ISDN Overview

by Roger Harrison

For a few years ISDN has been something that has been joked about.  Its acronym has stood for It Still Does Nothing, I Sure Don't kNow, and the correct term: Integrated Services Digital Network.

It started out as ISDN-1 and then evolved into ISDN-2 and ISDN-3.  The reason behind the sarcasm is because it is something that was almost as bad as vapor-ware.  It was promised but it never seemed to be delivered.

The AT&T "You Will" commercials are similar to this idea.  Laugh no more because ISDN is here... if you can convince those at your local phone company that it really exists.

ISDN is a digital service for both voice and data communications.  On POTS lines the maximum data transfer is about 30 kbps.  With ISDN you can reach 64-128 kbps for data.  This is all obtainable without changing your telephone lines.  How, you may ask?  It's done by changing your voice to data right at the phone line and combining it with up to two other data streams.

In the central office they give you a new ISDN line card for your phone line.  (Maybe they'll forget to reconnect the DNR in the process!)

Basic Rate Interface (BRI) is normally set up in 1B+D or 2B+D configuration.  It is equivalent to three POTS lines in your house.  The B stands for "Bearer" and the D for "Delta."

The B1 channel is used mainly as an 8-bit voice channel, although it can provide 64 kbps data.

The B2 channel is normally the 64 kbps data channel, but it also can provide voice.

The D channel is 16 kbps for X.25 packet data and also for out-of-band signaling to the switch in the central office.

Since there is a separate out-of-band signaling channel, this means that if you have Call Waiting you can use Caller ID on the person who just called.  In fact, you can do this many times to subsequent callers.

128 kbps data transmission is obtained by using two of the B channels.

What does this mean to you?  First of all, you can be talking on the phone with a friend on one B channel while sending them a virus on the other B channel while still being connected to the Internet on the D channel.

You can gain more information on ISDN by contacting the National ISDN hotline of Bellcore at: 1-800-992-ISDN, FAX: 201-829-2263, e-mail: isdn@cc.bellcore.com, WWW: info.bellcore.com.

The AT&T documentation guide has info you can get.  Obtain the guide by calling 1-800-432-6600.

Bellcore's Catalog of Technical Information also has documents.  Reach them at 1-800-521-CORE.  Your local company may have information too, but if you're in NYNEX territory, don't even bother with their 1-800-GET-ISDN number because the information isn't updated, therefore much of it is incorrect.