R functions are addressed and the format of the SAR Protocol 4. Cell Loss Priority (bit): Value = 1: Cell is subject to discard Data Unit (PDU) is defined for each type. depending on network conditions. Value = 0: cell has higher priority. 5. HEC field: operation described in I.432. 41.4.10.1 AAL Type 1 6. Reserved field: use not yet specified. Services provided to higher layer: 41.4.9 I.362 B-ISDN ATM Adaptation Layer 1. Transfer/delivery of constant bit-rate 'service data units'. (AAL) functional description 2. Transfer of timing information. 3. Indication when information is errored or lost. This classifies the services which may require AAL capabilities accessed through different Service Access Points (SAPs). SAR Functions in AAL type 1 are: The organisation of the AAL into two sublayers is defined. These are the Segmentation and Re-assembly (SAR) sublayer and the 1. Segmentation and re-assembly of user information. Convergence (CS) sublayer. 2. Handling of cell delay variation. The Service classification is based on three characteristics: 3. Handling of lost/misinserted cells. 4. Source clock frequency recovery at receiver. 1. Timing relationship between source and destination, needed or 5. Monitoring for/ handling of AAL Protocol control information not needed. bit errors. 2. Bit rate, constant or variable. 6. Monitoring for/ handling of user information field bit-errors. 3. Connection mode, connectionless or connection oriented. Segmentation and re-assembly (SAR) Protocol Data Unit (PDU): Four classes are defined as given in Table 41.3. Examples of the Type 1 SAR-PDUs contain a header having a 4-bit sequence number field to detect lost or misinserted cells, also a 4-bit sequence number Table 41.3 Four classes of service protection field. Class A Class B Class C Class D 41.4.10.2 AAL Type 2 Timing Required Not required Services provided to higher layer are: relationship between source 1. Transfer of variable source bit-rate SDUs. 2. As for AAL type 1. destination 3. As for AAL type 1. services in the four c