ysical (transmission) medium. How- ever, that is only a part of the physical layer. In the ATM protocol structure, the physical layer consists of two sub-layers: physical medium (PM) and transmission convergence (TC). PM is the lowest sub-layer and its functions are dependent on the physical medium. The main functions usually include bit alignment, line coding and electrical/optical conversion. TC lies above the PM and its main functions include maintaining the cell rate of the ATM cells. It does this by adding/extracting cells to adapt the rate of ATM cells. Other functions of TC include cell delineation, frame generation and frame recovery. ATM Layer This is the layer in which virtual paths and virtual channels are established. It is respon- sible for providing the connection-oriented service (i.e. it establishes connections before transmission). The features of this layer are independent of the physical layer below it. It is responsible for generic flow control (only for the UNI) and generation (and extrac- tions) of cell headers. Another important function includes cell VPI/VCI translations at the switching/cross-connect nodes. It is in this layer that cells from individual VPs and VCs are multiplexed into one resulting composite cell stream. ATM Adaptation Layer (AAL) As the name suggests, the ATM adaptation layer (AAL) 'adapts' the applications in a way to be understood by the ATM layers (or network) below. It also has two sub-layers: segmenta- tion and reassembly (SAR) and convergence (CL). The SAR layer takes the cells from upper layers and sends them to the ATM layer. Another function of this layer is 'reassembly'. It reassembles the cells it receives from the ATM layer and passes the result to the CL layer. The convergence sub-layer is an interface between the application and the SAR. It splits the cells for transmission towards the SAR and, on receiving the information from the SAR, it reconstructs the cells into the original message. CL is a service-dependent sub-layer making AAL also a service-depend