The purpose of the AA L is to interwork ATM networks with other non-ATM networks. It provides the necessary information to ensure proper handling of data from a variety of different networks. Keep in mind that protocol headers remain intact in some cases, treated by AAL as part of the user data. For example, bridging an Ethernet LAN with another geographically separated Ethernet LAN would require the header from the Ethernet packet to be passed through the ATM network to the remote Ethernet network for proper routing in that network.

The AAL resides above the ATM layer. No cells are formed at this layer, only PDUs which are then passed on to the ATM layer to be inserted into ATM cells. AAL is capable of viewing user data and is responsible for the segmentation and reassembly of user data.

There are two parts to the AAL. The convergence sublayer is the upper portion of the AAL. User data is first passed to the convergence sublayer, where it is encapsulated into a convergence PDU and passed to the lower part of AAL. The lower part of AAL is the SAR sublayer. The SAR is responsible for segmenting the user data, as well as any overhead added by the convergence sublayer and passing the segments to the ATM layer for encapsulation into ATM cells.

The convergence sublayer is divided into two parts (see figure 9.7). There is the Service Specific Convergence Sublayer (SSCS) and the Common Part Convergence Sublayer (CPCS). User data is first presented to SSCS, where it is encapsulated into an SSCS PDU. SSCS provides clock recovery and message identification. The CPCS provides message identification and detection of sequence errors. These are still evolving standards.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.