Short note: Architecture of ISDN network

ISDN (Integrated Services Digital Network) is an all-digital network defined by ITU-T that carries voice, data, fax and video over one subscriber line using standard interfaces. Its architecture has the user–network access, the ISDN exchange, and separate networks for circuit, packet and signalling traffic.

 Customer premises            |    Network
TE1 -S-+                      |
       [NT2]-T-[NT1]-U--------+--[LT/ET  ISDN exchange]
TE2-R-[TA]-S-+                |     |   |    |
                              |  circuit packet SS7
                              |  network network (CCS)

Functional groups

• TE1: ISDN-ready terminal (digital phone, ISDN PC card).
• TE2: non-ISDN terminal (analogue phone, RS-232 PC); needs a TA (terminal adapter).
• NT2: intelligent customer equipment – PBX, LAN controller; does switching and concentration.
• NT1: physical termination of the 2-wire loop; converts 2-wire U to 4-wire S/T, line coding, timing, power.
• LT/ET: line and exchange termination at the ISDN exchange.

Reference points: R (TE2–TA), S (TE–NT2), T (NT2–NT1), U (NT1–LT).

Access interfaces

• BRI: 2B + D = 2×64 + 16 = 144 kbps; for homes and small offices.
• PRI: 30B + D = 2.048 Mbps (E1) or 23B + D = 1.544 Mbps (T1); for PBXs and businesses.

Network side: the exchange routes B channels into the circuit-switched network or a packet-switched network, and carries D-channel signalling (Q.931 / LAPD) into common channel signalling (SS7) between exchanges. Signalling is thus out-of-band, giving fast call set-up and supplementary services.