ISDN
The Public Switched Telephone Network (PSTN) has been transformed into an Integrated Systems Digital Network (ISDN). Implementation of Signalling System 7 (SS7) in the PSTN backbone has made possible such widespread services as Caller-ID and Dialed-Number delivery, 1800 Directory Number lookup, Calling Card services, and Digital Data Services. Using BRI (Basic Rate Interface) and PRI (Primary Rate Interface) services, ISDN call switching can be extended to customer premises equipment (CPE) and provide end-to-end digital paths. Previous to ISDN availability, data connectivity over the Public Switched Telephone Network (PSTN) was via Plain Old Telephone Service (POTS) using analog modems. Connectivity over ISDN offers the internetworking designer increased bandwidth, reduced call setup time, reduced latency, and lower signal/noise ratios. ISDN is now being deployed rapidly in numerous applications including Dial-on-Demand Routing, Dial Backup, SOHO connectivity, and modem pool aggregation. ISDN has many applications in internetworking. Recently, ISDN has seen incredible growth in the support of mass Small Office/Home Office (SOHO) dial-up connectivity.

ISDN can be used as a backup service for a leased-line connection between the remote and central offices. If the primary connectivity goes down, an ISDN circuit-switched connection is established and traffic is rerouted over ISDN. When the primary link is restored, traffic is redirected to the leased line, and the ISDN call is released. Small Office and Home Office sites can be now be economically supported with ISDN BRI services. This offers to the casual or full-time SOHO sites the capability to connect to their corporate site or the Internet at much higher speeds than available over POTS and modems. ISDN itself does not solve internetworking problems but is a useful tool for smaller network applications and for backup services.