2 Routing How frames are routed over a packet-switched network depends on the instructions given by the users. Three basic styles, similar to the routing techniques employed in router driven networks, can be distinguished: ã Distributed routing: On the basis of information about traffic conditions and equipment status (network map, port status), each node decides which link the frame shall take to its destination. ã Centralized routing: A primary (and perhaps an alternate) path is dedicated to a pair of stations at the time of need. ã Permanent virtual circuit routing: A virtual connection is permanently assigned between two stations. Examples of each of these techniques are given in Figure 4.5: ã Frames 1, 2, and 3 are sent from A to C using distributed routing. On the basis of the traffic distribution (links AF and AG are assumed to be congested), frames 1 and 2 are launched on link AE. Although it is not the shortest, this is a link that will connect to C. When frame 3 is presented to A, the link AG is less congested than AE. A sends frame 3 over link AG. Because frame 3 takes the path AGC, and frames 1 and 2 take the path AEFGC, frame 3 arrives at C ahead of frames 1 and 2.