1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3#/2$).!4)/.G0G0/(b) that the use of such a radio link, in a long-distance telephone circuit, implies certain special conditions,which introduce particular difficult
2、ies 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 suffe
3、rs 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, toavoid disturbance of the radio r
4、eceiver 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 transmissionperformance, it is necessary to take special measures to ensure that the radio transmitter always operates,as far as possibl
5、e, under conditions of full loading, whatever may be the nature and the attenuation of thetelephone 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 b
6、e effectively transmitted by international landline circuits,has been determined by a study of the requirements of the human ear, this band (for a radiotelephonecircuit operating at a frequency below 30 MHz) may be limited by the necessity of obtaining the maximumnumber of telephone channels in this
7、 part of the radio-frequency spectrum and so that each telephonechannel 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 servicebetween two extended networks, and this fact is of great
8、importance from two points of view:i) on the one hand, international conversations, in general, are of great importance to the subscribersand, on the other hand, they are made in languages which are not always their mother tongue, so thathigh quality reception is particularly important;ii) the publi
9、c should not be deprived of a very useful service under the pretext that it does not alwayssatisfy the degree of excellence desirable for long-distance communication,unanimously recommends1 Circuits above 30 MHzthat between fixed points, telephone communications should be effected wherever possible
10、by means ofmetallic conductors, or radio links using frequencies above 30 MHz to make the allocation of radio frequencies less_1CCIR Recommendation 335 1.2Further reference to 30 MHz in this Recommendation means “about 30 MHz“.2 Fascicle III.2 - Rec. G.451difficult; where this can be realized, the o
11、bjective should be to attain the transmission performance recommended by theCCITT for international telephone circuits on metallic conductors;2 Circuits below 30 MHz2.1 that since it becomes necessary to economize in the use of the frequency spectrum, when consideringinternational circuits which con
12、sist 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 the300 to 3400 Hz recommended by the CCITT for landline circuits and, preferably, to reduce
13、the upper frequency of thespeech band to 3000 Hz or less, but not below 2600 Hz, except in special circumstances;2.2 that, although it will be necessary to tolerate large variations in noise level on such a radiotelephone circuit,every possible effort should be made to obtain minimum disturbance to
14、the circuit from noise and fading by the use ofsuch techniques as full transmitter modulation, directional antennas and single-sideband operation;2.3 that, during the time that such a radiotelephone circuit is connected to an extension circuit equipped with echosuppressors (voice-operated switching
15、device), the intensity of disturbing currents should not be sufficient to operate theecho suppressor frequently;2.4 that such a radiotelephone circuit should be provided with an echo suppressor to avoid singing or echodisturbance on the complete circuit, or, preferably, with terminals using the prin
16、ciples of constant overall transmissionloss, as set forth in CCIR Recommendation 455 2;2.5 that such a radiotelephone circuit should be equipped with automatic gain control to compensate automatically,as far as possible, for the phenomenon of fading;2.6 that the terminal equipment of such a radiotel
17、ephone circuit should be such that it may be connected, in thesame way as any other circuit, with any other type of circuit;2.7 that, where privacy equipment is used, this equipment should not appreciably affect the quality of telephonetransmission;2.8 that, when suitable automatic devices are not p
18、rovided, the circuit controls should be adjusted, as often asnecessary, by an operator to ensure optimum adjustment of transmitter loading, received volume and the operatingconditions of the echo suppressor.Note - Although the requirements contained in 2 are much less severe than those imposed on in
19、ternationallandline circuits, the objective remains to attain the same standards of telephone transmission in all cases. In view of this,it is desirable that the telephone systems connected to a radiotelephone circuit should conform to CCITTRecommendations covering the general conditions to be met b
20、y international circuits used for landline telephony,especially in respect of equivalent, distortion, noise, echoes and transient phenomena.Bearing in mind the recommendations contained in 1 and 2, it is desirable that in each particular case,Administrations concerned should first reach agreement on
21、 how far the standards usually employed for internationallandline circuits may be attained in the case considered. If the technique of 1 can be used, the objective should be toobtain, as far as possible, the characteristics recommended by the CCITT for international landline circuits. Otherwisethe A
22、dministrations concerned should study the best solution from the point of view of both technique and economy.References1 CCIR Recommendation Use of radio links in international telephone circuits, Vol. III, Rec. 335, Dubrovnik,1986.2 CCIR Recommendation Improved transmission system for HF radiotelephone circuits, Vol. III, Rec. 455,Dubrovnik, 1986.
