ed arises. Switch manufacturers provide many different routeing solutions; researchers have produced literally dozens of routeing algorithms or rules such as 'hot potato', 'selective flooding' and 'delay-based, single-path distributed adaptive per-packet' routeing. They are all decision making processes for deciding which output link is most appropriate for a given data packet, each allied to a type of routeing. 54.6.1 Fixed or static routeing Table 54.2 Routeing table for switch 3 of the example The most common overall routeing choice is between fixed, some- given in Figure 54.13 times called static or directory routeing, and dynamic routeing, with some variations between the two. End point device 3 address The simplest and least expensive strategy is fixed routeing, in which the network's packet switches are provided with routeing tables containing all the information they might need to route packets over that network. It is fairly common among X.25 PSNs to have fixed routeing. A routeing table for a packet switch comprises entries for each end point device connected to the network. In Figure 54.13 there are six, to the network means updating the table in each switch, a potentially Switch 1 Switch 3 burdensome task for managers of medium to large networks. Fixed routeing is straightforward, but depends on manual building and upkeep of the tables, for which a detailed knowledge of the network topology and traffic loadings is necessary. Software table building programmes are available to ease this task. Switch 2 Switch 4 Not all PSN links necessarily have the same capacity and an understanding of the particular network usage can be essential for efficient communications. For example, in Figure 54.13 there are two routes between switch 2 and switch 3, one direct, the other indirect through switch 1. The direct circuit may, in fact, be slower than the alternative or the traffic may be heavy enough to warrant splitting it between the two routes. Traffic for device E through switch 2 might best travel directly, while