1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G02%#/-%.$!4)/.3G0G0/.G0G04%,%0(/.%37)4#().G0G0!.$G0G03).!,).).4%2.!4)/.!,G0G0!54/-!4)#G0G0!.$3%-)G13!54/-!4)#G0G07/2+).2%$5#4)/.G0G0/ this refers in particular to conditions other than that of an est
2、ablishedconnection, viz. during set-up, clear-down and changes in a connection. Appropriate precautions must then be taken bythe switching services.Techniques applicable to analogue exchanges which will afford a reduction of the risk of instability for anational network have been shown in earlier ve
3、rsions of Recommendation Q.32 (Red Book 1985 and earlier). Fordigital exchanges these methods are as a rule not equally suitable, however, it should be noted that, with todays digitalnetworks giving 4-wire transmission down to the local exchanges and with corresponding terminating losses, thetransmi
4、ssion plan may often not require extra loss during setting-up, etc., conditions.Recommendation G.121, 6.2 calls for a sum of losses round the a-t-b path of at least 6 dB; calculatingaccording to Recommendation G.122, 2.2, this would be some four times the standard deviation, corresponding to arisk o
5、f about 3 in 10 000. (The six calls per thousand risk called for in Recommendation G.122 corresponds to about3.25 times the standard deviation.) The switching services thus only need to maintain this minimum loss in cases whereit is reduced in the conditions mentioned.The use of a restricted value o
6、f loss (rather than total interruption of the 4-wire loop) allows the passage ofinformation tones or recorded announcements or of communication with an operator, and of national use for non-chargeable calls. Although as a rule digital pads are deprecated, the reasons for this are all concerned with theirpresence in an established connection, and do not apply to their use for the present purpose.
