ATM Adaptation layers are classified based on their support for various service classes as well as following parameters:

• Timing between source and destination (required or not required)

• Bit rate (constant or variable)

• connection mode (whether connection oriented or connectionless)

• AAL service Classes- A/B/C/D

Class A CBR service with end-to-end timing, connection-oriented

Class B VBR service with end-to-end timing, connection-oriented

Class C VBR service with no timing required, connection-oriented

Class D VBR service with no timing required, connectionless

AAL Layer in ATM is divided into two sublayers viz. Convergence Sublayer (CS) and Segmentation and Reassembly Sublayer(SAR) based on functionality of AAL layer. The function of ATM Adaptation layer in ATM protocol stack are summarized below:

CS sublayer performs following functions:

• management of lost and wrongly delivered ATM cells

• Timing recovery

• Interleaving

SAR sublayer performs following functions:

• Fragmentation of bit stream from upper layers into size equal to ATM cells for transmission to ATM layer in the stack.

• Reassembly of ATM cells into bit stream for delivery to upper layers.

AAL1 supports CBR traffic, specifies how TDM-type circuits can be emulated over an ATM network. Circuit emulation is specified in detail for DS1, DS3, and n x DS0 support in ANSI T1.630.

AAL 2 specifies ATM transport of connection-oriented circuit and VBR high-bit- rate packetized audio and video (like the Motion PhotoGraphic Experts (MPEG) video encoding standard).

AAL3/4 supports VBR traffic, both connection-oriented and connectionless.

AAL. 5 supports lightweight VBR traffic, both connection-oriented and connectionless, for example, signaling SSCS protocols, FR, and multiple protocols (including IP) can be operated over AAL5.

ATM AAL1(AAL Type-1) Protocol Layer

ATM AAL2(AAL Type-2) Protocol Layer

ATM AAL3(AAL Type-3), AAL4(AAL Type-4) Protocol Layer

ATM AAL5(AAL Type-5) Protocol Layer