Integrated Services Digital Network

Internationally developed telecommunications network for sending signals in digital format that offers faster data transfer rates than traditional analogue telephone circuits. It involves converting the ‘local loop’ – the link between the user's telephone (or private automatic branch exchange) and the digital telephone exchange – from an analogue system into a digital system, thereby greatly increasing the amount of information that can be carried. The words ‘integrated services’ refer to the capacity for ISDN to make two connections simultaneously, in any combination of data (voice, video, and fax), over a single line. The data is sent and received in ‘digital’ format and ISDN is described as a ‘network’ because it extends from the local telephone exchange to the remote user. Before the availability of DSL (digital subscriber line), ISDN was used by many companies and educational institutions for internet connections.

ISDN has advantages in higher voice quality, better quality faxes, and the possibility of data transfer between computers faster than current modems. Speeds of roughly 128 kilobits per second are theoretically possible using existing telephone lines. With ISDN's Basic Rate Access, a multiplexer divides one voice telephone line into three channels: two B bands and a D band. Each B band offers 64 kilobits per second and can carry one voice conversation or 50 simultaneous data calls at 1,200 bits per second. The D band is a data-signalling channel operating at 16 kilobits per second. With Primary Rate Access, ISDN provides 30 B channels.