About ISDN

What is ISDN and why should I need it?

Integrated Services Digital Network (ISDN) sounds rather impressive. But if you pay a visit to the office of an ISDN user, expecting to be dazzled by the sight of some piece of techno-gadgetry, ablaze with twinkling lights, you're in for a disappointment.

What you will see is something that looks like an ordinary telephone socket. The reason for this is that an ISDN line is, in fact, a telephone line –albeit a digital one. But there the similarity ends, because digital lines are capable of far more than ordinary analogue phone lines. just as the cd revolutionised the record industry by providing a vehicle for delivering high quality, ‘crackle-free’ audio on a small disk, so ISDN is revolutionising business-to-business communication.

There are two types of ISDN service available throughout the world: basic rate interface (BRI) and broadband (primary rate). BRI service is by far the most common and is typically found in homes and businesses alike. the higher capacity broadband service is a central-site solution for extending applications to large numbers of remote users, who communicate through their BRI connections.

With its BRI and broadband services, ISDN has the flexibility to meet the bandwidth needs of a home office, branch office, or company headquarters. A small office can use a single ISDN BRI to support all of its voice and data communications requirements by installing the BT home or business highway. In a larger office, multiple ISDN BRI lines can be divided among multiple users and applications via a server or pbx. And users in a very large office can benefit from ISDN broadband capacity to provide a large quantity of multiple b channels.


The BT highway box has four ports - 2 analogue for any combination of telephones or fax machines and 2 digital - for connecting up your ISDN terminal adapter or ISDN card. The ISDN ports are marked blue. You will normally be given a minimum of three numbers - 2 analogue and one digital.

How does ISDN work?

A basic ISDN connection consists of two data ‘channels’, each of which can transfer data at speeds of 64kbps (about 200 pages of a4 text every minute). These channels, known as ‘b channels’, can either be used separately or together. If you use them separately you can do two things at once, for example sending a data file whilst simultaneously receiving a fax. If you use them together it doubles the rate at which you can send and receive files. There is also a third channel (the d channel) with a bandwidth of 16kbps, which is used for setting the calls up. When the two channels are bonded in a single connection, you get a speed of 128 kbps, which is about four times the actual top speed of the fastest analog modems. Using software that supports compression during file transfers, such as grand central pro you can increase the throughput to around 250kbps or more.

And ISDN for the small business user doesn't stop there. Inexpensive four channel solutions are available with twice the performance of the entry-level products. Again, you can use each channel separately or combine them for high-speed transfer of large files. There are also integrated analog,GSM and ISDN adapters available that allow faxing and analog connection as well as ISDN.

Telecommuting using ISDN

Telecommuters, for example, benefit immensely from ISDN. whether you access the corporate lan in the evenings or maintain a full-time, remote home office, ISDN is the next best thing to being there. e-mail, database access, and file transfers improve dramatically, making it seem like you're locally attached to the lan.

Internet Access

Internet access is another great application for ISDN. compared with even the fastest modem access, ISDN makes web graphics appear almost immediately, and can reduce download times by over 75%. ISDN can even provide advantages over shared, higher-bandwidth office connections; PC magazine advises that an ISDN connection can deliver better performance than a T1 shared among users on a lan. and in many markets, it's actually cheaper than an isolated analog business line.

But ISDN delivers even more than that. Phone, fax, e-mail, videoconferencing and access to the world wide web all become more efficient, more affordable and more manageable. Competitive advantage is all about doing things faster and smarter than everyone else. And all successful businesses have two things in common: good information and good communication. ISDN is an enabling technology. it lets you exploit digital technology to share information and communicate more effectively with your colleagues, clients and suppliers. once the domain of large corporations, ISDN has now fallen in price to a level where even sole traders can use it to gain a competitive edge.

The real advantage

In simple terms, ISDN is a replacement for plain old telephone service, which was never designed to meet the needs of the information age. ISDN uses the same wiring that currently serves homes and businesses. You get ISDN service from the same companies who provide telephone services, and you use it to connect telephones, computers and fax machines. The difference is that you get much faster, much more dependable connections for voice, data, fax, and even video - all through a single line. There is no other technology that comes close to delivering such communications benefits today!