Circuit Switching Links ISDN

Integrated Services Digital Network (ISDN) is used by telephone companies to carry digitized voice and data over the existing telephone system. ISDN has emerged as one of the leading technologies for telecommuting and remote office connection into the corporate headquarters.

On the seven layer model, ISDN operates at the physical, data link, and network layers. ISDN basic rate interface (BRI) provides two barrier channels (commonly referred as B channels). Each of these B channels carries data at 64 kbps. The D channel coexists along with B channels. The D channel operates at 16 kbps and is usually used for carrying control information.

The ISDN primary rate interface (PRI) service delivers 23 B channels and one 64-kbps D channel in the US and Japan with a total bit rate of 1.544 Mbps. In other parts of the world, the PRI service provides 30 B channels for a total bit rate of 2.048 Mbps.

ISDN consists of three network components: ISDN terminal equipment, termination devices, and reference points. ISDN terminal equipment has two basic terminal equipment types:

A specialized ISDN terminal that includes computers equipment and phones.

DTE equipment that connects to the ISDN line through a terminal adapter (TA).

ISDN Network Termination Devices are called network termination (NT) devices. There are three supported NT types:

Treated as a customer premises equipment in North America, elsewhere it is provided by carrier.

Found in PBXs, this device type provides Layer 2 and Layer 3 functions and concentration services.

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