Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

29.6 TRANSMISSION OF ATM CELLS

In ATM, no framing is imposed and a continuous stream of cells is transmitted. Hence synchronization is required—the receiver has to know where the cell starts. Header error control (HEC) field provides synchronization. A cell delineation algorithm is required to detect the boundaries of the cells. The algorithm is as follows: match the received HEC with the calculated HEC with an assumed cell structure. The cell delineation algorithm is successful if HEC matches x times consecutively, and it is lost if HEC mismatches y times continuously. The values of x and y are design parameters between 5 and 10. Note that greater x implies longer delay in synchronization but robustness against false delineation; greater y implies longer delay in recognizing a misalignment but robustness against false misalignment.

In an ATM network, the cells are transmitted as a stream. Hence, each switch has to identify the beginning of each cell. This is done through the cell delineation algorithm.