A block schematic diagram of the CCITT no. 7 signaling system is shown in fig. Signal messages are passed from the central processor of the sending exchange to the CCS system. This consists of the microprocessor based subsystem. The signaling control subsystems, the signaling termination subsystem and the error control subsystem. The signaling control subsystem structures the messages in the appropriate format and queues them for transmission. When there are no messages to send, it generates filler messages to keep the link active. Messages then passed to the signaling termination sub system, where complete signal units (SU) are assembled using sequence numbers and check bits generated by the error control subsystem. At the receiving terminal, the reverse sequence is carried out. The levels are as follows:
Level 1: The Physical Layer
Level 2: The Data Link Level
Level 3: The signaling network level
Level 4: The User Part
The relationship between these levels and the layers of the OSI model is shown in Fig. The user part encompasses layers 4 to 7 of the OSI model.
Level 1 is the means of sending bit streams over a physical path. It uses times lot 16 of a 2 M bit/s PCM system or times slot 24 of a 1.5 M bit/s system. Level 2 performs the functions of error control, link initialization, error rate monitoring, flow control and delineation of messages. Level 3 provides the functions required for a signaling network. Each node in the network has a single point odd, which is a 14 bit address. Every message contains point codes of the originating and terminating nodes for those messages. Levels 1 to 3 form the message transfer part (MTP) of CCITT no. 7 . Level 4 is the user part. This consists of the processes for handling the service being supported by the signaling system. The message transfer part is capable of supporting many different user parts. So far, three have been defined: the telephone user part (TUE), the data user part(DUP) and the (ISDN) user part (ISDN-UP).