B-ISDN and ATM

Introduction

During the last few years,the desire for broadband communication has been moving closer to reality.Exceeding the bandwidth limitations of the installed switching system is an ongoing challenge,driven by the users' demand and brought closer to reality by new technology.Broadband Integrated Services Digital Network or B-ISDN is a broadband communication network developed by International Telegraph And Telephone Consultative Committee(CCITT) that enables the transmission of design simulations and other multimedia transmission that include text,voice, video and graphics in one network.This B-ISDN would be able to provide end users with increased transmission rate,up to 155.54Mbits/s on a switching basis.This is a great improvement as compared to the earlier rate of 64kbits/s employed in the ISDN which is not suitable for high definition moving pictures.

Evolution of B-ISDN

When 'integrated' digital switching and transmission systems were first considered in the 1960's,one of the selling points was that digital operation would pave the way for a truly multi-service network that would carry data and facimile as well as voice service.At that time non-voice services were very much in the minority,and consequently the standard of the integrated digital network(IDN) were geared to the transmission of speech The choice of a 64kbit/s(European standard) channel was conditioned by the expected need for several changes between the analogue and digital transmission modes as calls pass through the network.As advances in micro-electronic technology made it possible to extend digital techniques to the subscriber's link with the local exchange and thence to the subscriber's terminal,over old copper wire,standards for an 'Integrated Service Digital Network' or ISDN were established.This was still based on the 64kbits/s circuit network channel. This was quite reasonable as apart from voice ,typescript could be sent fairly fast and large software could be downloaded within minutes.However users still were not satisfied with this speed as in this competitive world,information is expected to reach end users within the blink of an eye.The only service that didn't seem possible within the 64kbits/s channels were those based on high definition moving pictures.

Within the International Telegraph And Telephone Consultative Committee(CCITT), there has been the desire to explore at an early stage the possibility of producing recommendation for networks that would carry broadband services meaning network that enables the transmission of bits at a very high rate to accomodate high definition moving pictures,as well as all those services offered by the ISDN.

The first requirement was that the new type of network should be able to carry not only existing services but also those that might be needed well in the future such as mixed media services,High Definition Television(HDTV) as well as holographic images.The change from anologue to digital operation on an international scale is proving costly enough.Hence it is desirable that the new network should be capable of evolution from ISDN,rather than requiring a complete replacement of existing network.After much debate, the CCITT decided to provide a new network fitting all the criterions mentioned above called Broadband Integrated Service Digital Network(B-ISDN).This network was also decided to be based on a switching technique known as Asynchronous Transfer Mode(ATM). The term ATM network is sometimes used as a synonym for this B-ISDN network.

What is ATM ?

ATM is a general class of digital packet switching technology that relay and route traffic by means of an address contained within the header.Unlike more familiar packet technology,such as X.25 or frame relay,ATM uses fixed length packets called cells.A cell is 53 bytes long comprising of a 5-byte header containing the address and a 48-byte information field.The switching of the ATM circuit is statistically natured and switching is done purely on hardware level without the need for software interface.Hence enabling a high switching rate.

Atm in B-ISDN

One of the fundamental principles of the B-ISDN is to offer subscribers a large variety of services such as video telephony,video surveillance,high volume file transfer,High Definition Television(HDTV) and many more services not offered by ISDN.These services would require the transmission of data at different bit rates .Thus a technique was needed to switch information flows that have time constrain relating to video or voice as well as information flows that are more sporadic and have widely variable bit rate.In this context,it is critical that the multiplexing and switching technology used to support these B-ISDN services and the needed bandwidth management be extremely flexible.Traditional time division multiplexing which devide bandwidth into a number of fixed capacity channels,simply do not have the needed flexibility.Packet switching technology which allow variable length packets,may not meet the stringent delay requirements needed for some real time services such as voice and video.Since ATM cells are only 53 bytes long,real time services when given priority, need not wait longer than 53-byte cell time(3 microseconds for a 155Mbits/s ) before being given access to the communication channel.The variable bit rate offered by B-ISDN is due to the ATM technology used.The fixed length cells allows the prediction of the size of the buffers to be used and hence the bits could be transmitted at any convenient rate unlike the fixed rate(64Kbits/s) specified by ISDN

Using fiber optic cables

There are also plans for B-ISDN to provide fiber optic cables to replace existing twisted pair copper wires.This may be costly but it could certainly bring much improvement.For example,The fiber optic offers such a large bandwidth that it could accomodate 100Gbits/s data rate.This would certainly reduce delay caused by the limited bandwidth offered by the conventional twisted pair wire, to within a blink of an eye.Furthermore the fiber optic medium is designed to produce a bit error rate orders of magnitude lower than that provided by twisted pair copper wires(10^-12 versus 10^-6 for example) Hence the requirements for error detection/correction protocols might be relaxed by the elimination of the need for extensive layers of protection. The size of the 5-byte header could be reduced by reducing the number of bits required for error correction,making room for a larger information field.

Conclusion

The full implementation of B-ISDN is certainly something worth looking forward to.With the usage of ATM technology as well as the laying of fiber optic cables,the B-ISDN would certainly revolutionise the transmission of information.With the extremely high and flexible bit rate, users could be assured of a negligible delay between the sender and recipient.

reference

  • "Packet Power" by C.J.Hughes(F) and A.(Gill)Waters.IEE Review,October 1991 ,pp 357-360
  • "Broadband ISDN ATM Layer Management:Operations,Administration, and Maintenance Consideration" by Susumu Yoneda,IEEE Network Magazine,May 1991, pp 31-35
  • An Introduction to Broadband Networks by Anthony S. Acampora