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.

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The Data Link Layer

Data Link Layer Issues

• Functions of the Data Link Layer
– Provide service interface to the network layer
– Dealing with transmission errors
– Regulating data flow: Slow receivers not swamped
by fast senders
• To accomplish these goals, the data page link layer
encapsulates the packets into frames.

Framing

• To detect or correct errors in the raw bit stream
from the physical layer, the data page link layer
breaks the bit stream up into discrete frames and
computes the checksum for each frame.
• Four methods can be used to mark the start and
end of each frame:
– Character count
– Flag bytes with byte stuffing
– Starting and ending flags, with bit stuffing
– Physical layer coding violations