PRI (Primary Rate Interface) is an ISDN connection designed to be provided over T1 lines. In the United States, a PRI line consists of 24 (64 kbps each) DS0 channels. PRI lines are commonly configured so that 23 channels serve as B (Bearer) channels and one as a D (delta) channel. 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 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 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 PRI lines. This frees the 24th channel of each of the other PRI lines so that they can be used as additional bearer channels. 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. Integrated T1 PRI can come in many forms and variations, as mentioned above, but the most common integrated T1 PRI is a combination of PRI and data internet channels on one integrated T1 line. This service is very popular for small to medium companies which need all of the telephone functionality that PRI can offer, plus an extremely stable internet connection, but only have the budget for one T1 circuit.
Other advantages of channelized DS1 can include; one provider, one easy to read bill and one point of contact for all of the above mentioned services, flexibility in keeping your same phone numbers, much lower costs than if the above applications are purchased individually on separate T1 lines, and the flexibility to evolve as your business grows. Channelized DS1 is commonly confused with dynamic integrated T1, and occasionally confused with fractional t1. In actuality, one channelized DS1 circuit can include the functionality of multiple fractional T1 services, for far less money.