eneric flow control, and Cell delineation. Because the ATM layer creates the cell header, it is responsible for all of the functions that the header manages. The process, then, is fairly straightforward: The user's data passes from the services layer to the ATM adaptation layer, which segments the data stream into 48-octet pieces. The pieces are handed to the ATM layer, which creates the header and attaches it to the payload unit, thus creating a cell. The cells are then handed down to the physical layer. The physical layer consists of two functional sublayers as well: the transmission convergence sublayer and the physical medium sublayer. The transmission convergence sublayer performs three primary func- tions. The first is called cell rate decoupling, which adapts the cell cre- ation and transmission rate to the rate of the transmission facility by performing cell stuffing-similar to the bit stuffing process described earlier in the discussion of DS3 frame creation. The second responsibil- ity is cell delineation, which allows the receiver to delineate between one cell and the next. Finally, it generates the transmission frame in which the cells are to be carried. The physical medium sublayer takes care of issues that are specific to the medium being used for transmission, such as line codes, electrical and optical concerns, timing, and signaling. The physical layer can use a wide variety of transport options, includ- ing the following: DS1/DS2/DS3 E1/E3 25.6 Mbps UNI over UTP-3 51 Mbps UNI over UTP-5 (Transparent Asynchronous Transmitter/Receiver Interface, or TAXI) 100 Mbps UNI over UTP-5 OC3/12/48c Others, of course, will follow as transport technologies advance. 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 The ATM Cell Header As we mentioned before, ATM is a cell-based technology that re