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+).#/-0!2!4)6%G0G0!$6!.4!%3G0G0/b) completely separate, in which case signalling eq
2、uipment is not an integral part of the carrier system; thismay be termed “completely separate channel signalling“.1 Advantages of in-band signalling1.1 In-band signalling can be applied to any type of line plant. The application of out-band signalling, and built-in separate channel signalling, is li
3、mited to carrier systems.1.2 Through-signalling can be employed at transit points, and at carrier system terminals when a telephonecircuit comprises two or more carrier links. No direct current repetition and thus no delay and no distortion of signalsarises at such points. Out-band signalling and bu
4、ilt-in separate channel signalling require a direct current repetition atsuch points.1.3 Replacement of a faulty line section is easy. In the case of completely separate channel signalling,replacement of a faulty line section is based on security arrangements.1.4 It is impossible to set up a connect
5、ion on a faulty speech path. In the case of completely separate channelsignalling, a continuity check of the speech path is required.1.5 The full bandwidth of the speech channel is available for signalling. This facilitates the use of more than onesignalling frequency. Normally the full bandwidth pe
6、rmits faster signalling than with a smaller signalling bandwidth.With in-band signalling, realization of this advantage is limited to those signals not required to be protected againstsignal imitation due to speech currents.2 Advantages of out-band signalling2.1 Relative freedom from disturbances du
7、e to speech currents; freedom from disturbances due to echo-suppressors; freedom from disturbances which might arise from connections to other signalling systems. With in-bandsignalling it is necessary to take steps to guard against such disturbances.2.2 Possibility of signalling, during the setting
8、up of the call, by either discontinuous or continuous transmission,and the possibility of transmitting those signals during speech. Signalling during speech is not compatible with in-bandsignalling.2 Fascicle VI.1 - Rec. Q.202.3 Simplicity of terminal equipment due to 2.1 above and to the possibili
9、ty of continuous signalling.Out-band signalling (where the same channel carries both speech and signalling) also has the advantage of1.3 of in-band signalling.Built-in separate channel signalling has the advantages of 2.1, 2.2 and 2.3 of out-band, and the advantageof 1.3 of in-band signalling.Comple
10、tely separate channel signalling has the advantages of 2.1 and 2.2 of out-band signalling and,compared with out-band signalling and built-in separate channel signalling, has the additional advantages that no directcurrent repetition is necessary, and no distortion of signals arises, at carrier system terminals when a circuit comprisestwo or more carrier links.
