lies on a 48-octet payload field that contains actual user data, and a five-byte header that contains information needed by the network to route the cell and provide proper levels of service. The ATM cell header, shown in Figure 7-8, is examined and updated by each switch it passes through, and comprises six distinct fields: the generic flow control field, the virtual path identifier, the virtual channel identifier, the payload type identifier, the cell loss priority field, and the header error control field. Generic flow control (GFC): This four-bit field is used across the UNI for network-to-user flow control. It has not yet been completely defined in the ATM standards, but some companies have chosen to use it for very specific purposes. For example, Australia's Telstra Corporation uses it for flow control in the network-to-user direction and as a traffic priority indicator in the user-to-network direction. Virtual path identifier (VPI): The eight-bit VPI identifies the virtual path over which the cells will be routed at the UNI. It should be noted that because of dedicated, internal flow control capabilities Figure 7-8 ATM cell header details 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 within the network, the GFC field is not needed across the NNI. It therefore is redeployed: The four bits are converted to additional VPI bits, thus extending the size of the virtual path "field." This allows for the identification of more than 4,000 unique VPs. At the UNI, this number is excessive, but across the NNI it is necessary because of the number of potential paths that might exist between the switches that make up the fabric of the network. Virtual channel identifier (VCI): As the name implies, the 16-bit VCI identifies the unidirectional virtual channel over which the current cells will be routed. Payload type