ring between multiple routes is often a partial tion regarding the size of the packet queue with data from the answer, with automatic fall backs in the case of a network failure. switches to which it is directly connected. Alternatively, switches can refer to secondary and even tertiary At defined intervals a switch sends a delay table to every switch routeing tables if the primary options become unavailable, although to which it is directly connected. This contains an entry for every it demands more skill on the part of the network administrators. end point device on the network. On receipt of a delay table, a Fixed routeing with some degree of dynamic alteration in the switch combines it with its own current routeing table, producing a event of node failure is generally common in commercial network- new table. ing environments. For each end point device, each switch compares the delay in its routeing table with that indicated in the received delay table, ad- 54.6.2 Dynamic routeing justed for the effects of the link on which it was received. If the new delay is shorter than that for the route currently in use, the routeing Dynamic routeing is a complex solution favoured in academic or table entry for that end point device is updated to show a better route networks such as Arpanet in which security or the lack of commer- has been found. cial demands favour more esoteric methods. The result of this operation is that advice of alterations in network Rather than refer to predetermined routeing tables, packet swit- loading can be transmitted automatically through the PSN. As the ches capable of dynamic routeing make their own routeing deci- size of each routeing table is itself dynamic, any new end point sions based on prevailing network conditions. Network managers devices added to the network automatically appear in the switch and installers therefore do not need such a detailed knowledge of the tables soon after initial connection. The location of every end point live network; the implementation looks after itsel