SS7

Signaling System #7 (SS7) is the protocol that makes the public telephone networks operate, within themselves and across boundaries. Unlike Q.931, which is designed for simplicity, SS7 is a complete, Internet-like architecture and set of protocols, designed to allow call signaling and control to flow across a small, shared set of circuits dedicated for signaling, freeing up the rest of the circuits for real phone calls.

SS7 is an old protocol, from around 1980, and is, in fact, the seventh version of the protocol. The entire goal of the architecture was to free up lines for phone calls by removing the signaling from the bearer channel. This is the origin of the split signaling and bearer distinction. Before digital signaling, phone lines between networks were similar to phone lines into the home. One side would pick up the line, present a series of digits as tones, and then wait for the other side to route the call and present tones for success, or a busy network. The problem with this method of in-band signaling was that it required having the line held just for signaling, even for calls that could never go through. To free up the waste from the in-band signaling, the networks divided up the circuits into a large pool of voice-only bearer lines, and a smaller number of signaling-only lines. SS7 runs over the signaling lines.

It would be inappropriate here to go into any significant detail into SS7, as it is not seen as a part of voice mobility networks. However, it is useful to understand a bit of the architecture behind it.

SS7 is a packet-based network, structured rather like the Internet (or vice versa). The phone call first enters the network at the telephone exchange, starting at the Service Switching Point (SSP). This switching point takes the dialed digits and looks for where, in the network, the path to the other phone ought to be. It does this by sending requests, over the signaling network, to the Service Control Point (SCP). The SCP has the mapping of userunderstandable telephone numbers to addresses on the SS7 network, known aspoint codes. The SCP responds to the SSP with the path the call ought to take. At this point, the switch (SSP) seeks out the destination switch (SSP), and establishes the call. All the while, routers called Signal Transfer Points (STPs) connect physical links of the network and route the SS7 messages between SSPs and SCPs.

The interesting part of this is that the SCP has this mapping of phone numbers to real, physical addresses. This means that phone numbers are abstract entities, like email addresses or domain names, and not like IP addresses or other numbers that are pinned down to some location. Of course, we already know the benefit of this, as anyone who has ever changed cellular carriers and kept their phone number has used this ability for that mapping to be changed. The mapping can also be regional, as toll-free 800 numbers take advantage of that mapping as well.