ISDN Applications

ISDN: Digital Services and Applications

ISDN integrated digital telecommunication services through defined interfaces and procedures. Compared with a telephone modem, it provided, among other features, 64 kbit/s B channels and a D channel for signaling. Services are divided into two groups: bearer services, which describe information transport, and teleservices, which include network and terminal functions.

Basic-rate access (BRI) used two 64 kbit/s B channels and one 16 kbit/s D channel. Primary-rate access (PRI) used 23 B channels plus a 64 kbit/s D channel at 1.544 Mbit/s, or 30 B channels plus D at 2.048 Mbit/s. The D channel was primarily for signaling and could also carry packet data in defined arrangements.

Bearer Services: Transmission Channels

Bearer services define the transport capability of an ISDN network. They provide a digital channel between access points, with attributes describing the transfer mode, bit rate, and information-transfer method.

• Unrestricted 64 kbit/s digital service: This is a basic ISDN bearer service. It provides a synchronous channel with a bit rate of $64 \text{ kbit/s}$. "Unrestricted" means that the network does not interpret the bits as a particular data type. The channel can carry voice, computer data, fax images, and other information within the service parameters.
• Speech service (3.1 kHz audio): A B channel could carry voice through the unrestricted service, but this category informed the network that the content was speech. This supported the choice of processing such as echo cancellation or digital-to-analog conversion when interworking with the traditional telephone network.
• Multiple-rate service (N × 64 kbit/s): For applications requiring more than 64 kbit/s, this service allowed users to aggregate B channels into one connection with a higher bit rate. An early videoconferencing system could combine two B channels for a $128 \text{ kbit/s}$ connection or six channels for a $384 \text{ kbit/s}$ H0 channel. The higher rate allowed video to be sent with fewer restrictions.

ITU-T bearer categories distinguish the 3.1 kHz audio bearer from unrestricted digital service. Multiple-rate categories include 2 × 64, 384, 1536, and 1920 kbit/s profiles; the available rate is determined by the access and interface.

Teleservices: End-to-End Applications

Teleservices combine a bearer service with network and terminal functions. They define how the user accesses a service and which functions the terminal equipment must provide.

• Telephony (7 kHz): Standard telephony uses a 3.1 kHz audio band, while the ISDN teleservice covered up to 7 kHz, more precisely 50-7000 Hz. The wider band improved speech and sound quality for professional applications such as sound studios and teleconferencing.
• Group 4 Telefax: A high-resolution digital fax standard. Older analog faxes could take a minute or more per page, while Group 4 fax over a 64 kbit/s B channel could transmit a page in a few seconds under typical conditions. The time depended on the document content and transmission procedures.
• Videotex: A service developed before the widespread use of the World Wide Web that provided access to text and simple graphics from remote databases, such as train schedules, stock quotes, or electronic phone books. ISDN reduced exchange time compared with earlier dial-up connections.

ITU-T treats Group 4 Telefax as an automatic facsimile teleservice and Videotex as a retrieval and mailbox service. The 7 kHz telephony teleservice is distinct from the 3.1 kHz speech bearer, so the service definition also describes capabilities expected from the network and terminal.

Supplementary Services: Call and User Features

Supplementary services extended bearer services and teleservices. In ISDN, control messages used the D channel, supporting number identification, call management, and user information transfer.

Q.932 provides generic procedures for supplementary services used with an existing call and for procedures invoked outside a call. Their control information is exchanged on the D channel, while the B channel continues to carry the associated user service.