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SS7 is a protocol and set of functions used by telephone switches to communicate and pass call information. Most of us never realized that the SS7 signaling happening between switches makes possible many of the features on our phones.

Signaling between switches is a necessary function that provides dialing digits, dial tone, call-waiting tone and services like busy-number redial. SS7 signaling also provides services such as call establishment, billing, routing and information exchange, and enables special functions between the switches. What makes SS7 different from past signaling methods is its use of out-of-band instead of in-band signaling to pass information.

Before SS7, signaling took place on the same line that was used for the call itself. So once the call was set up, there was no way for switches to communicate. There was also no way for switches to communicate without a direct trunk connection. But SS7's out-of-band signaling occurs on a separate channel from the call itself. It's carried on a D (digital) channel, whereas the voice call or data traffic is carried on a B (bearer) channel of a circuit.

If you want to use SS7 on your network, you must have three elements in place: SSP, STP and SCP--all of which are usually duplicated within the network for redundancy. SSPs (signal switching points) are telephone switches equipped with SS7 software that can originate, terminate or switch calls. STPs (signal transfer points) are the packet switches of the network that receive and route the incoming signaling messages to their destinations. The last piece of the puzzle, the SCP (signal control point), is a database that provides information about the advanced call-processing capabilities that can be provided to or used by the network.