ATM ATM is simply a Data Link Layer protocol. It is asynchronous in the sense that the recurrence of the cells containing information from an individual user is not necessarily periodic. It is the technology of choice for evolving B-ISDN (Board Integrated Services Digital Network), for next generation LANs and WANs. ATM supports transmission speeds of 155Mbits / sec. In the future. Photonic approaches have made the advent of ATM switches feasible, and an evolution towards an all packetized, unified, broadband telecommunications and data communication world based on ATM is taking place. . Synchronous Transfer Mode (STM) was the first technique to be considered due to its compatibility with most existing systems and the desire to preserve the investment in existing equipment while evolving to a more flexible network. ATM has been proposed to overcome the limitations of STM and the large delay incurred by conventional packet switching. ATM is one of the general classes of digital packet technologies that relay and route traffic by means of an address contained within the packet. What makes packet technologies attractive for data traffic is that they exploit communication channels much more efficiently than the STM technologies common used to transmit digitized voice