What Is a PRI Line?

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

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.

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.

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 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.

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.

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.

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.

Are you interested in lower cost multi-line business telephone service to support your in-house phone system? If so, get prices and availability for PRI line service options. Single line service and VoIP SIP trunking are also often available.

ISDN's Primary Rate Interface

ISDN, the Integrated Services Digital Network, is a digital telephone standard that made voice and data services widely available long before the more recent deployment of Enterprise VoIP and MPLS networks. ISDN's Primary Rate Interface is more in demand today than ever. That's because it is tightly integrated with the installed base of telephone trunk lines that are almost universally available.

ISDN was the telcos' response to a change in technology that saw modems being installed on analog telephone lines to transfer data over long distances. The toll network was only designed for a limited voice bandwidth and tops out at about 56 Kbps when used as data bandwidth. In addition, analog modems use the same frequencies as telephone audio. You can use the line for data or voice, but only not both at the same time.

ISDN drew on existing digital carrier technology based on 64 Kbps channels. One channel can carry one telephone conversation, once the call is digitized using a CODEC or Coder / Decoder. These channels are the building blocks of T1, DS3, SONET and other digital carriers. The smallest unit is the DS0 which is a single 64 Kbps channel.

What ISDN did was to create two services for digitized voice and data. Note that once voice is digitized it actually is a data stream and can be carried by a digital trunk line side by side with computer data. Unlike VoIP, which consists of voice packets that all flow down the same line with data packets, ISDN keeps the voice and data channels separate.

The BRI or Basic Rate Interface consists of two 64 Kbps B or bearer channels plus one 16 Kbps D or delta channel. Each of the bearer channels can carry one telephone conversation or a 64 Kbps data link. The delta channel is used for signaling and other services such as Caller ID.

BRI was originally intended for residential and small business use. With a ISDN BRI connection you could have a telephone line and a 64 Kbps Internet connection, 2 telephone lines, or combined data of 128 Kbps. BRI was never widely adopted because other services such as DSL and Cable Internet became available with far greater bandwidths.

While BRI came too late to be a dominant digital line service, PRI or Primary Rate Interface has flourished. PRI consists of 23 bearer channels and 1 delta channel. Shorthand for this is 23 B + D. In this case all channels are 64 Kbps. The larger D channel can handle signaling for all 23 of the other channels. The total of 24 channels is exactly the right size for a T1 line, which is widely available. Thus, T1 lines are used to carry ISDN PRI. In this case, they are often called T1 PRI lines or T1 PRI service.

What has made T1 PRI such a popular service is that it is the perfect digital trunk to support PBX or Private Branch eXchange telephone systems used by businesses and call centers. Each T1 PRI line can replace up to 23 analog telephone lines. It's usually much more cost effective to order T1 PRI service than to bring in a dozen or more separate analog lines. Using the D channel to send signaling to the telephone company and receive Caller ID information results in faster call set-ups and disconnects, which can be important in busy call center operations.

Another popular use for T1 PRI has been to support modem banks. A dozen or more dial-up modems can be deployed by an Internet Service Provider at a single location and will communicate with the ISP via the T1 PRI connection. Each modem is supported by a single bearer channel.

Is ISDN PRI service priced right for your company? You can find out with a quick online price and availability search right now. Our friendly consultants will also be happy to review your needs and recommend the most cost effective solutions from a variety of competing carriers.