Wireless Ad Hoc and Sensor Networks

84 Wireless Ad Hoc and Sensor NetworksS2Link capacityS3D3SW1SW2SW3D2S1D1FIGURE 3.1AParking lot configuration.SourceVirtual dest/sourceVirtual dest/sourceDestinationFIGURE 3.1BHop-by-hop rate control.Most congestion schemes consist of adjusting the input rates to matchthe available link capacity (or rate) so that a desired QoS can be met.Depending upon the duration of congestion, different schemes can beapplied. For congestion lasting less than the duration of connection, anend-to-end feedback control scheme can be used. For short-term congestion,providing sufficient buffers in the switches is the best solution and adesign procedure is required. Though end-to-end rate control can be applicable,the round-trip delay can be very large. This problem is fixed bysegmenting the network into smaller pieces and letting the switches act as“virtual source” and/or “virtual destination.” Segmenting the networkusing virtual source/destination reduces the size of the feedback loops(Jain 1996), but the intermediate switches/routers have to be updated withsoftware so that the entire system is transparent. To overcome this problem,an end-to-end congestion control scheme is typically proposed.The most important step in preventing congestion is to estimate thenetwork traffic that is being accumulated at the switch in an intelligentmanner. In the worst-case scenario, every ATM switch could act as avirtual source/destination and one could apply hop-by-hop rate control,as shown in Figure 3.1b, for preventing congestion. Tracking queue length(whose exact value is determined based on QoS measurements) is desirableto avoid cell losses due to overflows and to maintain high networkutilization.A computer network can be viewed as a set of virtual source/destinationpairs and they can then be modeled as multiinput multioutput