ross the network. This eliminates the call setup time associated with the dial-up lines and the X.25 packet arrangements. The connection is always ready for the devices to ship data in a framed for- mat as the need arises. This takes away the need for the constant fine- tuning of a private-line or a dial-up network link arrangement. The Basic Data Flow In most popular synchronous protocols, data is carried across a communi- cations line based on very similar structures. The standard HDLC frame format is used in a myriad of these protocols and services. Frame Relay makes a very slight change to the basic frame structure, redefining the header at the beginning of the frame (2 bytes long). Figure 9-9 shows the basic data flow. Although Frame Relay purports to eliminate the operations at the net- work layer, it does not eliminate all network layer operations. Figure 9-9 illustrates one network layer operation that is essential for Frame Relay 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. PCUSN-to-SGSN Interface (Gb) PCUSN-to-SGSN Interface (Gb) Figure 9-9 The basic data flow. Router Router DLCI 1 DLCI 1 Switches Router DLCI3 DLCI 2 Router DLCI 3 DLCI 2 operations: the identification of virtual connections. Frame Relay uses the DLCI to identify the destination address. This 10-bit number corresponds to the virtual circuit number in the network layer protocol. The DLCIs are premapped to a destination node, as shown in Fig- ure 9-10. This simplifies the process at the routers because they only need to consult their routing table, check the DLCI in the table, and route the traffic to the proper output port based on this address. Figure 9-10 The mapping of the DLCI. Router Router DLCI = 700 Switches Router Router Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies.