ISDN architecture, services and channels

ISDN

ISDN (Integrated Services Digital Network) is an ITU-T standard digital network that provides end-to-end digital connections for voice, data, fax and video over the same subscriber line, using standard user–network interfaces and out-of-band (D-channel) signalling.

Architecture

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

Functional groups

• TE1 (Terminal Equipment 1): ISDN-compatible terminal (digital phone, ISDN PC card).
• TE2: non-ISDN terminal (analog phone, RS-232 device).
• TA (Terminal Adapter): converts TE2 signals to ISDN format.
• NT1 (Network Termination 1): layer-1 device at the customer premises; terminates the two-wire loop, line coding, power, timing.
• NT2: intelligent layer 2–3 device such as a digital PBX or LAN switch that concentrates terminals.
• LT/ET: line termination and exchange termination at the central office.

Reference points

• R: between TE2 and TA (non-ISDN interface).
• S: between TE1/TA and NT2 (4-wire, up to 8 terminals on a bus).
• T: between NT2 and NT1.
• U: between NT1 and the exchange (2-wire loop, 160 kbps for BRI using 2B1Q coding).

Services offered

1. Bearer services: transport of information without change: circuit-mode (64 kbps speech, unrestricted digital) and packet-mode (X.25 on B or D channel).
2. Teleservices: complete services including terminal functions: telephony, telefax (Group 4), teletex, videotex, videotelephony.
3. Supplementary services: added to basic services: calling line identification, call waiting, call forwarding, call hold, conference call, closed user group, advice of charge.

Channels

Interfaces

• BRI: 2B + D = 144 kbps user rate (192 kbps at S/T with overhead).
• PRI: 30B + D = 2.048 Mbps (Europe, Nepal) or 23B + D = 1.544 Mbps (North America).