Signaling

In the world of telephony, some information must be exchanged between the PBX and the end users, such as the phone number of the caller and the phone number of the callee, in order to place a call. This is referred to as the signaling information, and it usually contains much more than just phone numbers; for the sake of this section, however, it can be considered what controls the calls. The signaling information is how a call is placed, transferred, or ended. The actual voice traffic, or the audio a user speaks and hears, is considered the media. So in any call there is signaling information to control the call, and a media stream that is the actual audio heard. Keep in mind that the path taken by these two items does not have to be identical.

Signaling information can come in the form of in-band or out-of-band. In-band means the information shares the same channel or line as the media. The most common form of in-band signaling is Dual-Tone Multi-Frequency signaling (DTMF), which is what is sent when pressing keys on a phone. Each key transmits a unique tone, indicating a different piece of information to the PBX.

Signaling can also be carried out-of-band, which is typical for PBX trunk lines to the PSTN or when connecting directly to another PBX. Out-of-band signaling usually uses a dedicated channel for the signaling information while the media or actual voice traffic is carried in different channels. Take a North American T1 connection as an example; there are 24 channels each with 64Kbps of bandwidth available. For a primary rate interface (PRI), the first 23 channels will each carry voice traffic, so with one call per channel 23 simultaneous calls are supported. The 24th channel carries the signaling information for all the first 23 channels concurrently. This is considered out-of-band because the signaling and media are in separate channels on the connection. An example of this is shown in Figure 17.4.

Figure 17.4. Out-of-band signaling.