ISDN and its digital service model
defined digital bearer and teleservices for voice, video, data, and other applications over the public telephone network. Its access provided end-to-end digital connectivity between user-network interfaces. User information used B channels, while signaling used the D channel. Many ISDN accesses used existing copper telephone wiring, and an ISDN terminal did not require an analog modem. The access could support simultaneous user information and signaling on separate channels. The model is described in the I.210, I.230, I.231, I.240, and I.250 series, separating network capabilities from terminal functions.
ISDN service categories
ITU-T I.210 classifies ISDN telecommunications services into bearer services and teleservices. Supplementary services modify or supplement a basic bearer service or teleservice. They are offered in association with a basic service and cannot form a standalone service.
1. Bearer Services
Bearer services provide transfer capability between user-network interfaces (). They specify attributes such as information transfer rate, transfer mode, and connection type. The meaning of user bits and application behavior comes from terminal functions and the teleservice. Terminal and application behavior belongs to the teleservice. The network transfers the user information according to the selected bearer attributes.
2. Teleservices
Teleservices combine bearer capabilities with terminal functions to provide an end-to-end application. They define network transport and the functions of the end-user terminals. The terminals must support compatible service functions and formats. Telephony, telefax, videotex, and file transfer are examples of teleservices.
Circuit-mode bearer services
ISDN defines several circuit-mode bearer service categories. The selected category determines how the network treats the information on the connection:
- Unrestricted 64 kbit/s Digital Information: A transparent circuit-mode bearer service for unrestricted user information transfer at . User information is carried on a B channel.
- 3.1 kHz Audio: A circuit-mode bearer category for signals within the 3.1 kHz audio band, including voice-band modem and Group 1 to 3 fax signals. The network may apply processing such as echo cancellation. The category permits audio-specific processing, so its transfer properties differ from unrestricted bit-transparent 64 kbit/s service.
- Speech: A service for speech information. The network may apply speech encoding and processing such as echo cancellation or low-bit-rate coding. The network treats the information as speech, so arbitrary data, including modem data, requires the unrestricted category.
- N × 64 kbit/s: A multiple-rate service that combines B channels, each at 64 kbit/s, into one higher-rate connection. N denotes the number of combined B channels. It supports applications such as video conferencing that require more than .
Common teleservices
ISDN defines several standard user applications using bearer services:
- Telephony (3.1 kHz and 7 kHz): 3.1 kHz telephony provides speech transmission in a 3.1 kHz audio band. ISDN also defined a 7 kHz teleservice with a wider nominal audio bandwidth.
- Telefax Group 4: A high-speed, high-quality digital facsimile service that operates directly over the ISDN channels.
- Videotex: An online information service for access to remote databases.
- File Transfer (for example, EUROFILE or FTAM): Services and profiles defining end-to-end compatibility for transferring computer files.
ISDN supplementary services
Supplementary services modify or supplement basic bearer services and teleservices. The examples below cover number identification, call forwarding, call waiting, call hold, and direct dialling.
Number identification services
- CLIP (Calling Line Identification Presentation): Presents the calling number to the called party.
- CLIR (Calling Line Identification Restriction): Restricts presentation of the calling number to the called party.
- COLP (Connected Line Identification Presentation): Presents the number associated with the party that answered the call, including when the call was forwarded.
Call management and forwarding services
- CFU/CFB/CFNR (Call Forwarding): Redirects incoming calls to another number unconditionally, when the called party is busy, or when there is no reply.
- CW (Call Waiting): Indicates a new incoming call while another call is active.
- HOLD (Call Hold): Places an active call on hold temporarily.
Addressing services for businesses
- DDI (Direct Dialling In): Allows an external caller to dial an extension of a private telephone exchange (PABX) directly.
- MSN (Multiple Subscriber Number): Assigns multiple subscriber numbers to one ISDN access, for example for separate voice and fax terminals.