 for dial-up access to a host overcoming the inflexibility of point-to-point dedicated net- computer. works. Sender/source Destination Packet Switched Network Figure 54.1 The virtual circuit concept: multiple paths 54/4 Packet switching Destination 1 Sender/source Destination 2 Packet Switched Network DTE = Data Terminal Equipment DSE = Data Switching Equipment DCE = Data Circuit-terminating Equipment Destination 3 Figure 54.2 The virtual circuit concept: multiple destinations 6. Simultaneous calls allow PC users to access multiple windows Packet switched call to different remote applications. 7. Since many users can share transmission resources efficiently, Each call on a PSN is made up of several steps (Figure 54.3), similar the cost of intermittent data communications is reduced. in their progression to a switched telephone call. A terminal user 8. New calls can be added and old ones disconnected without enters a command for connection to the network, then the numeric affecting other users. network address of the destination. On hitting the Enter key, a specially formatted call request packet While suppliers have increased the number and sophistication of is generated and sent across the network; it holds the network switch functions over the years, supporting a myriad of terminal and address of both the source and destination. If the call is unaccept- host protocols, the X.25 specification of an interface to PSNs able, the destination host may reject the call and disconnect. If the remains standard and non-proprietary, offering users flexibility in call is accepted, a call connected packet is returned to the sender, equipment choice. indicating that an end-to-end switched virtual circuit (SVC) has been established for the duration of the call. The first recommendations for a standard PSN interface were laid To either end, the terminal and host, the SVC looks like a circuit down in 1976 by a subcommittee of the CCITT (International switched connection, but because packets can take any route in the Telephone an