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ISDN is  an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN 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 kb/s (bits per second).  There are two basic types of ISDN service: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 64 kb/s D channel for a total of 1536 kb/s.

To access BRI service, it is necessary to subscribe to an ISDN phone line. Customer must be within 18000 feet (about 3.4 miles or 5.5 km) of the telephone company central office for BRI service; beyond that, expensive repeater devices are required, or ISDN service may not be available at all. Customers will also need special equipment to communicate with the phone company switch and with other ISDN devices. These devices include ISDN Terminal Adapters and ISDN Routers.

Installing ISDN

The Central Office will provide the customer with a U interface. The U interface is a two-wire (single pair) interface from the phone switch, the same physical interface provided for POTS lines. It supports full-duplex data transfer over a single pair of wires, therefore only a single device can be connected to a U interface. This device is called an Network Termination 1 (NT-1).  The NT-1 is a relatively simple device that converts the 2-wire U interface into the 4-wire S/T interface. The S/T interface supports multiple devices (up to 7 devices can be placed on the S/T bus) because, while it is still a full-duplex interface, there is now a pair of wires for received data, and another for transmit data.  Technically, ISDN devices must go through an Network Termination 2 (NT-2) device, which converts the T interface into the S interface. Virtually all ISDN devices include an NT-2 in their design. The NT-2 communicates with terminal equipment, and handles the Layer 2 and 3 ISDN protocols. Devices that connect to the S/T (or S) interface include ISDN capable telephones and FAX machines, video teleconferencing equipment, bridge/routers, and terminal adapters. All devices that are designed for ISDN are designated Terminal Equipment 1 (TE1). All other communication devices that are not ISDN capable, but have a POTS telephone interface (also called the R interface), including ordinary analog telephones, FAX machines, and modems, are designated Terminal Equipment 2 (TE2). A Terminal Adapter (TA) connects a TE2 to an ISDN S/T bus. The diagram is shown below.

ISDN Network Interfaces Diagram


During installation, the central company will connect its end of the wire pair to digital equipment at the central office. The signaling conventions on the U-interface are designed to transfer data over a fairly long distance (up to 18000 feet), at a fairly high rate, on one pair of wires, in both directions at once. The company have to program the central office switch with the SPIDs that represent telephone number and type of service (data, voice, or both).

ISDN Routers and Bridges

AN ISDN router connects to an Ethernet instead of a COM port. This device will serve as the connection from the school to the central office.  The PC requires an Ethernet adapter card. A router filters out messages and only sends real data over the ISDN line. An external LAN-attached ISDN router provides a simple way for several computers to share a single ISDN line. There should also be a specific router support at the central site. The central site sees a single phone line and traffic from a single machine.