Fig. 11.34. Types of ATM adaptation layer.
AAL 2. It is designed to carry class B traffic. The 8 bit start field (SF) includes a 6 bit offset field (OSF), which indicates the number of bytes between the start field and the payload or between the start field. The 24 bit Header (H) includes the Length indicator (LI) for variable length payload.
AAL 3/4. It is started as two separate adaptation layers (AAL 3 and AAL 4). The specification of two AAL's merged and the newly formed type is called AAL 3/4. AAL 3/4 is designed to take variable length frames up to 64 kbytes and segment them into cells. The 2-bit ST or segment type is set as follows. ST = 10 for BOM, ST = 00 for COM, ST = 01 for EOM and ST = 11, if the packet fits in side a single cell. The 10 bit multiplexing identifier (MID) is used to distinguish among the cells originating from different data grams.
AAL 5. It is an efficient way to transfer information in ATM. The IP packet upto 64 k bytes is packaged into a CS packet that contains a length indicator and a 32 bit CRC calculated over the complete CS packet with the generator. AAL 5 provides services similar to those AAL 3/4, but uses fewer control fields.
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