Understand basic of Signalling System 7 (SS7)

Signalling system network is used for switching of messages pertaining to call handling of the network. All nodes in SS7 network are called signalling points. There are many types of signalling points exist like SCP, OAM centres, exchanges etc.

Signalling points are further divided in two parts:

  1. Signalling end points (SEP).
  2. Signalling transfer point (STP).

Messages from one SEP to another SEP travel through STP. STP does not examine messages, it just route them to next destination.

Two SEP are connected using at least with two STPs so that one STP remain active in case of failure of another STP. Each signalling points is identified with a code i.e. Point Code. Point code can be originating point code and destination point code.

Signalling links: Links between two signalling points are called signalling links.

                All the signalling links between two signalling points make a signalling link set.

                Group of signalling links between two signalling points are called signalling link group.

Signalling route: A pre-defined path between signalling points is called signalling route.

Like OSI model, SS7 layers are divided, and mainly in two parts i.e. Message transfer parts and User parts.

Message transfer part: this layer serves as a transport system and responsible of reliable transfer of messages. This layer is consist of three sub layers:

  1. MTP-1: This layer take care of functions related to physical and electrical information transfer between two signalling points.             
  2. MTP-2: Error detection and correction of messaged happen on this layer.
  3. MTP-3: This layer do routing, distribution and discrimination of message.

User Parts: There are several user parts which run on top of MTP layer. All user part information make use of MTP layer to flow information. User parts are divided further as below:

  1. SCCP: Signalling connection control part contains some extra functionalities like it can understand different type of information between signalling points e.g. understand global title, understand database connections etc.
  1. TCAP (Transaction capabilities application part): this function is used to connect and retrieve information from connected databases.
  2. MAP (Mobile application part): It transfer network information from one network to another.
  3. OMAP (Operations maintenance and administration part): It helps in administration and monitoring of network components.
  4. ISUP (ISDN user part): this is used for ISDN network.

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