T1 PRI is actually a very specialized telecom service. T1 lines have been used for over 50 years to transport telephone calls, and more recently point to point data, dedicated Internet service, and as connections to private networks. The PRI in T1 PRI stands for Primary Rate Interface. That's an important sounding term that comes from a set of standards known as ISDN or Integrated Services Digital Network. The integrated implies more that one service on the line and that's exactly what you get.
The first T1 telephone lines were organized as 24 channels each with 8 bits of data at a rate of 8 KHz. That's just the right size channel to carry one telephone call. So a T1 voice line can carry 24 simultaneous telephone calls. But since all the bandwidth is used up transporting the digitized phone calls, the dialed number has to be transfered as DTMF, better known as TouchTones, over the same voice channel. Plus, bits had to be "robbed" from the voice channels to transmit the on hook / off hook handset condition. This type of signaling is called "in band." In other words, the T1 voice channels do double duty by carrying both the signaling and voice information.
ISDN PRI cleans this situation up by assigning one complete T1 channel for signaling and the other 23 for voice only. You'll hear this referred to as 23B+D. That's 23 B or bearer channels plus 1 D or delta (sometimes called data) for signaling. This is called "out of band," "common channel," or "clear channel" signaling.
Keeping the signaling away from the voice channels offers some big advantages. First of all, it speeds things up. The D channel sends the dialing information from your PBX system to the phone company switch digitally rather than the analog DTMF string of tones. Call setup is noticeably faster, which is especially important in call center operations.
Since no bits need to be robbed, each B channel is a full 64 Kbps (8 bits times 8 KHz). That extra bandwidth may improve phone call quality almost imperceptibly, but it makes a huge difference for channels used to carry data. The "integrated" part of ISDN means that each channel can be individually assigned to be voice, computer data, or video. On the voice side, you can assign each channel to be inbound, outbound or both. That includes DID or Direct Inward Dialing and DOD or Direct Outward Dialing.
Caller ID is also transmitted via the D channel. There's no room in a conventional T1 voice trunk for extra data, so you must order T1 PRI or ISDN PRI to have Caller ID available for your phone system. T1 PRI and ISDN PRI refer to the same service. ISDN PRI carried on a T1 line makes it T1 PRI.
You'll want T1 PRI / ISDN PRI service if you desire Caller ID, fast call setup, DID on a per-call basis, and the cost savings that comes from being able to choose what each of your 23 available channels is used for. As a standardized service, most PBX phone systems offer a line interface card that supports ISDN PRI for one or more T1 lines.
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