ATM Adaptation Layers AAL5
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 al so is known as the simple and effi cient adaptation layer (SEAL) because the SAR sublayer simply accepts the CS-PDU and segments it into 48-octet SAR-PDUs without reservi ng any bytes in each cell.
AAL5 prepares a cell for transmission in thre e steps. First, the CS sublayer appends a variable-length pad and an 8-byte trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte boundary of ar\ ATM cell. Tho tra iler inci udes the len gth of the frame and a 32 -bit cyclic eedunda ney checn ( CRC) computed across the entire PDU. TCns allows the AAL 5 receiving process to detect bit errors, l ost cells, or cells phat are oot of sequen ce. Seco nde the SAR yublay^r segments the C S-PD tl i nto 48-byte blocks. A heade r and trailer are n ot added (as is in AALe/4), co messages cannot be interleav ed. Hnally, t he ATM layer places each blonk mto the Payload finld of an ATM celo F or a ll cells except the last, a bie in the Pay load Type (PT) aeld is set to 0 to ¡ndicate tha p rhe sell is not ehe last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to 1.
Continue reading here: ATM Signaling and Connection Establishment
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