There are many viewpoints as to how ATM fits within the OSI Model. If you examine the functions provided by ATM, the distinction is less clear. Perhaps Walter J. Goralski in his book Introduction to ATM Networking sums it up best: "The exact relationship of the ATM layers to the OSI layers is undefined." In other words, there is no alignment of ATM layers and functions to OSI layers and functions. This is because at the time OSI was developed, communications were very much different than they are today.

The OSI Model was developed for protocols used over unreliable facilities. Copper cable at fairly low speeds (in comparison with today's networks) was the only option available, and data transmission was very unreliable. This is why protocols such as X.25 have so much error detection/correction. The network nodes had to perform most of this function to get the data to where it belonged. If left to the endpoints, the networks would be clogged with retransmissions end to end, rather than node to node.

ATM does not suffer from these problems because today the transmission medium is highly reliable. Error detection/correction is not necessarily within the network nodes to transmit data quickly with very little processing. The endpoints are left with the chore of checking the user data for errors and managing retransmissions.

ATM is divided into layers (See Fig. 9.3).The physical layer is divided into two parts. The ATM physical medium sublayer is responsible for transmission of data over the physical medium, regardless of the type of medium used. ATM was originally designed to operate over fiber optics but because of the slow deployment of fiber, was later modified to operate over copper and coaxial facilities as well.

The physical medium sublayer is responsible for receiving and transmitting bit streams in a continuous method. This is important to channelized services with rely on constant bit streams to maintain synchronization. When the bit stream stops, channelized equipment interprets the condition as an error and releases the virtual connection. bit synchronization is also maintained by this sublayer.

The transmission convergence sublayer is responsible for the transmission and reception of frames over framed transmission facility, such as T-3. ATM cells are packed into these frames and unpacked at the remote end. This sublayer also performs error detection/correction but only on the ATM header. This prevents the cells from being sent to the wrong destination.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.