w arrival in the field of telecommunications a separate committee called the ATM Forum was formed and was given the job of accelerating the development of ATM standards. Broadband ISDN Services The services provided by the B-ISDN are classified by the ITU-T into two categories, the interactive services and the distribution services. Interactive services are those where there is a two-way exchange of information, other than control signal information, between the two end users or between a user and a service provider. Examples of this type of service include conversational services, messaging services and retrieval services. Distribution services are those in which the information transfer is primarily one way, i.e. from the service provider to the B-ISDN subscriber. These include broadcast services for which the user has no control over the presentation. Some examples of the services enjoyed by a B-ISDN user are telephone, tele-education, video confer- encing, communication via mailboxes (e-mails and video mails), high resolution image and document retrieval, electronic newspaper, TV program distribution and digital video library. The large and diverse services of B-ISDN produced a huge load on the circuit switching hardware and for this very reason another switching technique was employed. This fast switching ATM technique thus became the basic switching technique for B-ISDN. ATM Concepts The transfer of information across the user-network interface for B-ISDN uses the asynchronous transfer mode (ATM). This technology is embedded into a protocol reference model that defines the B-ISDN user-network interface. As discussed earlier, the ATM is a form of packet transmission across the user- network similar to the X.25. The basic difference however lies in the signalling between the two. The X.25 includes control signalling on the same channel as data transfer, whereas the ATM makes use of common channel signalling. The ATM packets are of fixed size, referred to as cells, as compared to Transmission Network Planning 