ells to the payload capacity of the transmission system. In other words, cells must be generated to exactly fill the payload of SDH/SONET, as an example, whether the cells are idle or busy. Section 20.12.5 gives several examples of transporting cells using the convergence sublayer. 20.6.2 The ATM Layer Table 20.4 shows the ATM layering functions supported at the UNI (U-plane). The ATM layer is completely independent of the physical medium. One important function of this layer is encapsulation. This includes cell header generation and extraction. In the transmit direction, the cell header generation function receives a cell information field from a higher layer and generates an appropriate ATM cell header except for the header error control (HEC) sequence. This function can also include the translation from a service access point (SAP) identifier to a VP (virtual path) and VC (virtual circuit) identifier. In the receive direction, the cell header extraction function removes the ATM cell header and passes the cell information field to a higher layer. As in the transmit direction, this function can also include a translation of a VPI and VCI into an SAP identifier. Table 20.4 ATM Layer Functions Supported at the UNI Functions Parameters Multiplexing among different ATM connections VPI/VCI Cell rate decoupling (unassigned cells) Preassigned header field values Cell discrimination based on predefined header field values Preassigned header field values Payload type discrimination PT field Loss priority indication and selective cell discarding CLP field, network congestion state Traffic shaping Traffic descriptor Source: Based on Refs. 3 and 8. 20.6 ATM LAYERING AND B-ISDN In the case of the NNI, the GFC is applied at the ATM layer. The flow control infor- mation is carried in assigned and unassigned cells. Cells carrying this information are generated in the ATM layer. In a switch the ATM layer determines where the incoming cells should be forwarded, resets the corresponding connection identifiers for the next link, an