What Are PRI Phone Lines?

May 18, 2012

What Are PRI Phone Lines?
The ISDN Primary Rate Interface and options for this multi-line business telephone service.

PRI lines are popular for providing multi-line phone service to businesses. But just what is a PRI line and what options are available?

All About ISDN
PRI is part of a telephone technology standard called ISDN (Integrated Services Digital Network). It was envisioned as a digital line service that would replace analog telephone lines for both residential and business users. At the time that ISDN was being introduced, connections to the Internet were mostly dial-up using 56 Kbps modems at best. Many of those 56K modems really connected at something around 30 to 40 Kbps due to noise and other limitations on most phone lines. All-digital lines offered basic ISDN service with 64 Kbps data plus one standard telephone service on a single line.

The Basic Rate Service
This basic service is called BRI for Basic Rate Interface. It consists of 3 channels. There are two “B” or Bearer channels at 64 Kbps each that can be configured as either voice or data. That can be a 64K digital Internet connection and a phone line, 2 phone lines, or 2 combined data lines for 128 Kbps Internet. The third channel is called a “D” or Delta channel. It has only 16 Kbps, but is used for signaling and control.

Faded to Obscurity
If you’ve never heard of ISDN BRI it’s because it never took off. By the time it was starting to be deployed in a big way, technology had moved on and broadband over DSL and Cable was taking over the role of Internet access.

The Primary Rate Service
The second standard introduced with ISDN is PRI for Primary Rate Interface. This much higher bandwidth service offers 24 channels configured as 23 B + D. In this case the D channel bandwidth has increased to 64 Kbps just like the B channels. Why stick with 64K channels? It’s because 64 Kbps is just the right amount of bandwidth to transport one digitized telephone conversation using the industry standard G.711 coding standard.

Supported By Most PBX Phone Systems
You can configure a PRI circuit to carry voice, data or a combination. Some ISPs used these to connect to modem banks in the dial-up days. Today, the most popular use of ISDN PRI is for PBX telephone trunking with 23 outside voice lines and one control channel that also handles Caller ID for the other 23. Most PBX switches come already configured for at least one PRI line or can be interfaced with a plug-in card.

Comes In On A T1 Line
BRI was designed to be provisioned over the same twisted pair cable that brings in analog phone service. PRI is provisioned over T1 lines because of its higher capacity. T1 was designed to use 2 twisted pair of telco cable and is almost universally available as a standard business-grade digital telecom service.

The Cable PRI Option
Recently, another PRI option has become available. That is business grade telephone trunking provided by Cable companies, such as Comcast. The coaxial cable used to bring in television has tremendous capacity and easily supports broadband services up to 100 Mbps. The same cable can transport individual analog telephone lines or ISDN PRI to support a business PBX phone system. With Cable PRI, you may have the option to order fractional PRI service with as few as 6 phone lines. You can then add more, even one at a time, as your business needs increase.

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