3.1. Signaling

The digital data stream contains two kinds of information: user data and signaling data. The signaling data is used to control the transmission of user data. For example, in telephony applications user data is usually digitally encoded voice data; in file transfer applications, user data is packets of High-level Data Link Control (HDLC) encoded information. Signaling data carries information such as the current state of the channel (for example, whether the telephone is on-hook or off-hook). Signaling data can also indicate who is calling, the type of call (voice or data), and the number that was dialed.

ISDN protocols use an out-of-band signaling method, which means the signaling data is carried on a channel or channels separate from user data channels. The user data is transmitted in eight bit samples on B or "bearer" channels, and the signaling data is transmitted outside of the eight bit sample on D channels.

In addition to carrying signaling data out-of-band, ISDN technology uses common channel signaling (CCS). This means that one signaling channel (D channel) carries signaling data for more than one user data channel (B channel). In BRI, two channels of user data for every one channel of signaling data are transmitted. In PRI T-1 lines, 23 channels of user data (B channels) and one channel of signaling data (D channels) are transmitted. In PRI E-1 lines, 30 B channels and 1 D channel are transmitted. Common channel signaling also allows the transmission of additional information, such as ANI and DNIS digits, over the signaling channel.

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