Public Networking: ISDN Features

ISDN evolved from the original analog-based telephony PSTN that provided end-to-end connectivity to support a wide variety of services to which users had access through a limited set of standard multipurpose customer interfaces. The major attributes of ISDN include:

• End-to-end digital connectivity

• Access and service integration

• User control of service features

• Standardized user-to-network interfaces

The CCITT (later the ITU) originally defined two major, globally standard interfaces: BRI and PRI. The BRI provides a station-direct interface to ISDN network facilities or ISDN-compatible customer premises equipment. The PRI provides an interface for switch connections, such as PBX-to-PBX. Both interfaces have similar format structures: one D-channel and multiple B-channels. The B-channel, or bearer channel, is a 64-Kbps transmission path for basic and primary rates and carries voice, data, and image or video communications traffic to and from the network or switch. The D-channel, or data channel, uses 64 Kbps for primary line and 16 Kbps for basic lines and carries packeted signaling and control information across the interface. The D-channel signaling is out-of-band because it supports calls on the separate B-channels. In North America, the PRI has 23 B-channels and 1 D-channel (23 B+D); outside of North America PRI has 30 B-channels (30B+D). ISDN services are carried over T1/E1 transmission circuits using the DS1 format structure: (24/32) 64-Kbps channels. Globally, the BRI has 2 B-channels and 1 D-channel (2B+D).

The ITU standard established for D-channel signaling is the Q.931 message-oriented signaling protocol. This protocol allows the use of the same access lines for many different services and the introduction of new services without requiring users to replace existing ISDN-compatible communications equipment. ISDN is defined internationally as the physical interfaces between communications equipment and networks and the signaling protocols exchanged by the elements.

Several PBX network features that work with ISDN PRI services, such as ANI and Station Identification (SID), CBCSS, non-facility associated signaling (NFAS), and D-channel backup.