366 Routing in Data Networks Chap.5 by means of a feedback mechanism shown in Fig.5.3.When the traffic load offered by the external sites to the subnet is relatively low,it will be fully accepted into the network,that is, throughput offered load When the offered load is excessive,a portion will be rejected by the flow control algo- rithm and throughput offered load-rejected load The traffic accepted into the network will experience an average delay per packet that will depend on the routes chosen by the routing algorithm.However,throughput will also be greatly affected(if only indirectly)by the routing algorithm because typical flow control schemes operate on the basis of striking a balance between throughput and delay (i.e.,they start rejecting offered load when delay starts getting excessive).Therefore,as the routing algorithm is more successful in keeping delay low,the fow control algorithm allows more traffic into the network.While the precise balance between delay and throughput will be determined by flow control,the effect of good routing under high offered load conditions is to realize a more favorable delay-throughput curve along which flow control operates, as shown in Fig.5.4. The following examples illustrate the discussion above: Example 5.1 In the network of Fig.5.5,all links have capacity 10 units.(The units by which link capacity and traffic load is measured is immaterial and is left unspecified.)There is a single destination (node 6)and two origins (nodes 1 and 2).The offered load from each of nodes I and 2 to node 6 is 5 units.Here,the offered load is light and can easily be accommodated with small delay by routing along the leftmost and rightmost paths,I -3-6 and 2-5→6，respectively.If instead.however,.the routes I一46and2→4→6are used,the flow on link(4,6)will equal capacity,resulting in very large delays. Example 5.2 For the same network,assume that the offered loads at nodes I and 2 are 5 and 15 units, respectively (see Fig.5.6).If routing from node 2 to the destination is done along a single path,then at least 5 units of offered load will have to be rejected since all path capacities Poor routing Good routing Figure 5.4 Delay-throughput operating Throughput curves for good and bad routing.366 Routing in Data Networks Chap. 5 by means of a feedback mechanism shown in Fig. 5.3. When the traffic load offered by the external sites to the subnet is relatively low, it will be fully accepted into the network, that is, throughput = offered load When the offered load is excessive, a portion will be rejected by the flow control algo￾rithm and throughput = offered load - rejected load The traffic accepted into the network will experience an average delay per packet that will depend on the routes chosen by the routing algorithm. However, throughput will also be greatly affected (if only indirectly) by the routing algorithm because typical flow control schemes operate on the basis of striking a balance between throughput and delay (i.e., they start rejecting offered load when delay starts getting excessive). Therefore, as the routing algorithm is more successful in keeping delay low, the flow control algorithm allows more traffic into the network. While the precise balance between delay and throughput will be determined by flow control, the effect of good routing under high offered load conditions is to realize a more favorable delay-throughput curve along which flow control operates, as shown in Fig. 5.4. The following examples illustrate the discussion above: Example 5.1 In the network of Fig. 5.5, all links have capacity 10 units. (The units by which link capacity and traffic load is measured is immaterial and is left unspecified.) There is a single destination (node 6) and two origins (nodes I and 2). The offered load from each of nodes I and 2 to node 6 is 5 units. Here, the offered load is light and can easily be accommodated with small delay by routing along the leftmost and rightmost paths, I -+ 3 -+ 6 and 2 -+ 5 -+ 6, respectively. If instead, however, the routes I -+ 4 -+ 6 and 2 -+ 4 -+ 6 are used, the flow on link (4,6) will equal capacity, resulting in very large delays. Example 5.2 For the same network, assume that the offered loads at nodes I and 2 are 5 and 15 units, respectively (see Fig. 5.6). If routing from node 2 to the destination is done along a single path, then at least 5 units of offered load will have to be rejected since all path capacities > co Q) Cl Poor routing Throughput Figure 5.4 Delay-throughput operating curves for good and bad routing