d forwards the cell. The ATM layer also handles traffic-management functions between ATM nodes on both sides of the UNI (i.e., single VP link segment) while the virtual channel identified by a VCI value = 4 can be used for VP level end-to-end (user user) management functions. What are flows such as "F4 flows"? OAM (operations, administration, and manage- ment) flows deal with cells dedicated to fault and performance management of the total system. Consider ATM as a hierarchy of levels, particularly in SDH/SONET, which are the principal bearer formats for ATM. The lowest level where we have F1 flows is the regenerator section (called the section level in SONET). This is followed by F2 flows at the digital section level (called the line level in SONET). There are the F3 flows for the transmission path (called the path level in SONET). ATM adds F4 flows for virtual paths (VPs) and F5 flows for virtual channels (VCs), where multiple VCs are completely contained within a single VP. We discuss VPs and VCs in Section 20.8. 20.6.3 The ATM Adaptation Layer (AAL) The basic purpose of the AAL is to isolate the higher layers from the specific character- istics of the ATM layer by mapping the higher-layer protocol data units (PDUs) into the payload of the ATM cell and vice versa. 20.6.3.1 Sublayering of the AAL. To support services above the AAL, some inde- pendent functions are required of the AAL. These functions are organized in two logical sublayers: the convergence sublayer (CS) and the segmentation and reassembly (SAR) sublayer. The primary functions of these layers are: SAR - The segmentation of higher-layer information into a size suitable for the infor- mation field of an ATM cell. Reassembly of the contents of ATM cell information fields into higher-layer information. CS -Here the prime function is to provide the AAL service at the AAL-SAP (service access point). This sublayer is service dependent. 20.6.3.2 Service Classification of the AAL. Service classification is based on the following parameters: Timing relatio