Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

26.2 OVERVIEW OF SIGNALING SYSTEM NO. 7

Signaling System No. 7 (SS7) defines the architecture of the data network to perform out-of-band signaling for call establishment, billing, and routing information of the switches in PSTN/ISDN/GSM. Out-of-band signaling is much more efficient than in-band signaling and also allows value-added services to be provided to the subscribers, and it is now being deployed extensively all over the world.

Signaling System No. 7 (SS7) defines the architecture for a data communication network to exchange signaling information between switches. SS7 facilitates fast and reliable exchange of signaling information.

The signaling information between the network users and the network elements contains the dialed digits, providing various tones such as the dial tone and the call waiting tone. To establish connection between two subscribers, network resources have to be assigned, and this is also a part of signaling. SS7 is the means by which network elements exchange information for call management. This information is exchanged in the form of messages. SS7 can be viewed as a data network that provides the signaling service to the PSTN/ ISDN/GSM. The important features of SS7 are high-speed packet data exchange for signaling information and out-of-band signaling. Separate paths, called signaling links, are used to exchange the signaling messages. The advantages of SS7 are:

• Signaling information can be exchanged even when a call is in progress.

• Messages are transmitted at very high speeds.

• It enables signaling between network elements that do not have trunk connections between themselves.

In SS7, a separate data network is used for signaling. This network consists of three components, as shown in Figure 26.1.

Figure 26.1: A smple SS7 network.

Signaling switching points (SSPs): These are telephone switches with SS7 software and terminating signaling links.

An SS7 network consists of three network elements: (a) signaling switching points (SSPs); (b) signaling transfer points (STPs); and (c) signaling control points (SCPs).

Signaling transfer points (STPs): These are packet switches of the SS7 network. They receive signaling messages and route them to the appropriate destination.

Signaling control points (SCPs): These are databases that provide necessary information for advanced call processing capabilities such as toll-free numbers (800 numbers).

Since continuous operation of SS7 networks is a must even for setting up calls, SS7 networks must have 100% availability. Redundancy is required in the network elements. STPs and SCPs are deployed in pairs for redundancy. Also, the communication links between the network elements are provided with redundancy. A typical SS7 network with SSPs, STPs, and SCPs is shown in Figure 26.1, along with voice trunks and signaling links. Note that in this figure, the STPs A and B perform the same function for redundancy. These STPs are called mated pairs. Each SSP has two links, one for each STP of a mated pair. STPs of a mated pair are joined with a link or a set of links. Two mated pairs of STPs are connected by four links, which are called quads. SCPs also are deployed in pairs, but they are not joined directly by a pair of links. Each link is generally a 56kbps or 64kbps bidirectional link. These links are classified into the following categories:

A links (access links): Interconnect STP and either STP or SCP and interconnect SSP and STP.

B links (bridge links): Quad of links interconnecting peer pairs of STPs.

C links (cross links): Interconnect mated STPs for redundancy.

D links (diagonal links): Quad of links interconnecting mated pairs of STPs at different hierarchical levels.

E links (extended links): SSP is connected to its home STP by a set of A links. To ensure reliability, additional links to a second STP pair called E links are provided. E links are optional.

F links (fully associated links): These links directly connect two signaling end points. F links allow only associated signaling. F links generally are not deployed.

STPs and SCPs are deployed in pairs to provide redundancy so that the signaling network will have 100% availability.