mission frame adaption Transmission frame generation/recovery Bit timing Physical medium Convergence sublayer Physical medium Segmentation and reassembly sublayer Transmission convergence Figure 20.7 B-ISDN/ATM functional layering. ASYNCHRONOUS TRANSFER MODE for cell delineation, according to the cell delineation mechanism previously described, and carries the operations, administration, and maintenance (OAM) information relating to this cell flow. The PM sublayer provides bit transmission capability including bit transfer and bit alignment, as well as line coding and electrical-optical transformation. Of course, the principal function is the generation and reception of waveforms suitable for the medium, the insertion and extraction of bit timing information, and line coding where required. The primitives identified at the border between the PM and TC sublayers are a con- tinuous flow of logical bits or symbols with associated timing information. 20.6.1.1 Transmission Convergence Sublayer Functions. Among the important functions of this sublayer is the generation and recovery of transmission frame. Another function is transmission frame adaptation, which includes the actions necessary to struc- ture the cell flow according to the payload structure of the transmission frame (transmit direction), and to extract this cell flow out of the transmission frame (receive direction). The transmission frame may be a cell equivalent (i.e., no external envelope is added to the cell flow), an SDH/SONET envelope, an E1/T1 envelope, and SO on. In the transmit direction, the HEC sequence is calculated and inserted in the header. In the receive direc- tion, we include cell header verification. Here cell headers are checked for errors and, if possible, header errors are corrected. Cells are discarded where it is determined that headers are errored and are not correctable. Another transmission convergence function is cell rate decoupling. This involves the insertion and removal of idle cells in order to adapt the rate of valid ATM c