Signaling

To facilitate users who are able to call each other, there must be some information exchanged between the PBX and the end users, such as the phone number of the caller and the phone number of the callee. This is referred to as the signaling information, and usually contains more than just phone numbers. However, for the sake of this text, 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.

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 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 uses a dedicated channel for the signaling information while the media or actual voice traffic is carried in different channels. Using a T1 connection as an example, there are 24 channels each with 64 kbps of bandwidth available. The first 23 channels carry the voice traffic, so 23 simultaneous calls are supported. The channel 24 carries the signaling information for all of the first 23 channels. This is considered out-of-band because the signaling and media are in separate channels on the connection, as shown in Figure 17.4.

Figure 17.4 Out-of-Band Signaling