SS7 (Signaling System Number 7) is the network control signaling protocol utilized by the Integrated Services Digital Network (ISDN) services framework. ISDN control information for call handling and network management is carried by SS7. SS7 is a large and complex network designed to provide low latency and to have redundancy in many network elements. The SS7 control-signaling network consists of signaling points, signaling links and signaling transfer points. Signaling links or SS7 links interconnect signaling points. Signaling points (SSP) use signaling to transmit and receive control information. A signaling point that has the ability to transfer signaling messages from one link to another at level 3 (SS7 level 3 will be described in detail later) is a Single Transfer Point (STP). There is a fourth entity, the Service Control Point (SCP), which acts as a database for the SS7 network. The STP queries the SCP to locate the destination of the calls. The design of the SS7 protocol is such that it is independent of the underlying message transport network. The design of the signaling network is very important in that it will directly impact the availability of the overall system. In general, the network will be designed to provide redundancy for signaling links and for STPs. Figure 1 shows a basic SS7 network.
Figure 1: SS7 Signaling Endpoints in a Switched-Circuit Network
A typical call can be illustrated using Figure 1. User A goes off-hook in New York and begins dialing. User A is calling User C in San Francisco. The dialed digits are transmitted across the local loop connection to a local switch that has signal point functionality (SSP). The local switch translates the digits and determines the call is not local to itself. The local switch will use its signal point functionality to signal into the SS7 network to a Signal Transfer Point (STP). The STP queries a SCP to locate the destination local switch. The STP signals to the destination local switch to alert it of the incoming call. The destination local switch rings the phone of User C. User C answers and the two local switches signal across the SS7 network and determine the bearer path through the PSTN. Once the path is setup the call begins. When either user goes on hook, the network signals the other end to tear down the bearer path and the call is terminated. The worldwide SS7 network is divided into national and international levels.This allows the numbering plans and administration to be separated.