Whole idea of ISDN is to digitize the telephone network to permit transmission of video, audio and text over existing telephone lines. The purpose of ISDN is to provide fully integrated digital services to users.
Use of ISDN for accessing the Internet has breathed new life into ISDN service. ISDN's slow acceptance was due mostly to a lack of a need for its capabilities. Being a digital interface, ISDN has provided a means for accessing web sites efficiently and quickly. In response to this new demand, telephone companies are speedily adding ISDN services.
ISDN standard defines three channels types, each with different transmission rate: bearer channel (B), data channel (D) and hybrid channel (H) (see the below table)
|
Channel
|
Data Rate (Kbps)
|
|
B
|
64
|
|
D
|
16, 64
|
|
H
|
384, 1536, 1920
|
B channel is defined at a rate of 64 Kbps. It is the fundamental user channel and can carry any type of digital information in full duplex mode as long as required transmission doesn't exceed 64 kbps. A data channel can be either 16 or 64 kbps depending on the needs of user used to carry control signals for B channels.
Of two basic rate B channels, one is used to upload data to Internet and one to download from the Internet. D Channel helps in setting up connection and sustaining flow control. There are 3 ways ISDN can be used to interface to Internet, by employing an adaptor, modem or bridge/router. ISDN adaptors and modems limit access to a single user. Both terminate line into an ISDN service. Main difference between them is that ISDN modem takes Internet traffic and pushes it through computer serial port, whereas , faster ISDN adaptor connects directly to computer's buses.
ISDN routers/bridge allow for local network connections to be made by ISDN to Internet. ISDN termination is made into an Ethernet-type LAN so that numerous users can achieve access to Net by a single access address. Transfer rates between user and Internet are between 56 and 128 Kbps.