l rate decoupling. Cell rate decoupling maintains synchronisation and inserts or suppresses idle (unassigned) ATM cells to adapt the rate of valid ATM cells to the payload capacity of the transmission system. After the functions of different sublayers of the physical layer are known, it is possible to dealt with its most important job: transmission of ATM cells. The ITU-T Recommendation I.432 specifies that the ATM cells may be transmitted at one of the several data rates: 25.6, 51.84, 155.52 or 622.08 Mbps. According to this Recommendation, two approaches used for transmitting can be followed: a cell based physical layer or an SDH based physical layer. Cell Based Physical Layer In this approach, no external framing is imposed and the 53-byte cells are transmitted as a continuous stream of data, as shown in Figure 3.31. However, no framing would lead to asynchronous receiving of these cells as a continuous stream and thus there should be some means of synchronising them. Synchronisation is achieved on the basis of the HEC field in the cell header. Cell delineation is performed at different times at different stages to confirm the correct occurrence of the HEC field. Once a cell undergoes these transitions it is assumed to be correct. The advantage of using a cell based transmission system is that the simple interface of framing and transfer are based on a common structure, which is the 53-byte cell. SDH Based Physical Layer This method of transmission employs time division multiplexing while packing the low capacity SDH containers into high capacity ones. This type of transmission also imposes a structure on the ATM cell stream; i.e. the cells are packed in the basic signal STM-1 frame format whose capacity is 155.52 Mbps. The payload can be offset from the beginning of the frame, indicated by the pointer in the section overhead of the frame. The synchronous payload envelope (SPE) consists of a 9-byte path overhead and Transmission Network Planning and Optimisation the remaining 2340 bytes are used to store the i