Common Channel Signaling System no. 7 (CCS7) was initially designed for fixed line networks. As we will learn in the following topics, CCS7 is also a basis for signaling traffic in the GSM core network and plays an important role in 3G networks after suitable adaptation. An understanding of CCS7 is required to grasp the signaling concepts in wireless networks. This topic introduces the CCS7 network architecture, its layered protocol architecture, and the user parts. CCS7 is also commonly known as Signaling System 7 (SS7).
By definition, signaling is the process of transferring information over a distance to control the setup, holding, charging, and releasing of connections in a communication network. In the past, several different types of signaling system were in use. Some examples of signaling used in core networks are: CCITT, R1, CCITT R2 (National network), CCITT C5, and CCITT C6 (International network).
Prior to CCS7, Channel-Associated Signaling (CAS) was used. In CAS. a dedicated signaling link is required for each speech channel. For example, if a 30-channel PCM is used to interconnect two telephony exchanges, the dedicated signaling channel for each bearer is multiplexed and carried in one of the channels, e.g., in time slot 16. This is not an efficient utilization of resources and is slow, resulting in long call setup time. With the advent of CCS7, a logically separate signaling network is established to transport the signaling information from a large number of bearers. For example, one 64-Kb/s signaling link can carry signaling information for the control of 4096 speech circuits. In addition to its economical use of PCM channels, CCS7 can support a wide range of services and more message types and is much faster. CCST is used both in national and international networks.
