Cabletron Systems manual B-8 BRIM-A6 User’s Guide

Appendix B: ATM Overview

Layer 3: The ATM Adaptation Layer vs. The Network Layer

Even though these two layers have the same “physical” location in their perspective models, they are not alike. In the OSI model, the Network Layer formats data to be sent out on a particular network (for example, IP, or IPX). In the ATM model, since the Physical and ATM Layers take care of all the information routing normally performed by the Networking Layer, the ATM Adaptation Layer (AAL) acts as the first link between the ATM cell and the higher layer services.

In this role, the AAL provides the convergence sub-layer (CS) and the segmentation and reassemble (SAR) sub-layer. When transmitting, the CS prepares data from the higher layers to be converted into cells. The SAR then segments the CS data into several 53 byte cells. Conversely, when receiving ATM cells, the SAR recombines data received from the ATM Layer and sends it to the higher layers.

Page B-8 BRIM-A6 User’s Guide