ISDN is a digital telephone and internet service that uses digital signals to increase data throughput and improve signal quality. It was popular in the mid-1990s but has been largely replaced by affordable DSL service. It is still used for professional broadcast and audio applications and may be preferred by small businesses that frequently use two voice lines.
ISDN (Integrated Services Digital Network) is a type of digital telephone/data and Internet service that preceded ADSL (Asynchronous Digital Subscriber Line) and has been largely superseded by it.
Regular telephone lines carry analog signals that must be amplified and converted to digital signals by the telephone company. This process introduces not only a slight delay, but also distortion into the signal. Modems and dial-up phones are examples of equipment that use analog signals. ISDN makes use of digital signals running along existing copper lines to increase data throughput, reduce line noise, and improve signal quality.
In the mid-1990s, ADSL was very expensive and not widely available. Businesses and individuals alike wanted a faster way to connect to the Internet, but the technology behind dial-up modems had reached its threshold. ISDN has become a popular alternative for providing speeds of up to 128 kilobits per second (kbps), compared to the standard 30-53 kbps connection with a dial-up modem.
The most common type of ISDN service for connecting to the Internet is the Basic Rate Interface, or ISDN BRI. This technology creates two B channels on existing copper lines at 64 kbps each, along with a single 16 kbps D channel for the telephone line. This separates the data channels from the voice channel, allowing you to use your phone or fax while online.
While ISDN is inexpensive and about twice as fast as dial-up service, it has largely been replaced by affordable DSL service. A cheap ADSL service offers speeds of up to 384 kbps, while more expensive versions continuously improve in speed. As of fall 2005, standard ADSL speeds range between 1.5 and 3.0 mbps (megabits per second), or 1536-3072 kbps.
While ISDN may not be the best choice for packet switched networks such as the Internet, it is still widely used for professional broadcast and audio applications where digital clarity with integrated telephone services is specifically required. Small businesses that frequently use two voice lines, such as telephone and fax, and require only limited Internet connectivity of, say, an hour or less per day, may prefer ISDN. ISDN may also be a better choice for high-speed connections to intranets for video conferencing or remote networks other than the Internet.
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