milar to X.25). X.25 networks can be connected to sessions can be re-routed and data recovered with little or no non-X.25 networks via a gateway or router which is just a type of inconvenience to the end users. protocol converter. Some of the larger public networks offer an Maintainability. The node or network is intelligent and can integrated PAD service. A large PAD is resident at the node and provide statistical and diagnostic information to aid fault trac- subscribers rent stop/start lines to the PAD (dial-up or modem link). In this configuration the X.25 interface is actually within the net- OSI Compatible. X.25 is part of the OSI model and as such can work and not available to the subscribers. be used as part of an OSI networking strategy. Another method of accessing an X.25 network is via a high speed leased line link, such as KiloStream in the UK. KiloStream operates The disadvantages of X.25 are: at speeds of up to 64kbit/s (but only 48kbit/s when connecting to the PSS X.25 network). Network Delay. Although networks can provide the same (or very close) throughput figures to private wires, there is always 40.6.3 Services provided by X.25 a finite delay caused as packets are routed (stored and for- warded) through the network. This is a product of the number Recommendation X.25 defines three types of data transmission of nodes in the route across the network, the traffic load on services for use with PDNs: leased circuit, circuit switched, packet different parts of the network and various other factors. switched. The packet switched service is primarily considered here. 40/12 Multiplexers X.25 Network Alien network Protocol converter Gateway, router or bridge Non-X.25 DTE X.25 DTE Network PAD Asynchronous devices Asynchronous devices Figure 40.12 An X.25 based network The packet switched service is subdivided into three services: 40.6.3.2 Switched virtual circuits Permanent Virtual Circuits (PVCs), Switched Virtual Circuits (SVCs) and Datagrams. PVCs and SVCs are essential services i.e. Switched virtual ci