InnoWave Telecommunications

Common Channel Signalling

Introduction of SPC digital switching systems with high speed processors in the telecom network has necessiated modernisation of signalling. Also, in order to meet the transfer of varieties of informations for call management and network management and to satisfy the subscribers requirement on various features, the uninterrupted, high speed signalling has become inevitable. The rapid dvelopment of digital systems paved way for the new signalling system called common channel signalling.

Instead of using the same link for signalling information and message as in Inchannel signalling, the common channel signalling (CCS) uses a dedicated line for the signalling information between the stored program control elements of switching systems. Fig. 7.7 shows the basic schematic of CCS.

Exchange A or Exchange B

data link

Fig. 7.7. Basic schematic of CCS.

Exchange A or Exchange B

data link

Fig. 7.7. Basic schematic of CCS.

The data link sends messages that identify specific trunks and events. Two signalling channels, one for each direction are used in a dedicated manner to carry signalling information. Hence, they are capable of carrying information for a group of circuits. At the bit rate of 2.6 kbps, CCS can carry signals for 1500-2000 speech circuits. CCS network is basically a store and forward network. In CCS network, the signalling information travels in a link-by-link basis along the route. The information arrived at a node is sotred, processed and forwarded to the next node in the route. The CCS technique is also called the transparent mode for signalling.

7.4.1. CCS Signalling Message Formats

CCS signalling information is transferred as signalling units, which is of varying length with one or more fixed length. A signalling unit is divided into a number of fields. The fields may be address unit, centralised service message unit, acknowledgement unit, synchronisation or idle unit, management message unit etc. The signalling information includes data related to routing, addressing digits, inter exchange information, routing status to the originating exchange maintenance request/details etc. Fig. 7.8 shows the typical CCS signalling message format.

h— 5 bits —4 bits -+H-11 bits--8 bits-H

Header			Signalling information	Circuit label	Error check
(a) Single unit message (SUM)
Header			Signalling information	Circuit label	Error check

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