The international Engineering Consortium defines Signaling System 7 (SS7) as "a style of architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signaling-system network and a protocol to enable their performance." In this brief glimpse into SS7, we will try to deliver a useful overview of the many elements and core competencies that make up Signaling System 7.
1. What is signaling?
Signaling refers to the exchange of information between call components required to provide and maintain service.
As users of the PSTN, we exchange signaling with network elements all the time. Examples of signaling between a telephone user and the telephone network include: dialing digits, providing dial tone, accessing a voice mailbox, sending a call-waiting tone, dialing *66 (to retry a busy number), etc.
SS7 is a means by which elements of the telephone network exchange information. Information is conveyed in the form of messages. SS7 messages can convey information such as:
· I'm forwarding to you a call placed from 214-555-1234 to 817-555-5678. Look for it on trunk 067.
· Someone just dialed 800-555-1212. Where do I route the call?
· The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone.
· The route to XXX is congested. Please don't send any messages to XXX unless they are of priority 2 or higher.
· I'm taking trunk 143 out of service for maintenance.
High-speed packet data and out-of-band signaling characterize SS7.
2. What is Out-of-Band Signaling?
Out-of-band signaling is signaling that does not take place over the same path as the conversation.
We are used to thinking of signaling as being in-band. We hear dial tone, dial digits, and hear ringing over the same channel on the same pair of wires. When the call completes, we talk over the same path that was used for the signaling. Traditional telephony used to work in this way as well. The signals to set up a call between one switch and another always took place over the same trunk that would eventually carry the call. Signaling took the form of a series of multifrequency (MF) tones, much like touch-tone dialing between switches.
Out-of-band signaling establishes a separate digital channel for the exchange of signaling information. This channel is called a signaling link.