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February 24, 2018
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Learn the hows and whys of Integrated Services Digital Network and what it can do for your business telephone needs

By: John Shepler

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?

The Most Popular PBX Telephone Line Standard
As you might suspect, ISDN is an acronym. It stands for Integrated Services 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.

How ISDN is Constructed
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.

A Word About ISDN BRI
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.

Get 23 Telephone Lines Plus Caller ID
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.

Connecting to ISDN PRI
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.

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