Data Communications Networking Devices - 4th Ed.pdf

260 __________________________________________________________ WIDE AREA NETWORKSFigure 2.55The ATM cell headerthe User-to-Network Interface UNI) and is used only at the UNI. When cells aretransmitted between switches, the four bits become an extension of the VPI ®eld,permitting a larger VPI value to be carried in the cell header.Two modes of GFC based ¯ow-control are speci®edÐuncontrolled access andcontrolled access. When uncontrolled access is speci®ed, all bits in the GFC ®eldare set to zero. In the controlled access mode the ®eld is set when congestion occurs.Virtual Path Identi®er ®eldsThe Virtual Path Identi®er VPI) identi®es a path between two locations in anATM network that provides transportation for a group of virtual channels, where avirtual channel represents a connection between two communicating ATM devices.Thus, a virtual path can be considered to represent a bundle of channels betweentwo endpoints. When an endpoint has no data to transmit, the VPI ®eld is set to allzeros to indicate an idle condition. As previously explained, when transmissionoccurs between switches, the GFC ®eld is used to support an extended VPIvalue.Virtual Channel Identi®er ®eldThe Virtual Channel Identi®er VCI) can be considered to represent the secondpart of the two-level routing hierarchy used by ATM, where a group of virtualchannels VCs) are used to form a virtual path VP). The virtual channel identi®eridenti®es a virtual channel, which, in turn, represents the ¯ow of a single networkconnection data ¯ow between two ATM endpoints. ATM standards de®nes thevirtual channel as a unidirectional connection. Thus, two virtual channels must beestablished between ATM endpoints to support a bidirectional data ¯ow.Figure 2.56 illustrates the relationship between virtual paths and virtualchannels. Here the virtual channel represents a connection between two communicatingATM entities, such as an endpoint to a central of®ce switch, or between two