ISDN service connections

ISDN (Integrated Services Digital Network) gives digital end-to-end connections over the subscriber line for many services (voice, data, fax, video) through one standard interface. The connections and services ISDN offers can be described by (1) the types of connection it can set up, (2) the access channels and interfaces used, and (3) the service classes (bearer, tele- and supplementary services) defined in ITU-T I.200 series.

Architecture behind the connections

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

• TE1 ISDN terminal, TE2 non-ISDN terminal with TA, NT2 PBX/LAN, NT1 line termination, LT/ET at the exchange.
• Reference points R, S, T, U separate these groups.
• The exchange sends user B-channel traffic to the circuit-switched or packet-switched network and the D-channel signalling to the common channel signalling (SS7) network.

1. Types of ISDN connection

1. Circuit-switched connection: a B channel (64 kbps) is set up on demand using D-channel signalling (Q.931 SETUP, ALERTING, CONNECT, RELEASE). Used for telephony, fax, video calls, file transfer.
2. Packet-switched connection: user data is carried as X.25 packets either on a B channel to a packet handler or directly on the D channel (low-speed data, e.g. POS terminals, alarm systems).
3. Semi-permanent (leased) connection: a B channel is set up by the operator for a long time without call signalling; acts like a 64 kbps leased line.
4. Frame-mode connection: frame relay service with LAPF framing over B or H channels.

2. Channels and access interfaces

• BRI = 2B + D = 144 kbps user rate (192 kbps line rate at S/T): homes, small offices.
• PRI = 30B + D = 2.048 Mbps (E1) or 23B + D = 1.544 Mbps (T1): PBXs, businesses.

3. ISDN services

(a) Bearer services – only transport the information across the network between interfaces (OSI layers 1–3); the user chooses the terminal.

• Circuit-mode: 64 kbps unrestricted digital, 64 kbps speech, 3.1 kHz audio (for modems and fax), 2×64 kbps, 384/1536/1920 kbps.
• Packet-mode: virtual call and permanent virtual circuit (X.25, connection-oriented); a separate connectionless packet service; user signalling.
• Frame-mode: frame relay and frame switching.

(b) Teleservices – complete services including terminal functions (layers 1–7):

• Telephony (3.1 kHz and 7 kHz wideband speech)
• Group 4 facsimile
• Teletex and telex
• Videotex, videotelephony, video conferencing

(c) Supplementary services – added to a bearer or teleservice, not sold alone:

• Calling line identification presentation/restriction (CLIP/CLIR)
• Call waiting, call hold, call forwarding (busy / no reply / unconditional)
• Three-party and conference calling
• Closed user group, direct dialling-in, advice of charge, user-to-user signalling

Example

A small office has a BRI line. One B channel carries a phone call (circuit-mode telephony teleservice) while the other carries a 64 kbps Internet/file-transfer connection; the D channel handles signalling for both and also shows the caller's number (CLIP supplementary service). A PC with only an RS-232 port connects through a TA at the R point.

Advantages of ISDN connections

• Fully digital, so clear speech and error-free data.
• Fast call set-up (1–2 s) because of out-of-band D-channel signalling.
• Several services and calls on one line at the same time.
• Standard interfaces let terminals from different makers work together.