Troubleshooter’s Guide for Ordering ISDN

Before embarking on your ISDN ordering adventure, you’d better learn how to talk to the phone company.

Digital phone lines can be set up or “provisioned” in a variety of ways, not all of which are compatible with your videoconferencing system. So before embarking on your ISDN ordering adventure, you’d better learn how to talk to the phone company.


A typical ISDN line consists of three separate channels: two wide bearer-channels for carrying information (B channels) and a single data-channel for handling phone-company network chores (the D channel). Each B channel can accommodate 64 Kbps of data, yielding 128 Kbps of total bandwidth. This “2B + D” configuration is known as a Basic Rate Interface (BRI). It’s the configuration you should order when calling your local telephone company for service.

To get an ISDN line into your home or office, all the phone company does is to drag a line into the building from the telephone box on the street. Installation runs anywhere from $100 to $250; monthly service typically costs around $40 (plus local and long-distance charges).

After this, the fun begins. To get the line set up to work with your videoconferencing system, you have to tell the phone company how you want the line provisioned. Some basic terminology:

CSV/D: Circuit Switched Voice/Data is an option on each B channel that allows it to handle either voice or data traffic. You want each B channel to be CSV/D, giving you voice or data on demand.

NT-1: A Network Terminator 1 is a small black box about the size of a paperback book, which gives you separate transmit and receive lines so you can use multiple devices (phone and ISDN). Some systems, such as Intel’s ProShare, require an NT-1. Check with the vendor. If you need one, try to buy it from the phone company.

SPID: Service Profile Identifier. When you make a call, the SPID tells the phone company the type of connection you need and the kind of routing that must be done (it differs for voice and data). Ask the phone company if you need to add any extra numbers to the SPIDs in your software setup. For the switch to recognize the call, you may have to add a 0 and a 1 to the end of the number.


Switch Type: To make a call connection, your software must know what kind of switch it’s dealing with, such as an AT&T 5ESS switch, Northern Telecom DMS 100 switch, or an NT-1 compliant switch. Ask the phone company which one you’re on.

Coordinates For more information on ISDN and whom to call in your area to get service, look up ISDN*tek;

See Also:

  • “I See How to Work Together”
  • “Quain’s Top Ten – VideoPhone”
  • “Show Me Your Data, Not Your Face”