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Signalling System 7 (SS7)


Bellcore developed SS7 as a signalling protocol to operate on CCS T1/E1 circuits within the ISDN and Broadband ISDN environment. SS7 replaces trunk inband signalling between PSTN switches, Local Exchange Carrier (LEC) switches and International Exchange Carrier (IEC) switches. The signalling protocol uses a separate channel from the data/voice (hence CCS) and is used for:
  • Setting up calls
  • Traffic management
  • Fault management
  • Network management
  • Routing
  • Billing
  • Information Exchange
  • Special call treatment
No voice or data is run across the signalling channel, only signalling information. The voice and data are run across the bearer channels.


SS7 requires an architecture and this is illustrated below:

SS7 architecture

  • Switching Service Points (SSP) are switches containing SS7 signalling software and signalling links to pairs of Signalling Transfer Points (STP).
  • Signalling Transfer Points (STP) route signalling traffic. They work together as mated pairs, sharing the traffic load. The links are known as Link Sets because they are always in pairs.
  • System Control Points (SCP) are databases that provide special call processing and routing information for mobile phones, call centre applications etc. SCPs link to STPs as mated pairs.

Point Codes (PC)

Each signalling point is an SS7 node and has a unique address called a Point Code (PC). International PCs are 1 x 4-bit in length. The US and China use 2 x 4-bit PCs, however they are incompatible with each other. The PCs allow the SS7 nodes to identify who is sending the message and who is to receive the message.

The PC has a hierarchy to it which has the following components:
  • Network Code - identifying the network
  • Network Cluster - the group of nodes using the same mated pair of STPs
  • Network Cluster Member - individual node.
Along with the PC you also get a Network Indicator (NI) that shows the type of call being made. An example could be that an NI of 0 would route a call to the pair of STPs that act as the international gateway. NIs are also used where international carriers use the same PCs. The NIs can be used to distinguish one from the other.

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