ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits

pdf标准 ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits

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ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits  第 1 页 ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits  第 2 页 ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits  第 3 页 ITU-T G.451-1988 在国际电话电路中无线链路的使用 Use of radio links in international telephone circuits  第 4 页
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国外分类,ITU,ITU-T(电信标准分局),itu,国际,电话,电路,无线,链路,使用,use,of,radio,links,in,international,telephone,circuits

INTERNATIONAL TELECOMMUNICATION UNION )45 4 ' TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU ).4%2.!4)/.!, !.!,/'5% #!22)%2 3934%-3 #//2$).!4)/. /& 2!$)/4%,%0(/.9 !.$ ,).% 4%,%0(/.9 53% /& 2!$)/ ,).+3 ). ).4%2.!4)/.!, 4%,%0(/.% #)2#5)43 )45 4 Recommendation ' (Extract from the "LUE "OOK) NOTES 1 ITU-T Recommendation G.451 was published in Fascicle III.2 of the Blue Book. This file is an extract from the Blue Book. 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 conciseness to indicate both a telecommunication administration and a recognized operating agency. © 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 in writing from the ITU. Recommendation G.451 Fascicle III.2 - Rec. G.451 USE OF RADIO LINKS IN INTERNATIONAL TELEPHONE CIRCUITS1) The CCITT, considering (a) that, at the present time, radiotelephone systems connecting the various countries often employ carrierfrequencies below about 30 MHz2) ; (b) that the use of such a radio link, in a long-distance telephone circuit, implies certain special conditions, which introduce particular difficulties not encountered when purely metallic connections are used; (c) that such a radiotelephone circuit differs from a metallic circuit in the following ways: 1) such a radiotelephone circuit is subject to attenuation variation with the special difficulty of fading; 2) such a radiotelephone circuit suffers from noise caused by atmospherics, the intensity of which may reach, or even exceed, a value comparable with that of the signal which it is desired to receive; 3) special precautions are necessary in the setting up and maintenance of such a radiotelephone circuit, to avoid disturbance of the radio receiver by any radio transmitter and especially by its own radio transmitter; 4) to maintain the radiotelephone link in the best condition from the point of view of transmission performance, it is necessary to take special measures to ensure that the radio transmitter always operates, as far as possible, under conditions of full loading, whatever may be the nature and the attenuation of the telephone system connected to the radiotelephone circuit; 5) it is necessary to take measures to avoid or correct conditions of abnormal oscillation or cross-talk; 6) although the recommended frequency band, to be effectively transmitted by international landline circuits, has been determined by a study of the requirements of the human ear, this band (for a radiotelephone circuit operating at a frequency below 30 MHz) may be limited by the necessity of obtaining the maximum number of telephone channels in this part of the radio-frequency spectrum and so that each telephone channel does not occupy a radio-frequency band larger than necessary; 7) in general, such a radiotelephone circuit is a long-distance international circuit giving telephone service between two extended networks, and this fact is of great importance from two points of view: i) on the one hand, international conversations, in general, are of great importance to the subscribers and, on the other hand, they are made in languages which are not always their mother tongue, so that high quality reception is particularly important; ii) the public should not be deprived of a very useful service under the pretext that it does not always satisfy the degree of excellence desirable for long-distance communication, unanimously recommends 1 Circuits above 30 MHz that between fixed points, telephone communications should be effected wherever possible by means of metallic conductors, or radio links using frequencies above 30 MHz to make the allocation of radio frequencies less _______________ 1 CCIR Recommendation 335 [1]. 2 Further reference to 30 MHz in this Recommendation means "about 30 MHz". Fascicle III.2 - Rec. G.451 1 difficult; where this can be realized, the objective should be to attain the transmission performance recommended by the CCITT for international telephone circuits on metallic conductors; 2 Circuits below 30 MHz 2.1 that since it becomes necessary to economize in the use of the frequency spectrum, when considering international circuits which consist mainly of single long-distance radio links operating at frequencies less than 30 MHz, it is desirable to use single-sideband transmission to the maximum extent possible, to employ a speech band less than the 300 to 3400 Hz rec

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