four octets in the header) and the HEC octet. This octet is produced at the originating end using a generating polynomial covering those first four octets of the cell. The generating polynomial is X8 + X2 + X + 1. There must be correlation at the receiving end between those first four octets and the HEC octet, which we can call a remainder. This is only true, of course, if there is no error in the header. When there is an error, correlation cannot be achieved, and the processor just goes to the next cell. There must be a constant cell flow rate to keep the far-end node synchronized and aligned. 20.6 ATM LAYERING AND B-ISDN 20.6 ATM LAYERING AND B-ISDN The B-ISDN reference model was given in Figure 20.3, and its several planes were described. This section provides brief descriptions of the ATM layers and sublayers. Figure 20.7 illustrates B-ISDN/ATM layering and sublayering of the protocol reference model. It identifies functions of the physical layer, the ATM layer, and the AAL (ATM adaptation layer), and related sublayers. 20.6.1 Physical Layer The physical layer consists of two sublayers. The physical medium (PM) sublayer includes only physical medium-dependent functions. The transmission convergence (TC) sublayer performs all functions required to transform a flow of cells into a flow of data units (i.e., bits) which can be transmitted and received over a physical medium. The service data unit (SDU) crossing the boundary between the ATM layer and the physical layer is a flow of valid cells. The ATM layer is unique (independent of the underlying physical layer). The data flow inserted in the transmission system payload is physical medium-independent and self-supported. The physical layer merges the ATM cell flow with appropriate information Higher layer functions Higher layers Convergence Segmentation and reassembly Generic flow control Cell header generation/extraction Cell VPI/VCI translation Cell multiplex and demultiplex Cell rate decoupling HEC header sequence generation/verification Cell delineation Trans