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Each B channel carries data, voice, and other services. The D channel carries control and signaling information for the B channels. B channels can be used as independent channels to simultaneously provide a variety of services such as data, voice, or imaging. They also have the flexibility to be bonded, as needed, to provide higher bandwidth for special needs such as video conferencing. A significant difference between T1 PRI and traditional integrated T1 is that integrated T1 requires a portion of each channel to be allocated for signaling functions. This results in a reduction in effectiveness, efficiency, potential and bandwidth for most functions of each channel of traditional T1. With T1 PRI, all signaling functions are provided by the 24th channel. This leaves each of the other 23 channels with a full 64 kbps which can be fully dedicated to bearer functions. One D channel can provide signaling functions for several other PRI lines. It is not uncommon for two D channels (one as primary, one for backup) to provide the signaling functions for a span of as many as 20 T1 PRI lines. This frees the 24th channel of each of the other PRI lines so that they can be used as additional bearer channels. Because PRI lines serve as a direct connection to the telephone company’s central office, PRI is often the preferred choice of PBX customers. T1 PRI provides many advantages over traditional T1. These include: dynamic channel allocation, call by call service selection, more PBX users can be supported, caller ID, messaging, and video conferencing. Because of increased efficiency provided by PRI, companies which switch to PRI from traditional T1 often find that they not only receive a much greater variety of services but often do not need to pay for as many trunks as they needed prior to the change.
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