1.2. ISDN Signaling Concepts

ISDN protocols use an out-of-band signaling method, carrying signaling data on a channel or channels separate from user data channels. This means that one signaling channel (D channel) carries signaling data for more than one bearer channel (B channel). This signaling technique is referred to as common channel signaling (CCS). Signaling data carries information such as the current state of the channel (for example, whether the telephone is on-hook or off-hook). Common channel signaling allows the transmission of additional information, such as ANI and DNIS digits, over the signaling channel.

An ISDN Primary Rate Interface (PRI) trunk provides a digital link that carries some number of TDM (Time Division Multiplexed) channels:

• a T-1 trunk carries 24, 64 Kbit channels - 23 voice/data channels (B channels) and one signaling channel (D channel), on a single 1.544 MHz digital link
• an E-1 trunk carries 32, 64 Kbit channels - 30 voice/data channels and two additional channels: one signaling channel (D channel) and one framing channel to handle synchronization, on a single 2.048 MHz digital link.

The ISDN digital data stream contains two kinds of information: user data and signaling data used to control the communication process. For example, in telephony applications user data is digitally encoded voice data. Voice data from each time slot is routed to a separate B channel. Signaling data carries information such as the current state of the channel (for example, whether the telephone is on-hook or off-hook). The signaling information for all B channel information is routed to the D channel of the device.

Global Call's primary rate implementations comply with most switch protocols worldwide. For the most up-to-date list of available protocols, contact your nearest Sales Office or visit our web site.

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