Signaling System 7 (SS7)

On the public switched telephone network (PSTN), Signaling System 7 (SS7) is a system that puts the information required to set up and manage telephone calls in a separate network rather than within the same network that the telephone call is made on.

On the public switched telephone network (PSTN), Signaling System 7 (SS7) is a system that puts the information required to set up and manage telephone calls in a separate network rather than within the same network that the telephone call is made on. Signaling information is in the form of digital packets. SS7 uses what is called out-of-band signaling, meaning that signaling (control) information travels on a separate, dedicated 56 or 64 Kbps channel rather than within the same channel as the telephone call. Historically, the signaling for a telephone call has used the same voice circuit that the telephone call traveled on (this is known as in-band signaling). Using SS7, telephone calls can be set up more efficiently and with greater security. Special services such as call forwarding and wireless roaming service are easier to add and manage. SS7 is now an international telecommunications standard.

SS7 is used for these and other services:

  • Setting up and managing the connection for a call
  • Tearing down the connection when the call is complete
  • Billing
  • Managing call forwarding, calling party name and number display, three-way calling, and other Intelligent Network (IN) services
  • Toll-free (800 and 888) and toll (900) calls
  • Wireless as well as wireline call service including mobile telephone subscriber authentication, personal communication service (PCS), and roaming

SS7 messages contain such information as:

How should I route a call to 914 331-4985?

The route to network point 587 is crowded. Use this route only for calls of priority 2 or higher.

Subscriber so-and-so is a valid wireless subscriber. Continue with setting up the call.

Because control signals travel in a separate network from the call itself, it is more difficult for anyone to violate the security of the system. (See 2600 and phreak for cracking techniques that are defeated by SS7.)

The Integrated Services Digital Network (ISDN) also uses out-of-band signaling, extending it all the way to the end user on the ISDN D-channel while voice and data flow on B channels.

Briefly How It Works

SS7 consists of a set of reserved or dedicated channel known as signaling links and the network points that they interconnect. There are three kinds of network points (which are called signaling points): Service Switching Points (SSPs), Signal Transfer Points (STPs), and Service Control Points (SCPs). SSPs originate or terminate a call and communicate on the SS7 network with SCPs to determine how to route a call or set up and manage some special feature. Traffic on the SS7 network is routed by packet switches called STPs. SCPs and STPs are usually mated so that service can continue if one network point fails.

This was first published in April 2005

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