ATM Adaptation Layer 5 - Convergence, Segmentation, and Reassembly

Convergence, Segmentation, and Reassembly

When an application sends data over an ATM connection using AAL5, the host delivers a block of data to the AAL5 interface. AAL5 generates a trailer, divides the information into 48-octet pieces, and transfers each piece across the ATM network in a single cell. On the receiving end of the connection, AAL5 reassembles incoming cells into a packet, checks the CRC to ensure that all pieces arrived correctly, and passes the resulting block of data to the host software. The process of dividing a block of data into cells and regrouping them is known as ATM segmentation and reassembly (SAR).

By separating the functions of segmentation and reassembly from cell transport, AAL5 follows the layering principle. The ATM cell transfer layer is classified as "machine-to-machine" because the layering principle applies from one machine to the next (e.g., between a host and a switch or between two switches). The AAL5 layer is classified as "end-to-end" because the layering principle applies from the source to the destination - AAL5 presents the receiving software with data in exactly the same size blocks as the application passed to the AAL5 on the sending end.

The AAL5 on the receiving side knows how many cells comprise a packet because the sending AAL5 uses the low-order bit of the "PAYLOAD TYPE" field of the ATM cell header to mark the final cell in a packet. This final cell header can be thought of as an "end-to-end bit". Thus, the receiving AAL5 collects incoming cells until it finds one with an end-of-packet bit set. ATM standards use the term "convergence" to describe mechanisms that recognize the end of a packet. Although AAL5 uses a single bit in the cell header for convergence, other ATM adaptation layer protocols are free to use other convergence mechanisms.