When you place a telephone call, the signal travels in analog form until it reaches a switch where it is converted into digital format. The signal remains digital as it is switched and transported to other switches, either local or long distance. It is only converted back to analog for the last leg from the final switch in the chain to the person you're calling.
ISDN simply moves the line card typically found in our switch to a box on your desk, so the conversion from analog to digital happens right away, instead of in the switch. That feature alone would be attractive, but ISDN does much more. The most important is that you get two 64 kbps channels over the existing pair of copper wires to your home or business. These are called the "B" channels. You also get a third channel, called the "D" channel, that operates at 16 kbps; it's normally used for signaling to set up calls, although it can handle other data such as home energy usage and ATM machines.
The "Two B plus D" configuration is called Basic Rate ISDN. ACS offers both a business and a consumer version of this service. There's also Primary Rate ISDN, which is made up of 23 B channels and one 64 kbps D channel.
Primary Rate is of interest to large businesses. Among other things, it allows Caller ID to work to and from extensions from a company switchboard, something which is otherwise not possible.
But the big demand for Basic Rate ISDN is it provides so much bandwidth over existing wires at a competitive price. A big reason for the service's popularity is its flexibility: the two B channels can each be used for different purposes, such as faxing and talking on the phone simultaneously, or they can be "bonded" together into one 128 kbps channel. ISDN has the capacity to assign multiple telephone numbers to each B Channel.
Here are four of the most popular uses for ISDN:
- Telecommuting: One channel is used for telephone, and the other is connected to a company's computer network. This allows employees to work at home.
- Videoconferencing: Both channels are bonded together; the resulting 128 kbps channel is fast enough to generate jerky but recognizable color video in a small window on a computer screen at each end.
- Telemedicine: ISDN allows digitized X-rays and other medical images to be transmitted far faster than the typical modem/phone line arrangement.
- Internet Access: Possible either with one B channel, reserving the other for telephone, or bonded at 128 kbps. As many Internet users are learning, the faster the access, the better.