d Telegraph Consultative Committee). They have network, call set-up time is less, often measured in tenths of a evolved into the X.25 international standard, approved by the Inter- second compared to several seconds. national Standards Organisation in 1984 8208), for accessing a host over a wide area network. Typically, the first PSE in the chain determines to which particular link the call request packet will be forwarded and sends it to the next Geographically scattered standalone LANs can be interconnected PSE along that route. This continues until the data reaches its to form a single logical LAN using X.25 over WAN links, taking destination. advantage of its shareable, protocol independent data transport Once connection is made, the data transfer begins: electronic mail, mechanism. Built under the Open Systems Interconnection frame- updates to databases and so on. It is the network's responsibility to work, the network can be upgraded to take advantage of new ensure fast, error-free data delivery, transparently between the two developments, leaving the software affecting the upper layers in end points. place. Network operations at this stage include sorting the packets into Significantly, the recommendations say nothing about the internal the correct sequence at the destination, flow control (ensuring data protocols and algorithms needed to run a PSN but describe the input rates match delivery rates) and signalling, i.e. notifying the interface between Data Terminal Equipment (DTE) and Data Cir- source or destination of any unusual network events. cuit-terminating Equipment (DCE) for devices designed to run in The final phase of a packet call is disconnection or call clearing, packet mode. Rather than being a protocol in itself, the X.25 by either the source or destination. A unique packet notifies the specification describes an interface using individual protocols at the network and the opposite end that the call is to be cleared and the three lowest layers of the OSI model. SVC freed. The packet switched n