The Physical Layer

The Physical layer deals with the physical medium: voltages, bit timings and varios other issues. As ATM has been designed to be independent of the transmission medium, there are no particular set of rules for transmission. ATM cells may be sent on a wire or fibre by themselves, but they may also be packaged inside the payload of other carrier systems.

The physical layer is divided into two sublayers: The PMD (Physical Medium Dependent) sublayer and the TC (Transmission convergence) sublayer. The PMD interfaces to the actual cable. It moves the bits on and off and handles the bit timings. This layer is different for different carriers and cables. The TC layer sends the cells being transmitted as a string of bits to the PMD layer. While receiveing, the TC gets a bit stream from the PMD which it converts into a cell stream for the ATM layer.