With the increasing usage of computer network to transmit data, voice and video, old network transmission can not provide the high speed that needed. We want a faster network service. B-ISDN stands for Broadband Integrated Services Digital Network. It is a fast network link service.
B-ISDN comes from ISDN. ISDN is a switched digital, rather than analog, service that is also capable of transmitting voice and data now. It is a service offered by telephone company and similar to telephone line. Compare ISDN to telephone line. Which is digital but the telephone line is analog. A digital phone or fax machine can be used for the common telephone line service. While a ISDN data/voice modem is needed to change the digital signal to analog signal. It's bandwidth is about 64Kbps. When the speed of modem is just about 2400bps or 14.4Kbps, ISDN is a powerful tool. While now V.90 modem has a bandwidth of 56Kbps. The speed of standard ISDN is not so fast comparing with telephone line. So ISDN is not so attractive now. It is called "It still does nothing" or "I still don't need".
B-ISDN comes in this case. The name means two things: broad bandwidth and digital. B-ISDN is a kind of combination of SONET and ATM technologies to transmit digital information of data, voice, video and image package.
SONET (Synchronous Optical Network) is the optical transmission interface and mechanism that will deliver broad-band ISDN. SONET is not the only one that can be used to deliver broad-band information on B-ISDN. T-1, T-3 lines, frame relay and some future technology like HDTV support B-ISDN.
ATM is used as the switching architecture that will ensure that all these kinds of information can be transferred by B-ISDN. ATM and SONET formed the underlying network architecture of the B-ISDN of the future. In B-ISDN, SONET’s synchronous payload envelope provides empty boxes for ATM’s cells or frames, which can manage all these information into ATM cells for transferring on the board-band lines of B-ISDN.
Now B-ISDN is mostly used on the lines like T-1, T-3. T-1 is a kind of digital service that can be consider as a kind of B-ISDN. On current T-1 lines, it uses 24 64Kbps channels for the data transmission. That’s 23B+D named as primary rate interface. The architecture of T-1: 24bytes a frame, 8000 frames per second. The speed is 1.544Mbps. At both ends of T-1 line, CSU/DSU is used to send the information to customer. In ISDN’s BRI (basic rate interface), it is 2B+D. In Europe, ISDN’s PRI is offered as 30B+D as E-1. That offers the bandwidth of about 2Mbps. With the increasing of network transmission, B-ISDN will be widely used in the future because it supports existing services as well as future services.