nted variable bit rate services. 1Computer Technology, August 1997, p. 16-26. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies Figure 7-7 ATM protocol Upper Layer Services stack Convergence Sublayer ATM Adaptation Layer SAR Sublayer ATM Layer TC Sublayer Physical Layer PMD Sublayer The ATM adaptation layer, or AAL, has four general responsibilities: Synchronization and recovery from errors, Error detection and correction, Segmentation and reassembly of the data stream, and Multiplexing. The AAL comprises two functional sublayers. The convergence sub- layer provides service-specific functions to the services layer SO that the services layer can make the most efficient use of the underlying cell-relay technology that ATM provides. Its functions include clock recovery for end-to-end timing management; a recovery mechanism for lost or out-of- order cells; and a timestamp capability for time-sensitive traffic such as voice and video. The segmentation and reassembly sublayer, or SAR, converts the user's data from its original incoming form into the 48-octet payload "chunks" that will become cells. For example, if the user's data is arriv- ing in the form of 64-kilobyte IP packets, SAR chops them into 48-octet payload pieces. It also has the responsibility to detect lost or out-of-order cells that the convergence sublayer will recover from, and to detect single-bit errors in the payload chunks. The ATM layer has five general responsibilities: Cell multiplexing and demultiplexing, Virtual path and virtual channel switching, Creation of the cell header, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 G