The integrated Services Digital Network (ISDN) is a set of digital transmission standards which are used for end-to-end digital connectivity. "Integrated Services'' referring to its ability to sustain numerous applications. "Digital Network'' relating to its end-to-end digital connection. In general, ISDN networks extend from the local telephone exchange to the remote user and include all the telecommunications and switching equipments in between. ISDN supports voice and data. In the past, video, audio, voice and data services required atleast four separate networks. ISDN integrates all four over the same network.
History. ISDN is based on technology developed during the 1970's designed to address the problem of how to transport digital services across a telephony infrastructure based on copper wiring originally intended to carry analog signals only. To meet the customer needs, initially, the telephone companies created Integrated Digital Networks (IDN). IDN is a combination of networks available for different purposes. Access to these networks is by digital pipes which are time multiplexed channels sharing very high speed paths. By this network, customers can use their local loops to transmit both voice and data to their telephone company's central office. The office then directs these calls to the appropriate digital networks via the digital pipes. ISDN integrates customer services with the IDN. With ISDN, all customers will become digital rather than analog.
Standards. ISDN technology is standardised according to recommunications of the CCITT, now ITU, which describe the protocols and architectures to implement a world wide digital communication network. The CCITT was comprised to study groups (SGs). Each SG has its own area of expertise. The following are the SGs related ISDN. SG VII — Public data network (X.25) X-series standards SG VIII — Terminal equipment for telematic services SG XI — ISDN and telephone network switching and signalling SG XII — Transmission performance of telephone networks and terminals SG XV — Transmission systems
SG XVII — Data transmission over public telephone networks
SG XVIII — Digital networks including ISDN.
Types of ISDN.There are two types of ISDN:
Narrow band ISDN (N-ISDN) and Broadband ISDN (B-ISDN)
N-ISDN — Carry data rating upto 64 kbps, ranging up to T1 rates. Sometimes used to refer to regular telephone and nonvideo capable systems.
B-ISDN — The communication standards being dveloped by the ITU to handle the high bandwidth applications such as video. B-ISDN will use ATM technology over SONET based transmission units to provide data rates of 155 Mbps to 622 Mbps and beyond.
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