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AAL - technical definition

In the ATM reference model, a layer that comprises two sublayers.The Convergence Sublayer (CS) functions are determined by the specifics of the service class supported by that particular AAL.The Segmentation and Reassembly (SAR) sublayer functions to segment the user data into payloads for insertion into cells, on the transmit side. On the receive side, the SAR extracts the payload from the cells and reassembles the data into the information stream as originally transmitted. There exist defined AAL Types 1, 2, 3/4, and 5, each of which supports a specific class of traffic. See also ATM, ATM reference model, CS, and SAR.

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(ATM Adaption Layer) The part of the ATM protocol that breaks up application packets into 48-byte payloads which 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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