 identifier (PTI): The three-bit PTI field is used to indicate network congestion and cell type, in addition to a number of other functions. The first bit indicates whether the cell was generated by the user or by the network, while the second indicates the presence or absence of congestion in user-generated cells, or flow-related operations, administration, and maintenance (OA&M) information in cells generated by the network. The third bit is used for service-specific, higher-layer functions in the user-to-network direction, such as to indicate that a cell is the last in a series of cells. From the network to the user, the third bit is used with the second bit to indicate whether the OA&M information refers to segment or end-to-end-related information flow. Cell loss priority (CLP): The single-bit CLP field is a relatively primitive flow-control mechanism by which the user can indicate to the network which cells to discard in the event of a condition that demands that some cells be eliminated, similar to the DE bit in frame relay. It can also be set by the network to indicate to downstream switches that certain cells in the stream are eligible for discard, should that become necessary. Header error control (HEC): The eight-bit HEC field can be used for two purposes. First, it provides for the calculation of an eight-bit CRC that checks the integrity of the entire header. Second, it can be used for cell delineation. Addressing in ATM ATM is a connection-oriented, virtual circuit technology, meaning that communication paths are created through the network prior to actually 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 7-9 Addressing in Virtual Channel ATM with Bidirectional Virtual Path virtual channels, Virtual Channel paths sending traffic. Once established, the ATM cells are routed bas