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+).30,)44).G0G0!22!.%-%.43G0G0!.$3).!,G0G02%#/.)4)/.G0G04)-%3).G0G0 ).G13“!.$ G0G03
2、).!,).G0G03934%-3)45G134G0G0RecommendationG0G01 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation Q.25 was published in Fascicle VI.1 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book versi
3、on, thecontents 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 atelecommunication administration and a recognized operating agency. ITU 1988, 1
4、993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VI.1 - Rec. Q.25 1Recommendation Q.25Fascicle VI.1 - Rec. Q.25SPLITTING AR
5、RANGEMENTS AND SIGNAL RECOGNITION TIMESIN “IN-BAND“ SIGNALLING SYSTEMS1 GeneralIn each “in-band“ signalling system precautions should be taken so that, when the signalling in that system istaking place:1.1 no interference in the voice-frequency range from outside the system can pass into the system
6、(i.e. into thetransmission path between the sending end and the receiving end of the voice-frequency signals), and1.2 as far as possible, no signalling current used in the system can pass into other systems, connected in tandem.2 Sending-end splitting arrangements2.1 In order to satisfy the conditio
7、n in 1.1 above, care should be taken that the correct operation of the signalreceiver at the other end of the circuit is not disturbed by:- surges (transient currents) caused by the opening or closing of direct current circuits connected to thespeech wires of the switching equipment, whether these s
8、urges precede or follow the sending of a signal;- noise, speech currents, etc., coming from tandem switched circuits, preceding or during the sending of asignal.2.2 For this reason the following arrangements have been made in the Signalling Systems No. 4 and No. 5 for thetransmission of voice-freque
9、ncy signals on the international circuit:i) The exchange side of the circuit shall be disconnected 30 to 50 ms before a voice-frequency signal is sentover the circuit.ii) The exchange side of the circuit will not be reconnected for 30 to 50 ms following the end of the sendingof a voice-frequency sig
10、nal over the circuit.2.3 Arrangements of the same type are required on System R1 and on national “in-band“ systems see 3.4.1 b)below.3 Receiving-end splitting arrangements3.1 General3.1.1 In order to satisfy the condition in 1.2 above, the length of the part of a signal which passes into anothersyst
11、em is limited by splitting the speech wires beyond the signal receiver when a signal is received and detected bythis receiver.2 Fascicle VI.1 - Rec. Q.25The time during which the first part (sometimes called spillover) of a received signal passes into anothersystem, until the splitting becomes effec
12、tive, is called “splitting time“.Too long a splitting time may result in interference to signalling on a tandem system depending on the signalrecognition time on the tandem system.Too short a splitting time may result in an increase in the number of false operations of the splitting device byspeech
13、currents (signal imitation) and so impair speech transmission.The splitting time must therefore be a compromise between the above two factors.The splitting device also serves to limit the duration of signals on one path of the 4-wire circuit from returningover the other path by reflections at the te
14、rmination; these reflections may give rise to faulty operation of signallingequipment on the other path.3.1.2 The protection against mutual interference between in-band signalling systems in international serviceinvolves limitations of the length of any part of:3.1.2.1 the international signal that
15、may be able to pass:a) from the international signalling system into a national signalling system (protection of the nationalsystem);b) from one international signalling system into another international signalling system, when they areswitched in tandem (protection of the international systems);c)
16、from one international circuit into another international circuit of the same system when they areswitched in tandem in the case of link-by-link signalling.3.1.2.2 the national signal that may be able to pass:a) from the national signalling system into an international signalling system (protection
17、of the internationalsystem);b) from one national signalling system into the national signalling system of another country via aninternational connection (protection of the national system).3.2 Protection of national and international systems against international systemsConditions in 3.1.2.1 above a
18、re met because international signalling systems have a splitting device on eachcircuit. The splitting times of such systems are:55 milliseconds for the compound signal element in System No. 4;35 milliseconds for a signal in System No. 5;20 milliseconds for a signal in System R1.3.3 Protection of the
19、 international system against national systemsThe condition in 3.1.2.2 a) above is generally covered because:- the values given in the specifications of the CCITT standard systems as the minimum recognition time ofa line signal are in general greater than the splitting times of national systems (see
20、 the tables giving thebasic characteristics of national signalling systems in Supplement No. 3 at the end of this fascicle);- the signalling frequencies used in the international systems are, in the majority of countries, differentfrom those used in national systems.It may be necessary, if the split
21、ting time of a national signalling system is greater than the minimum signalrecognition time of an international system and the signalling frequencies used in the national system and internationalsystem are the same or nearly the same, to insert a device at the international exchange which will prev
22、ent a part of thenational signal from passing into the international circuit for longer than this recognition time.Fascicle VI.1 - Rec. Q.25 33.4 Interference between national signalling systems when they are interconnected via an international circuit3.4.1 To ensure protection of national signallin
23、g systems one against the other protection defined under 3.1.2.2 b) above, it has been recommended by the CCITT since 1954 that new national “in-band“ signalling systemsshould comply with the following two clauses:a) not more than 35 milliseconds of a national signal should be able to pass into anot
24、her country;b) the connection between an international circuit and a national circuit should be split on the nationalcircuit at the international exchange 30 to 50 milliseconds before that exchange sends any signal over thenational signalling system.Note - The object of these two clauses is to avoid
25、 interference, especially in conditions that may exist oninternational automatic connections.3.4.2 The requirement of 3.4.1 a) permits the signalling system used in country A to have a minimum signalrecognition time based on this value of 35 milliseconds. It will then be possible to ensure, without
26、taking any otherprecautions at the incoming end of an international circuit, that no fraction of a signal coming from country B, andbeing of the same, or nearly the same, frequency as that used in country A, will be wrongly recognized as a signal incountry A.One method of meeting the requirement of
27、3.4.1 a) is to adopt a splitting time of less than 35 millisecondsfor the national systems.Another method exists which does not involve such a limitation in the splitting times of national systems, andwhich might be preferred when the design of the national signalling system is such that a short spl
28、itting time is notnormally justified for that system alone. This second method involves the introduction, in the international exchange,of an arrangement for limiting the length of national signals which are liable to pass into the international circuit. Suchan arrangement would be used only on circuits to those countries where there is a danger that interference might arise.3.4.3 The requirement of 3.4.1 b) avoids the false operation of the guard circuit of a signal receiver situated at thedistant end of a national circuit.
