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.

So, What Does ISDN Stand For?

If you are involved with business telephone systems, you’ve no doubt heard the term ISDN bandied about. But what is ISDN and what can it do for your company?

As you might suspect, ISDN is an acronym. It stands for Integrated Serviced Digital Network.

I guess that’s not much help in and of itself. ISDN is a digital telephone standard that was originally intended to replace analog telephone lines for both residential and business users. It never really took off on the consumer side because DSL, Cable broadband and VoIP came along with higher bandwidths and lower prices. But ISDN did become a popular trunking service for PBX telephone systems. It also has a specialty niche application.

There are two ISDN standards you should know about. The low bandwidth version is called BRI or Basic Rate Interface. The higher bandwidth version is called PRI or Primary Rate Interface. They are constructed from the same elements. PRI just has more of them than BRI.

The two basic building blocks of ISDN are the B or bearer channel and D or delta channel. B channels have a bandwidth of 64 Kbps each. That’s exactly enough for one digitized telephone call or a low bandwidth digital modem. Remember 56 Kbps dial-up modems? ISDN BRI was supposed to replace them with one B channel for telephone service and a second B channel for modem service. The D channel is 16 Kbps and is used for signaling and control of the two bearer channels.

This arrangement of two bearer channels plus a delta channel is called 2B+D in telephony lingo. You have the option to use both bearer channels for telephone service so that you’ll have two digital phone lines. Or both bearer channels can be assigned to modem data with a bandwidth of 128 Kbps. That was considered fast when ISDN BRI was introduced, but quickly became obsolete with the introduction of broadband Internet service.

While ISDN BRI flopped as a digital home telephone service, it has found a new lease on life with radio stations and audio production. Standard analog telephone lines don’t have the bandwidth to reproduce natural sounding voices. But ISDN lines with the right CODECs (Coders/Decoders) can transport studio quality microphone signals from sports venues and other remote locations. They are also used extensively by voiceover artists and for high fidelity call-in interviews on-air.

ISDN PRI has the same B and D channels that make up BRI service. Only PRI is configured as 23B+D. That’s 23 bearer channels and one delta channel. The delta channel is still used for signaling and call control. Each B channel can support one telephone conversation. So, if you order ISDN PRI service for your PBX telephone system you can have up to 23 outside lines. They can be individually configured as incoming, outgoing, local, long distance, toll free or some combination of these.

Most PBX phone systems come with at least one ISDN PRI interface standard. Some have multiple ISDN ports so that you can add more phone lines if you exceed the capacity of 23 lines on one ISDN PRI trunk. Larger companies and call centers can make good use of the extra ports. The D channel handles call switching much faster than a standard analog phone line and also provides Caller ID for all B channels.

Is ISDN the service that is right for your telephony needs? If you have more than a half-dozen outside lines, you might be able to save money by converting to ISDN PRI. Even if you already have PRI service, prices have plunged in recent years and you may be able to cut your monthly phone expenses significantly with a competitive service provider. Find out right now by checking ISDN PRI prices and availability for your business location.

What is ISDN and Why Do I Want It?

You may have heard the term ISDN related to telephone services. So what is it and what can it do for you?

ISDN stands for Integrated Services Digital Network. It's a fancy term for digital telephone service. But not just any digital phone service. This one has very specific standards.

The story behind ISDN goes back to the days when the standard for Internet access was a dial-up modem using an ordinary analog telephone line. ISDN was envisioned as a way for users to have a combination of separate telephone service and Internet service on a single ISDN telephone line with three channels. Two of these are called "B" or bearer channels and are 64 Kbps each. The third is the "D" or delta channel that is used for switching and signaling. One B channel can be used for a telephone handset. The other can be used for a modem. Or both channels can be combined for 128 Kbps Internet service. This arrangement is called ISDN BRI. The BRI means Basic Rate Interface.

The telephone companies expected ISDN BRI to be a hit with consumers as an upgrade to their analog phone service. But by the time it was ready for mass deployment, Internet standards had moved on and everybody really wanted DSL or Cable broadband. After all, 128 Kbps may be impressive when compared to a dial-up modem, but it pales in comparison with always-on broadband at 512 Kbps or higher. Today, even mobile users want bandwidth above 1 Mbps, so ISDN BRI is of little interest to most users.

The exception is in the field of broadcasting and professional audio. ISDN BRI lines can be used to transmit high quality audio through the telephone network by using a CODEC (Coder / Decoder) to convert the audio to and from the ISDN format at each end of the call. Guests around the country can join a panel discussion and sound like they are right in the studio instead of hundreds or thousands of miles away. Voice over artists can contribute to a production from their home studios.

Where ISDN flourishes today is in its second standard, called PRI or Primary Rate Interface. ISDN PRI consists of 24 channels configured as 23 bearer channels plus 1 delta channel. The 23 bearer channels can each support 1 telephone call each. That's 23 digital phone lines, plus the delta channel used for signaling and Caller ID data.

ISDN PRI is the telephone service of choice for medium and large company telephone systems. Most PBX and IP PBX business phone systems offer built-in support for at least one ISDN PRI line. Other systems can be configured for one or more ISDN PRI trunks with a plug-in circuit card.

In addition to office PBX telephone, ISDN PRI is preferred by company contact centers and independent call centers. There is a significant cost reduction realized by using a single ISDN PRI instead of a dozen or more individual analog phone lines. Each channel on the PRI can be configured to be local and/or long distance, incoming and/or outgoing calls. If more than 23 lines are needed, more can be added to the system to double, triple or quadruple the number of active phone lines.

ISDN PRI is delivered on a T1 line in the United States. For this reason, it is sometimes known as T1 PRI or T1 telephone service. In other countries, an E1 line might be used to provide PRI service.

Now that you know what ISDN is good for, can this service help your business or organization to provide high quality voice communications for perhaps less than you are spending now? Find out by checking ISDN PRI service pricing now.