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Advantages of ISDN PRI
What is PRI and how does it relate to T1 lines and PBX telephone systems?

By: John Shepler

You may be familiar with the terms PRI, T1 PRI, Telephone PRI, PRI Trunk or ISDN PRI. But what do they mean and why would you select this type of service?

The Primary Rate Interface
PRI stands for Primary Rate Interface. It's a part of the ISDN or Integrated Services Digital Network and the big brother of BRI or Basic Rate Interface. In simple terms, PRI is a digital trunk line that connects to your PBX telephone system. It combines multiple phone lines into a single digital line. More importantly, it can save you money on your monthly phone expenses.

All these terms: PRI, T1 PRI, Telephone PRI, PRI Trunk and ISDN PRI really mean the same thing. You'll most often hear of T1 PRI and ISDN PRI. The T1 part refers to a T1 line, which is a digital communications line that can carry voice, data, or video. T1 lines can be organized differently to support different applications. The ISDN PRI service is one of those. It has a very specific protocol.

How PRI is Organized
PRI is organized in a format called 23 B + D. Looks a little like a math problem, doesn't it. Actually, it's just shorthand for 23 B or bearer channels and 1 D for delta or data channel. The T1 line is split into 24 equal size channels of 64 Kbps each for a total of about 1.5 Mbps total bandwidth. It's full duplex, so you get the same bandwidth in both directions.

Why carve up the line capacity into 64 Kbps chunks? That happens to be exactly the size required to transport one digitized telephone call using PCM or Pulse Code Modulation, the telephone company standard. It actually breaks down into and 8 bit digitized sample x 8,000 bits per second. The entire switched telephone network is organized around these channels, designated as DS0. They are carefully synchronized so that one call will not bleed into another.

Purpose of the D Channel
Now, with 23 bearer channels, each "bearing" a single telephone call, you can get the equivalent of 23 separate telephone lines on one T1 PRI line. The remaining channel, the D channel, is used for call control and data. This is how the PBX system and the telephone service provider exchange information know when a call is coming in, when a line is being requested, what number is dialed, and the extent of any auxiliary information such as Caller ID.

Fortunately, you don't need to dig in any deeper than this to be able to put PRI to work in your business. You order your PRI service with so many incoming and outgoing phone lines, local and/or long distance. There a lot of customization you can do with this. Then it's a matter of plugging your T1 PRI line from the telco demarcation point (usually in a back room) into the socket on the PBX circuit card designed to accept PRI service. There can be more than one PRI interface on larger systems. Each line will deliver 23 outside phone lines.

Applications for ISDN PRI
Who uses PRI? Medium and larger size businesses, or even smaller businesses that are telephone-intensive. Sales offices, customer support, call centers of all sizes, and corporate headquarters all use PRI phone service. Why do they use it? Because it connects and disconnects telephone calls faster than the older T1 phone line service, it consolidates many analog lines into one or a few digital lines, it offers exceptional voice quality and reliability, and it's cheaper. Any business that uses 10 or more analog phone lines can potentially save money by switching to PRI service. The more lines you have, the larger the savings.

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