fore be kept both Datagrams, however, can still be effective internal network proto- simple and fast. cols. The idea is to combine the speed and simplicity of datagram If the amount of data to be sent can be confined to a single packet switches with the advantages of a virtual call network, producing a topology in which the domain outside the cloud is datagram packet transferred between devices, it can also be very connection oriented, but inside is connectionless. Perhaps the lar- efficient. As call set-up of a virtual call is one of the most time consuming operations for such networks, if the call period is only gest example is the US Department of Defence's military oriented research network, Arpanet. short, but set-up rates are high, the PSN performance can be seri- ously impaired. Described as a lean and mean method of shifting data, datagram bearer networks place responsibility for error correction, resequenc- 54.6 Routeing over packet switched ing packets on delivery and lost packets from the PSN to the end networks point node or user device. In terms of the OSI model, these functions are left to the Transport Layer, one above X.25. A packet entering a PSN cloud from an end point device has to be Data is broadcast into the network and left to fend for itself, routed to its destination device by the packet switches within the sometimes described as spray and pray, bringing with it some network according to various techniques, each with their own ad- disadvantages: vantages and drawbacks. The choice is usually a balance between Routeing over packet switched networks 54/13 the complexity of the solution and the breadth of routeing function- ality provided. Table 54.1 Routeing table for switch 2 of the example The most common PSN architecture is an irregular mesh, reflect- given in Figure 54.13 ing the gradual, if not actually haphazard growth of any large PSN End point device 2 address aiming to meet changing user demands. A routeing strategy must be flexible enough to cope with a different topology as the ne