Integrated Services Digital Network (ISDN) is comprised of digital telephony and data-transport services offered by regional telephone carriers. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. The emergence of ISDN represents an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. ISDN applications include high-speed image applications (such as Group IV facsimile), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and videoconferencing. Voice service is also an application for ISDN.
ISDN devices include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals, such as DTE, that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a TA. The ISDN TA can be either a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device that typically is found in digital private branch exchanges (PBXs) and that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists as a single device that combines the functions of an NT1 and an NT2.
Reference points
A set of reference points are defined in the ISDN standard to
refer to certain points between the telco and the end user ISDN
equipment.
R - defines the point between a non-ISDN device and a terminal
adapter (TA) which provides translation to and from such a device
S - defines the point between the ISDN equipment (or TA) and
a Network Termination Type 2 (NT-2) device
T - defines the point between the NT-2 and NT-1 devices
U - defines the point between the NT-1 and the telco switch
ISDN in Business
For business users and even residential subscribers, videoconferencing is the biggest communication advancement that ISDN has to offer. With the simultaneous high speed transfer of voice and video, ISDN can provide real time video communication on a PC that once was only capable on sophisticated systems costing upwards of $100,000.
A shared electronic chalk board is another tool available through ISDN. Ideas and illustrations can be distributed in real time to remote locations so people in other cities or other countries can participate in meetings.
Telecommuting is becoming a rule more than an exception; more and more people are working from home. ISDN provides the facilities for users to tap into central network resources from the privacy of their own homes and do so with the functionality of a network node. Node connections are possible with Serial Line Interface Protocol (SLIP) and Point-to-Point Protocol (PPP).