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Signalling system 7

SS7 stands for Signalling System No. 7. It is a set of telephony signalling protocols used to set up, manage, and terminate circuit-switched telephone calls in public switched telephone networks (PSTNs).

SS7 was developed in the 1970s by the International Telecommunication Union (ITU) to replace earlier signalling systems like SS1 and SS2. It was standardized in 1988 with the recommendation Q.700 series. SS7 was designed to provide more efficient and reliable signalling for the growing complexity of telephone networks.

SS7 operates by using a separate network of signalling links that run parallel to the voice and data circuits. It uses a common channel signalling (CCS) method, where signalling information is transmitted over a separate channel from the voice or data traffic. This allows for more efficient call setup and management.

SS7 uses a variety of signalling messages to control call setup, maintenance, and termination. These messages are transmitted between signalling points, which can be switches, service control points, or signal transfer points. The protocol supports features such as call routing, call forwarding, and conference calling.

In a simple example, when a user dials a number on a telephone, the following SS7 signalling process occurs:

  1. The originating switch sends an Initial Address Message (IAM) to the destination switch, containing the dialed number and other call setup information.
  2. The destination switch sends an Address Complete Message (ACM) back to the originating switch, indicating that the call is being set up.
  3. Once the call is established, the originating switch sends a Connect Message (CON) to the destination switch.
  4. The destination switch sends a Connect Acknowledge Message (CAK) back to the originating switch, confirming the connection.
  5. The call is now active, and voice traffic can flow between the two switches.
  6. When the call is terminated, a Release Message (REL) is sent from one switch to the other, followed by a Release Complete Message (RLC) to confirm the termination.