lasses are given in Table 41.4. SAR Functions in AAL type 2: all as for AAL type 1, items 1 to Segmentation and re-assembly (SAR) Protocol Data Unit (PDU): 41.4.10 I.363 B-ISDN ATM Adaptation Layer Type 2 SAR-PDUs contain a header having a sequence number field (AAL) specification to detect lost or misinserted cells and an information type field to indicate Beginning of Message (BOM), Continuation of Message This describes the interactions between the AAL and the next higher (COM), End of Message (EOM); also a trailer containing a Length layer and between the AAL and the ATM layer, and also AAL peer Indicator (LI) field and a CRC code field. 41/16 Asynchronous Transfer Mode 41.4.10.3 AAL Type 3 Message), a Sequence Number Field, and a Reserved Field; also a trailer containing a Length indicator field and a CRC code field. Services provided to higher layer are: 1. Message mode service: transports single AAL-SDUs in one or 41.4.10.4 AAL Type 4 (optionally) more CS-PDUs. 2. Streaming mode service: transports one or more fixed size Services provided to higher layer: transfer of the AAL-SDU for one AAL-SDUs in one CS-PDU. The AAL-SDU is the atomic unit AAL user to one or more AAL users through the ATM network. of data recognised by the application and is therefore delivered Message and streaming modes with assured/non-assured operation as a unit. procedures are defined as for AAL type 3. SAR Functions in AAL type 4: as for AAL type 3. Both service modes may offer the following procedures: Segmentation and re-assembly (SAR) Protocol Data Unit (PDU): Type 4 SAR-PDUs are as for type 3, except that the reserved field 1. Assured operations: flow control and re-transmission of cor- is used for Multiplexing Identification, to assist in interleaving rupted/missing CS-PDUs ensures delivery of correct AAL- ATM-SDUs from different CS-PDUs. SDUs. 2. Non-assured operations: Integral AAL-SDUs may be lost/cor- 41.4.11 I.413 B-ISDN User-Network interface rupted and will not be corrected. Options are delivery of cor- rupted AAL-SDUs a