5 and 4 dBr were used as the reference points. The concept of relative levels (dBr) and associated terms are defined and explained in Recommendation G.101 and Annex AiG.101. 2 3 1.2 Traffic-weighted mean values of the distribution of send and receive loudness ratings, SLRs and RLRs An object
24、ive for the mean value is necessary to ensure that satisfactory transmission is given to most subscribers. Transmission would not be satisfactory if the maximum values permitted in clause 2 were consistently used for every connection. An appropriate subdivision of the overall loudness requirement is
25、 obtained by the following long-term objectives referred to the VICP: - SLR: 7 tO9dB; - RLR: 1tO3dB. NOTES 1 In some networks the long-term values cannot be attained at this time and appropriate short-term objectives are: - SLR: 7 to 15 dB; - RLR: 1to6dB. 2 3 In some networks the actual traffic dist
26、ribution is known only incompletely. In such cases, subscribers generating heavy traffic, like PBXs, should be given special consideration. The long-term traffic weighted mean values of LRs should be the same for each main type of subscriber categories, such as urban, suburban and rural. Only consid
27、ering the mean value for the whole country in the transmission plan might lead to a discrimination of some important customer groups. 4 5 The ranges stated for SLR and RLR are for planning and do not include measuring and manufacturing tolerances. Some Administrations have found it advantageous in s
28、ome circumstances to include a manual volume control in the receive part of the digital telephone set. See the remarks made in 3.2/G. 11 1. Recommendation 6.121 (OW3) 1 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*G.LZL 9
29、3 4862591 0585563 T38 Country size a) 2 Maximum Send and Receive Loudness Ratings, SLR and RLR NO. of national b) O dBr point VASP circuits in the 4-w chain above the primary centre SLR RLR SLR RLR 2.1 Values for each direction of transmission Average Large Large The maximum SLRs and RLRs given belo
30、w in Table 1 mainly apply when the national system is predominantly analogue. When modernizing networks by digital techniques, efforts should be made to avoid having those maximum values for the national system. up to 3 I 6.5 13 20 9 4 17 13.5 20.5 9.5 5 17.5 14 21 10 TABLE VG.121 Nominal maximum LR
31、s recommended for national systems 2.2 Difference in transmission loss between the two directions of transmission in national systems The difference in nominal loss between the two directions of transmission on an international connection should not exceed 8 dB, preferably not 6 dB. It has been foun
32、d practical to have a certain asymmetry in the nominal loss in the two directions of transmission in the national part of the connection. It is recommended that the difference between loss a-t and loss t-b (see Recommendation G.122) should be 3-9 dB. In a national network where all 4-wire circuits a
33、re digital and extends down to the local exchange, this difference will be equal to the difference between the T and R pads in the local exchange (see Figure 1). Pads chosen by different Administrations are shown in Annex C. The following points should be noted: 1) Bearing in mind that most administ
34、rations allocate the losses of their national extension circuits in much the same sort of way connections set up in practice should not exhibit differences much in excess of 3 dB. 2) As far as speech transmission is concerned, from the studies carried out by several Administrations in 1968-1972, it
35、is clear that for connections with overall LRs falling within the range found in practice, no great disadvantage attaches to any reasonable difference in LR between the two directions of transmission. 3) When devising national transmission plans, Administrations should take into account the needs of
36、 data transmission between modems complying with the pertinent Recommendations. 2 Recommendation G.121 (03/93) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*G*323 93 4862593 0585564 974 = 3 Minimum SLR Administrations must
37、 take care not to overload the intemational transmission systems if they reduce the attenuation in their national trunk network. Provisionally a minimum nominal value of SLR = +2 dB referred to the VICP is recommended in order to control the peak value of the speech power applied to international tr
38、ansmission systems. It should be noted that the imposition of such a limit does not serve to control the long-term mean power offered to the system. In some countries a very low sending loudness rating value may occur if unregulated telephone sets are used. Furthermore, the speech power applied to t
39、he international circuits by operators sets must be controlled so that it does not become excessive. NOTE -The value of +2 dB is under further study. 4 Loudness Ratings and their properties and uses are explained in Annex NG. 1 1 1. There it is explained how the LR of a complete connection can be de
40、termined as the sum of the CLRs of the circuits and the SLR and RLR of the local systems (telephone sets and subscriber lines). The loudness loss between two electrical interfaces in a connection or a circuit, the Circuit Loudness Rating (CLR), is equal to the composite loss at the reference frequen
41、cy 1020 Hz, when each interface is terminated by its nominal impedance which may be complex. Determination of nominal Loudness Ratings “Nominal value” here signifies a “reasonable engineering average” for typical conditions as exemplified in what follows, excluding “worst cases”. Variations with tim
42、e, temperature, etc. are not included in the nominal CLRs. For telephone sets, most Administrations today have to accept a large variety of types which comply with some national specification having rather wide limits. The requirements for SLR and RLR usually refer to a measuring setup with a variab
43、le artificial line terminated by a feeding bridge and a nominal impedance which may be complex or, most often, 600 ohms. The specification is often drawn up in the form of upper and lower limits for SLR, and RLR, as functions of line length (or possibly line current). The “nominal” SLR, and RLR, of
44、telephone set plus subscriber line may then be interpreted as the arithmetic mean between the upper and lower limit curves. In practice, the subjective quality impression of the overall loudness changes rather insignificantly for fairly large variations of OLR around the optimum value and it is unli
45、kely that sets with the worst possible LRs are associated with limiting line lengths. Therefore, rather wide manufacturing tolerances, commonly about f3 dB, can be accepted for the individual set SLR (set) and RLR (set). (SLR (set) and RLR (set) refer to set measurements without the subscriber line
46、but as function of line current, including the 1 dB bandwidth correction.) Note however, that the sum of SLR (set) + RLR (set) for an individual 2-wire telephone set must be controlled more carefully so that is does not decrease below a certain minimum value. The reason is that, under certain circum
47、stances. subscribers react very unfavourably to strong sidetone and talker echo. Both effects depend directly on this LR sum in addition to the unavoidable network impedance variations. This minimum limit is often translated into a minimum limit for STMR as measured against a specified impedance. Se
48、e clause 5 for a discussion. 5 Sidetone 5.1 General Especially for those connections approaching the limits for high Loudness Ratings and/or noise, further transmission impairments should be avoided. One important precaution is to ensure that an adequate sidetone performance is maintained for the va
49、rious circuit combinations occurring in the telephone system. (“Adequate” is in most cases to be interpreted as a sufficiently high sidetone loss.) Recommendation 6.121 (0393) 3 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECHN*G*LZL 93 4862591 0585565 800 For 2-wire telephone sets, the sidetone performance is basically dependent on set sensitivity and impedance variation limits as explained in Annex NG. 11 1. Thus, a national transmission plan should not only give rules for allocation of losses in the network but
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