Short note: ISDN

ISDN (Integrated Services Digital Network) is a set of ITU-T standards (I-series) for a fully digital network that carries voice, data, fax and video over the same digital subscriber line, using standard interfaces and common-channel signalling. It gives end-to-end digital connectivity.

Features

• Digital transmission from the user to the exchange.
• Several services on one line, with several simultaneous calls.
• Out-of-band signalling on a separate D channel (DSS1 to the user, SS7 inside the network).
• Standard user–network interfaces and fast call setup.

Channels

Access types

• Basic Rate Interface (BRI): 2B + D = 2×64 + 16 = 144 kbps (192 kbps with framing at the S/T point); for homes and small offices.
• Primary Rate Interface (PRI): 30B + D = 2.048 Mbps (Europe/Nepal, E1) or 23B + D = 1.544 Mbps (North America, T1); D = 64 kbps; for PBXs and large users.

Architecture (reference model)

        R        S        T        U
 [TE2]---[TA]--+
               |--[NT2]---[NT1]=======[LT/ET]
 [TE1]---------+  (PBX)   (NTU)  2-wire  exchange
               S bus    4-wire   loop

• TE1: ISDN terminal; TE2: non-ISDN terminal with TA (terminal adapter).
• NT1: physical/line termination at the customer site; NT2: intelligent device such as a PBX.
• R, S, T, U: reference points between these units; U is the two-wire loop to the exchange.

Services

• Bearer services: circuit-mode and packet-mode transport.
• Teleservices: telephony, fax, videotex, teletex.
• Supplementary services: caller ID, call waiting, call forwarding, conference.

Advantages and limitations

• Faster and better-quality connections than analog modems; several services on one pair; quick call setup.
• Data rates are low by modern standards; costly equipment; largely replaced by DSL and fibre. Broadband ISDN (B-ISDN), based on ATM, was the high-speed extension.