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COMMON-CHANNEL SIGNALING In channel-associated signaling (CAS) systems, the signaling information for a trunk is carried by the trunk itself. In common-channel signaling (CCS), a common signaling link (SL) carries signaling messages for a number of trunks. Just as multi-frequency (MF) signaling became feasible with the introduction of the second-generation (common-control) switching systems, CCS was developed for the third-generation (stored program controlled, SPC) exchanges that were introduced in the 1960s. . | Signaling in Telecommunication Networks. John G. van Bosse Copyright 1998 John Wiley Sons Inc. ISBNs 0-471-57377-9 Hardback 0-471-22415-4 Electronic INTRODUCTION TO COMMON-CHANNEL SIGNALING In channel-associated signaling CAS systems the signaling information for a trunk is carried by the trunk itself. In common-channel signaling CCS a common signaling link SL carries signaling messages for a number of trunks. Just as multi-frequency MF signaling became feasible with the introduction of the second-generation common-control switching systems CCS was developed for the third-generation stored program controlled SPC exchanges that were introduced in the 1960s. There are several reasons for the move from multi-frequency signaling to CCS 1 1. It is often less costly to interface the processing equipment of SPC exchanges with a relatively small number of signaling links than to provide pools of MF registers and line-signaling hardware for the individual trunks. 2. Common-channel signaling is much faster than multi-frequency signaling. Early CCS systems already reduced post-dialing delays on long-distance calls from 10-15 s to around 3 s. 3. New telecommunication technology and services require the transfer of additional signaling information for processing a call. In Chapter 4 we have already encountered some additional information items echo suppressor information calling line category etc. . In CCITT and CCITT-R2 signaling this information is in the form of digits with special meanings see Section and Tables and . Commonchannel signaling messages provide a more flexible way to transfer both 109 110 INTRODUCTION TO COMMON-CHANNEL SIGNALING the classical supervision and address signals and other types of callcontrol information. 4. Subscribers cannot access the CCS signaling links. This avoids the bluebox fraud problems that have plagued many frequency-division multiplexed FDM trunk groups that use channel-associated signaling with in-band signal .

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