ISDNn

ISDNn is a non-standard term sometimes used in telecommunications literature and vendor documentation to describe an Integrated Services Digital Network interface configured to carry a variable number of bearer channels, denoted by n. The designation is not a formal ITU-T specification in its own right, but a shorthand for discussing ISDN implementations that use a configurable number of B channels alongside a control D channel.

In ISDN terminology, B channels carry user data while the D channel handles signaling and control. An ISDNn interface thus represents a family concept where the total capacity scales with the number of B channels. In common ISDN deployments, a basic rate interface (BRI) provides 2 B channels plus a D channel (2B+D), whereas primary rate interfaces (PRI) offer many more B channels (for example, 23B+D in North America or 30B+D in Europe). The exact rates differ by region, with typical B channels at 64 kbps and D channels at 16 kbps for BRIs, or 64 kbps for D channels in PRI configurations.

Signaling for ISDNn follows established ISDN protocols, primarily Q.931 for call control and Q.921 for lower-layer signaling, with the D channel used to set up and tear down calls and manage network resources. Compatibility depends on adherence to the general ISDN framework rather than a separate ISDNn standard.

Today, ISDN usage has declined in favor of IP-based services and broadband access. ISDNn remains a useful framing concept in historical reviews, legacy systems documentation, and some vendor-specific configurations where a flexible, channel-count-based approach to ISDN is described. See also ISDN, BRI, PRI, and related ITU-T standards.