functions: bits are converted into cells, the transmission and receipt of bits on the physical medium are controlled, ATM cell boundaries are tracked and cells are packaged into the appropriate types of frames for the physical medium. The ATM physical layer is divided into two parts: 1. Physical medium dependent (PMD) sublayer 2. Transmission convergence (TC) sublayer The PMD sublayer provides two key functions. Firstly, it synchronises transmission and reception by sending and receiving a continuous flow of bits with associated timing information. Secondly, it specifies the physical media for the physical medium used, including connector types and cable. Examples of phys- ical medium standards for the ATM include synchronous optical network/synchronous digital hierarchy Digital Hierarchies - PDH and SDH 53 Bytes Figure 3.31 Cell based ATM cell transmission (SONET/SDH), DS-3/E3, 155 Mbps over a multimode fibre (MMF) using the 8B/10B encoding scheme and 155 Mbps 8B/10B over shielded twisted-pair (STP) cabling. The TC sublayer has five functions: 1. Cell delineation. The cell delineation function maintains ATM cell boundaries, allowing devices to locate cells within a stream of bits. This also helps to de-scramble the cells at the destination, which were scrambled at the source for transmission purposes. 2. Transmission frame adaptation. The transfer of information at the ATM layer is always in terms of the ATM cells. Transmission frame adaptation packages these ATM cells into frames acceptable to the particular physical layer implementation. The cells can also be simply transmitted and received without framing them. 3. Transmission frame generation and maintenance. This function generates and maintains the frame structure appropriate for a given data rate for the physical layer. 4. Header error control (HEC) sequence generation and verification. Each cell header is protected by a header error control (HEC) code. HEC sequence generation and verification generates and checks the HEC code to ensure valid data. 5. Cel