Level 1 is 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 a1.5 M bit/s system.
Level 2 performs functions of error control, error rate monitoring, link initialization, delineation of messages andflow control.
Level 3 provides functions essential for a signaling network. Every node in the network has a single point odd that is a 14 bit address. Every message comprises point codes of the originating and terminating nodes for those messages.
Levels 1 to 3 form message transfer part (MTP) of CCITT no. 7.
Level 4 is user part. This comprises the processes for handling the service being supported by signaling system. Message transfer part is capable of supporting many different user parts. So far, three have been defined: telephone user part (TUE), data user part(DUP) and (ISDN) user part (ISDN-UP).