 enough single bytes to equal 48 bytes. AAL Type 2 The type 2 deals with the variable bit rate information and is intended for analogue applications, such as video and audio, that require timing information but do not require a constant bit rate. An initial specification for this type has been withdrawn. AAL Type 3/4 AAL3/4 supports both connection-oriented and connectionless data. It was designed for network service providers and is closely aligned with the switched multimegabit data service (SMDS). AAL3/4 is used to transmit SMDS packets over an ATM network. AAL3/4 prepares a cell for transmission in four steps. Firstly, the convergence sublayer (CS) creates a protocol data unit (PDU) by prepending a beginning/end tag header to the frame and appending a length field as a trailer. Secondly, the segmentation and reassembly (SAR) sublayer fragments the PDU and attaches a header before it. Then, the SAR sublayer appends a CRC-10 trailer to each PDU fragment for error control. Finally, the completed SAR PDU becomes the payload field of an ATM cell to which the ATM layer appends the standard ATM header at the start. An AAL 3/4 SAR PDU header consists of type, sequence number and multiplexing identifier fields. Type fields identify whether a cell is the beginning, continuation or end of a message. Sequence number fields identify the order in which cells should be reassembled. The multiplexing identifier field determines Digital Hierarchies - PDH and SDH which cells from different traffic sources are interleaved on the same virtual circuit connection (VCC) SO that the correct cells are reassembled at the destination. AAL Type 5 AAL5 is the primary AAL for data and supports both connection-oriented and connectionless data. It is used to transfer most non-SMDS data, such as classical IP, over ATM and LAN emulation (LANE). AAL5 also is known as the simple and efficient adaptation layer (SEAL) because the SAR sublayer simply accepts the CS-PDU and segments it into 48-octet SAR PDUs without adding any additional fields. AAL