abstract

In this context, the Consultative Committee on International Telephone and Telegraph (CCITT), which has since evolved into ITU-T, began working on modernization of the PSTN in the 1970s, but the original ISDN guidelines were not produced until 1984 in CCITT recommendation I.120, and this was far from complete.
ISDN lines come in two varieties, BRI and PRI. BRI consists of two 64 Kbps B (bearer) channels plus one 16 Kbps D (data) channel. In ISDN shorthand the interface is called 2B+D. The D channel carries signaling between the central office switch and terminating equipment, which could be a telephone set, personal computer, video conferencing system, router, or similar device. The B channels are used for so-called bearer services such as voice, data, or video.
Through a process known as bonding, the two channels of a BRI can be tied together to provide 128 Kbps of bandwidth. Most LECs offer BRI as a line-side option from local switches, and most major PBXs support BRI line cards. A special station set is required for ISDN, or as shown in Figure 15-1, ordinary station equipment can be installed behind a terminal adapter (TA).
The objectives of ISDN are
• To provide end-to-end digital connectivity
• To gain the economies of digital transmission, switching, and signaling
• To provide users with direct control over their telecommunications services
• To provide a universal network interface for voice and data
ISDN network architecture is based on ITU-T standards. ISDN standards are formulated on the first three layers of the OSI Model.
In the United States, the two major switch manufacturers, Nortel and Lucent Technologies, developed their own ISDN protocols initially. This first edition was known as NI-1, which defines the most commonly needed set of features for regular telephone service plus several Centrex features. The main objectives of NI-1 are terminal portability and switch interoperability. Switch interoperability enables switches of different manufacture to communicate over SS-7. Unfortunately, these objectives were not all realized because the switches were not completely compatible.
A subsequent version, NI-2, expands on NI-1 by defining the PRI standard and expanding BRI capabilities. In NI-1, some BRI features were proprietary. In NI-2, they are standardized. Features such as D-channel backup and non facility associated signaling are introduced. The current version of ISDN is ISDN 2000.
An NT-2 is an optional terminator connecting between the NT1 and ISDN compatible devices. A terminator that includes both NT1 and NT-2 functions is known as an NT12. The NT-2 allows up to eight devices to share a passive bus.
The D channel can be used for data communications from other devices on the bus. The NT-2, which may be built into a PBX, multiplexer, local area network, or terminal controller, performs the data link and network layer functions.
Equipment that is not BRI compatible is known as TE-2 equipment, and must plug into a TA.
ISDN standards define five points of demarcation:
• R interface is a link between non-ISDN equipment and an ISDN TA.
• S interface connects ISDN terminals to NT-2 and NT12 devices.
• T interface connects NT-2 and NT1 devices.
• U interface connects NT1 and NT12 devices to the public network.
• V interface, located in the ISDN node, separates the line termination equipment from the exchange termination equipment.