Basic ISDN services and architecture

ISDN (Integrated Services Digital Network) is an ITU-T standard all-digital network that provides voice and non-voice services over the subscriber line through a small set of standard user–network interfaces.

Basic services in ISDN

1. Bearer services – carry information between user–network interfaces without changing it (OSI layers 1–3).

• Circuit-mode: 64 kbps unrestricted data, 64 kbps speech, 3.1 kHz audio, multiples of 64 kbps.
• Packet-mode: X.25 virtual calls and permanent virtual circuits on B or D channel.
• Frame-mode: frame relay.

2. Teleservices – complete services including the terminal (layers 1–7).

• Telephony, Group 4 fax, teletex, videotex, videotelephony.

3. Supplementary services – extra features added to the above.

• Caller ID (CLIP), call waiting, call forwarding, call hold, conference calls, closed user group, advice of charge.

Channel services (access)

B = 64 kbps user channel; D = signalling and low-speed packets; H channels (384, 1536, 1920 kbps) for high-speed data.

ISDN architecture

 Customer premises                   Network
TE1 --S--+
         [NT2]--T--[NT1]--U--+--[ISDN exchange]
TE2-R-[TA]-S--+              |   |     |     |
                             | circuit packet CCS/SS7
                             | switched switched signalling

Functional groups

• TE1: ISDN terminal (digital phone, ISDN fax).
• TE2: non-ISDN terminal (analogue phone, PC with RS-232).
• TA: terminal adapter; converts TE2 signals to ISDN.
• NT2: intelligent premises equipment (PBX, LAN controller) – switching, concentration, layer 2–3 functions.
• NT1: physical termination of the line – 2-wire to 4-wire conversion, line coding, timing, power feeding.
• LT/ET: line and exchange termination at the central office.

Reference points

• R: TE2–TA (non-ISDN).
• S: TE1/TA–NT2, 4-wire 192 kbps bus (up to 8 terminals).
• T: NT2–NT1.
• U: NT1–exchange over the 2-wire loop (2B1Q, 160 kbps).

Network side: the ISDN exchange connects to a circuit-switched network, a packet-switched network and a common-channel signalling (SS7) network. User signalling on the D channel (Q.931) is converted to SS7 ISUP between exchanges, giving fast call set-up and the supplementary services.