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ATM Adaptation Layer


(ATM Adaption Layer) The part of the ATM protocol that breaks up application packets into 48-byte payloads that become ATM cells when the 5-byte headers are attached. The AAL resides between the higher layer transport protocols and the ATM layer. The AAL comprises two layers: Convergence Sublayer (CS) and Segmentation & Reassembly Sublayer (SAR). There are four types of AALs, which are summarized below. See ATM.

AAL-1 Connection-oriented, Constant Bit Rate (CBR), such as DS1 and DS3.

AAL-2 Connection-oriented, Variable Bit Rate (VBR).

AAL-3/4 Connection-oriented and connectionless, Available Bit Rate (ABR).

AAL-5 Connection-oriented, Unspecified Bit Rate (UBR). Least amount of error checking and retransmission.

AAL-6 Connection-oriented, MPEG-2 video streams.

ATM Adaption Layer
The SSCS is a service dependent layer, while the CPCS provides common functions such as CRC checking and padding to fill out a 48-byte payload. The SAR converts the output of the CS into cells for the ATM layer.
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