ISDN, which stands for Integrated Services Digital Network, is a system of digital phone connections. This system allows data to be transmitted simultaneously across the world using end-to-end digital connectivity. With ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth of 64 Kbps (bits per second). Some switches limit B channels to a capacity of 56 Kbps. A data channel (D channel) handles signaling at 16 Kbps or 64 Kbps, depending on the service type. Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices are called ISDN Terminal Rate Adapters or Terminal Adapters.
There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 kbps B channels and one 16 kbps D channel for a total of 144 kbps. This basic service is intended to meet the needs of most individual users. PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kbps D channel for a total of 1536 kbps. In Europe, PRI consists of 30 B channels plus one 64 kbps D channel for a total of 1984 kbps. The original version of ISDN employs baseband transmission of digital data. Baseband refers to a type of digital transmission in which each wire carries only one signal, or channel at a time. Another version, called B-ISDN, uses broadband transmission (a single wire, or channel can carry multiple signals simultaneously). Broadband can support transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available.