Si

Address digits

Error check

2 bits 2 bits

(b) Multi unit message (MUM)

2 bits 2 bits

(b) Multi unit message (MUM)

Fig. 7.8. Basic CCS signalling message formats.

7.4.2. CCS Network

CCS network improved the performance of the existing network and also established a plat from for the introduction of new facilities. The first CCS of AT & T were installed in the toll network between a No. 4 A cross bar switch in Madison, Wisconsin and a No. 4 ESS switch in Chicago, Illinois in 1976. The CCS network links between SPC switching offices increases the speed of long distance call connect timings and reduces the cost compared to the inchannel SF/ MF signalling facilities. Fig. 7.9 shows a typical CCS network.

Fig. 7.9. CCS basic structure.

The CCS network consists of two types of nodes. They are signal transfer points (STP) and Signal control points (SCP). These nodes are interconnected by signalling links. The original communication protocols used between CCS entities was CCITT signalling system No. 6 (CCS 6). The modified protocol introduced in 1980 by CCITT is signalling system No. 7.

The STP's are the packet switching nodes of the CCS network. They receive and route incoming signalling messages towards the proper destination. They also perform specialized routing functions. Signal control points (SCP) are data bases that provide information necessary for advanced call processing capabilities. SCP also serves how to route calls, verify credit cards, process special services etc. The same basic structure is also installed within local Access Transport Area (LATA)'s to extend CCS feature all the way to end offices or local exchanges (LE).

7.4.3. CCS Implementation

Common channel signalling may be implemented in three ways. They are (a) Channel Associated mode (b) Channel Non-Associated mode and (c) Quasi-Associated mode. Fig. 7.10. shows all the three modes of CCS signalling.

In associated CCS signalling mode, there is a direct link between two exchanges. In this mode, the signalling path passes through the same set of switches as does the speech path. Network topologies of the signalling network and the speech network are the same. This mode of operation is simple, economic and easy to control. This involves in delayed operation for long distance communication.

In non-associated CCS signalling, there are separate control of the networks from the switching machines themselves. In multiexchange network, signal message passing through several intermediate nodes is referred as non-associated signalling. The network topologies for the signalling and the speech networks are different. Between exchanges, many STP's are placed. This approach is flexible as far as the routing is concerned. It demands more comprehensive scheme for message addressing than is needed for channel associated signalling.

(a) Associated signalling

(a) Associated signalling

(b) Non-associated signalling

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