1.1. SS7 and Computer Telephony

Signaling System 7 (SS7) is a common-channel signaling (CCS) system that defines the procedures and protocol by which network elements (signaling points) in the public switched telephone network (PSTN) exchange information over a digital signaling network to facilitate wireline and wireless (cellular) call setup, routing and control.

In an SS7 network, control messages (packets) are routed through the network to perform call management (setup, maintenance, and termination) and network management functions. Therefore, the common-channeling signaling SS7 network is a packet-switched network, even though the network being controlled can be a circuit-switched network (PSTN).

An SS7 network is comprised of network elements connected together using signaling links. Such a network element that is capable of handling SS7 control messages is called a Signaling Point (SP). All signaling points in a SS7 network are identified by a unique code known as a point code.

There are three different basic types of network elements:

• Signaling Transfer Point (STP) - A signaling point that is capable of routing control messages; that is, a message received on one signaling link is transferred to another link.
• Service Control Point (SCP) - Contains centralized network databases for providing enhanced services. An SCP accepts queries from an SP and returns the requested information to the originator of the query. For example, when an 800 call is initiated by a user, the originating SP sends a query to an 800 database (at the SCP) requesting information on how to route the call. The SCP returns the routing information to the SP originating the query and the call proceeds.
• Service Switching Point (SSP) - A signaling point in a switching office, either a local exchange or a tandem office. An SSP has the capability to control voice circuits via a voice switch. The SSP can either integrate the voice switch or can be an adjunct computer to the voice switch.

Network elements are interconnected using signaling links. A signaling link is a bidirectional transmission path for signaling, comprised of two data channels operating together in opposite directions at the same data rate. The standard rate on a digital transmission channel is 56 or 64 kilobits per second (kbps), although the minimum signaling rate for call control applications is 4.8 kbps. Network management applications may use bit rates lower than 4.8 kbps.

Figure 1 shows an example of an SS7 network that carries signaling information for the underlying PSTN network nodes.

Figure 1. Signaling and Information Transfer Networks

The signaling network is independent of the circuit-switched network. Signaling links can be physically located on trunks that carry voice circuits, but can also be completely independent, or even use a different transmission medium (for example, serial V.35). SSPs are the bridges between both networks.

To ensure reliable transfer of signaling information in an environment susceptible to transmission disturbances or network failures, an SS7 network employs error detection and error correction on each signaling link. An SS7 network is normally designed with redundant signaling links and includes functions for the automatic diversion of signaling traffic to alternative paths in case of link failures.

Another type of network element that appears in an Intelligent Network (IN) is the Intelligent Peripheral (IP). An IN is a service-independent telecommunications network, that is, a network in which intelligence is taken out of the switch and placed in computer nodes that are distributed throughout the network. An IP is an SP that provides enhanced services to the SSP, usually under control of an SCP. Those services range from providing user-input prompts and collecting digits to providing a complete service application.

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