ISDN is accessed through one of two services, named Basic Rate Access (BRA) and Primary Rate Access (PRA). Basic Rate Access (BRA) provides an ISDN user with simultaneous access to two 64 kbps data channels using the existing twisted pair copper telephone cable. Each data channel is referred to as a B-channel and can carry voice or data. Another channel, the D-channel, operates at 16 kbps and is used for signalling to the ISDN. The total data rate of BRA is therefore 144 kbps. The two B-channels and the single signalling channel give rise to the term '2B+D'. Basic rate ISDN is intended for low capacity usage, such as that required for small businesses.
Primary rate access can carry 30 independent voice or data channels, each at 64 kbps. The structure has a 64 kbps D-channel for signalling between devices and the network, and a 64 kbps channel for synchronisation and monitoring. The total data rate of PRA is 2.048 Mbps. Although primary rate ISDN is overkill for many single user applications, it is a cheap way of providing several basic rate lines to one site.
The ISDN passive bus, which can be a maximum of 1 km in length, is a cable which in user premises. It enables up to eight user devices to be attached to the basic rate ISDN interface. Since there are only two B-channels, only two of the eight devices can communicate at any one time. For this reason, each device must contend for access to the passive bus.
ISDN signalling information, carried in the D-channel, is used to establish, monitor and control ISDN connections between users as well as instigating, the audible ringing or engaged tones. The D-channel is available to users equipment in some countries. The use of the D-channel by routing equipment could reduce call costs if small link acknowledgement data packets could be sent over the D-channel.
A Terminal Adapter (TA) is to an ISDN line what a modem is to a normal analogue telephone line. It is an external device which connects a PC to a ISDN circuit, allowing non-ISDN equipment to use ISDN. A disadvantage of terminal adapters is that not all the information from the D-channel of the ISDN line passes through the TA, so that the non-ISDN equipment cannot take full advantage of ISDN facilities, such as Calling Line Identification. A terminal adapter has to change the format of the data to conform to one of two standards used with asynchronous transfer of data down the B channel of the ISDN line - V.110 or V.120 and provide means of setting up and clearing calls. Terminal adapters usually use an extended range of the Hayes AT commands to do this. When used for LAN to LAN routing terminal adapters are operated synchronously.
A router is used to connect two or more dissimilar networks together which may be LANs, WANs or a mixture of both. It operates at the network layer of the OSI model using a network based address such as an Internet address or a telephone type number to route the data to the correct destination. A specific internetworking protocol (such as IP or IPX) is used by all routers in a network. Network based addresses such as Internet address "bbc@bbc.house.uk" are converted into local area network adapter addresses such as "H508t6045678" via an addressing table. The router can be more selective than the bridge and as a result is able to filter out unwanted data which is meant only for local computers on one side of the router. Thus unnecessary traffic across an ISDN link is removed costs reduced and LANs belonging to different organisations may be safely isolated.
Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) provide a standard method for allowing any two devices to communicate over a serial link (byte-by-byte). However, this is being gradually replaced by PPP which adds error correction and compression techniques.
PPP is defined in several RFCs (requests for comment). Since PPP has many features not all of which may be implemented by a developer the only way to see if two pieces of equipment using PPP work together is to test them.