Skip to main content

Short note: ISDN structure, services, reference points

Short answer

ISDN provides end-to-end digital services over standard interfaces. Reference points are R, S, T, U; channels are B (64 kbps), D (signalling), and H (high-speed). BRI is 2B+D, PRI is 30B+D. Services include bearer, teleservices, and supplementary services.

Write a short note on ISDN structure, services, reference points and channels.

ISDN (Integrated Services Digital Network) is an ITU-T standard digital network that gives end-to-end digital connections for voice, data, fax and video over the telephone subscriber line, through a small set of standard user–network interfaces.

Structure (reference model)

TE1 ------------ S --[NT2]-- T --[NT1]-- U --[LT/ET]
TE2 -R-[TA]----- S
  • TE1: ISDN terminal (digital phone); TE2: non-ISDN terminal; TA: terminal adapter.
  • NT2: customer switching (PBX, LAN); NT1: line termination at customer premises.
  • LT/ET: line and exchange termination at the exchange.

Reference points: R (TE2–TA, non-ISDN e.g. RS-232), S (TE–NT2), T (NT2–NT1), U (NT1–exchange, 2-wire loop).

Channels

ChannelRateUse
B64 kbpsUser voice/data
D16 or 64 kbpsSignalling (Q.931), low-speed packets
H0 / H11 / H12384 / 1536 / 1920 kbpsHigh-speed data, video
  • BRI: 2B + D(16) = 144 kbps (192 kbps with framing).
  • PRI: 30B + D(64) = 2.048 Mbps (Europe/Nepal) or 23B + D = 1.544 Mbps (USA).

Services

  • Bearer services: pure transport – circuit-mode 64 kbps speech/data, packet-mode, frame-mode.
  • Teleservices: complete services incl. terminals – telephony, Group 4 fax, teletex, videotex.
  • Supplementary services: add-ons – calling line ID, call waiting, call forwarding, conference, closed user group.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Telecommunications chapter 8 questions →