 to be routed indi- end devices on that specific occasion; congestion at a node, for vidually, any routeing time saved is particularly significant. instance, may well increase the cost so that a cheaper route is found Large PSNs even create sub-domains within domains, just as the for the rest of the virtual call. digits in an orthodox international telephone number can be sub- Packet switches can exchange route status information at regular divided into national, regional and individual areas. As a packet intervals with adjacent or more distant nodes to build up a picture switch only needs routeing information about its immediate do- of current network conditions a capability known as distributed main, the domain addressing technique can be practical for large adaptive routeing. Arpanet, for instance, employs this form of rou- PSNs without the burden of enormous routeing tables. teing and is able to circulate information regarding link availability However, static routeing is poorly equipped to cope with line and route loading estimates to end point devices. failures. As the routeing tables are fixed, the link will remain Routeing tables in these type of switches contain an entry for each unavailable until it is restored. Even though there might physically end point device attached to the network. When the network is first appear to be more than one route from A to B, packets and virtual installed, these entries contain only estimates of route loadings to calls cannot suddenly re-routed if the normal route fails. the end devices. As a result, network management systems capable of informing The network's aim is always to make use of the optimum route managers of any untoward events as soon as possible are of para- between communicating end point devices: a route in which the mount importance, as manual intervention to alter the routeing transit delay is as low as possible. Once in action, the network tables is the only option. estimates the delay of any one route by combining its own informa- In practice, load sha