ISDN (Integrated Services Digital Network)ISDN offers businesses and consumers digital telephone service. ISDN provides static- and error-free communication at speeds as high as 128 Kbps.
ISDN is one of a number of new digital network technologies. ISDN offers home offices and small businesses inexpensive high-speed data transmission with the ability to send and receive voice, data, and still and moving images through the same connections. A standard telephone line that would normally carry only one voice, computer, or fax "conversation" can carry as many as three separate "conversations" at the same time.
Computers traditionally require a modem to translate binary computer data to and from analog form for transmission/reception via telephone lines, even though the only analog part of a modern phone system is the "last few miles of wire" from a home or business to the nearest phone company equipment. ISDN is totally digital; no modem is unnecessary. The switch to a purely digital signal results in increased bandwidth for greater transmission speed and the static- and error-free transmission and reception associated only with a digital signal.
ISDN contains a number of bearer (B) channels or "pipes" coupled to one data (D or Delta) channel. B channels, which transmit at relatively high speeds, can be used for normal voice telephone service or to connect fax machines, video conferencing equipment, computers, networks and other digital equipment. The D channel is primarily for call set-up, signaling, or information.
ISDN is delivered in two types of user interface: Basic Rate Interface (BRI, also called Basic Rate Access or BRA) and the Primary Rate Interface (PRI, also called Primary Rate Access or PRA).
Basic Rate Interface offers two 64-Kbps bearer channels using existing twisted-pair copper telephone cable. The channels can be used independently as separate voice or data channels are bonded into a single 128-Kbps channel for increased bandwidth (sometimes called inverse multiplexing) . The D channel operates at 16 Kbps and is used for signaling to the ISDN. The two B channels and the single D channel give rise to the term '2B+D'. Basic rate interface is intended for low capacity use by small businesses.
Primary Rate Interface in the United States consists of twenty-three 64-Kbps B channels and one 64-Kbps D channel, or a 23B+D connection for a total bandwidth of 1.544Mbps. (European and Pacific Rim Primary Rate Interface consists of either 30B+D or 31B+D.) PRI is designed for transmission through a standard dedicated T1 trunk line to connect medium and large locations to a telephone company central office.
Since ISDN uses digital technology, it is not subject to the static and noise associated with analog lines. ISDN provides essentially static- and error-free communications.
Unlike analog systems, ISDN always runs at its rated speed, a speed in excess of those possible with an analog signal. The increase in bandwidth (and the ability to increase effective bandwidth through bonding) have fostered the growth of various new telecommunication applications.
One ISDN line can provide multiple lines to handle multiple telephone devices and telephone numbers. Up to eight separate devices can be linked to a single ISDN connection, with up to 64 "call appearances" of the same or different telephone numbers. ISDN supports up to three calls at the same time: two voice, fax, or PC "conversations," and one data "conversation."
Although the specifics of ISDN implementation differ slightly from nation to nation, ISDN is defined by a set of digital transmission protocols that are standard throughout the world.
For you to obtain ISDN service, your Internet Service Provider must be equipped to provide ISDN service and the telephone company must provide an ISDN line with associated digital switching capability. The sole remaining requirement is the hardware necessary for hooking up your computer or Ethernet network to the ISDN line:
Just as there are internal and external modems, so there are internal and external ISDN adapters. Many newer TAs include a built-in network termination device.
ISDN service generally costs a minimum of $20-$30 per month more than existing telephone service, with an additional $100-$500 for installation. Combined TA/NT-1s cost around $250.
ISDN technology and availability is in flux. Telephone companies are rapidly expanding the availability of ISDN service and the price of new equipment continues to drop. For those with a real need for the increased bandwidth, ISDN is the best way to go until cable-TV or other high-speed access becomes a reality. Businesses seeking full-time connection are often better served by 56Kb/sec Frame Relay.