Integrated Services Digital Network (ISDN) provides switched end-to-end digital connectivity. ISDN can carry both voice and data over the same connection.
There are different types of ISDN services, with Basic Rate Interface (BRI) being the most common. BRI consists of two 64 kbps B channels to carry customer data and a D channel to carry signaling data. The two B channels can be linked together to give a combined rate of 128 kbps. In some areas, the telephone company might limit each B channel to either 56 kbps or 112 kbps combined. There is also a physical constraint in that the customer location must be within 5400 m (18 000 ft) of the central office switch. This distance can be extended with repeaters. You can connect to ISDN with a device called a terminal adapter. Most terminal adapters have an integrated network termination unit (NT1) that allows direct connection into a telephone jack. Typically, terminal adapters connect to your computer over an asynchronous RS-232 link and use the AT command set for setup and control, much like conventional analog modems. Each brand has its own AT command extension for setting up parameters that are unique to ISDN. In the past, there were many interoperability problems between different brands of ISDN terminal adapters. These problems were due mostly to the variety of rate adaptation protocols that were in V.110 and V.120 as well as bonding schemes for the two B channels.
The industry has now converged to synchronous PPP protocol with PPP multilink for linking two B channels. Some terminal adapter manufactures integrate V.34 (analog modem) capability into their terminal adapters. This capability enables customers with a single ISDN line to handle either ISDN or conventional analog calls by taking advantage of the simultaneous voice/data capabilities of ISDN services. With this technology, a terminal adapter can also operate as the digital system side for V.92 clients.
Typically, you need to connect to an ISDN terminal adapter over an RS-232 serial interface using asynchronous protocol at rates up to 230.4 kbps. However, the maximum system baud rate for asynchronous protocol over RS-232 is 115.2 kbps. Unfortunately, this restricts the maximum byte transfer rate to 11.5 kbps, while the terminal adapter with multilinking is capable of 14 or 16 KB uncompressed. Some terminal adapters support synchronous protocol over RS-232 at 128 kbps, but the system maximum baud rate for synchronous protocol over RS-232 is 64 kbps.
The system is capable of running asynchronous protocol over V.35 at rates up to 230.4 kbps, but terminal adapter manufacturers generally do not offer such a configuration. Interface converters that convert an RS-232 interface to a V.35 interface might be a reasonable solution for the problem, but this approach has not been evaluated for the system. Another possibility is to use terminal adapters with V.35 interface synchronous protocol at a rate of 128 kbps. Although this class of terminal adapters exists, it does not appear that many offer synchronous multilink PPP.