Wireless Ad Hoc and Sensor Networks

Distributed Power Control and Rate Adaptation 275In the case of high congestion, the radio channel experiences demandfor access from numerous sources. Thus, it is necessary to increase themodulation rate for completing the transmission quicker. This will clearthe channel sooner allowing other nodes to transmit data reducingcongestion. Notably, this subsequent increase of modulation rate shouldnot cause increase in burst size because it will defy the purpose ofreleasing the channel quicker. Moreover, the receiving node has limitedamount of data it can accept because of buffer constraints and, thus,increasing burst size will cause uncontrolled increase in queue size atthe receiver.6.9.5 Implementation ConsiderationIn the DP-based scheme, the nodes use the standard backoff interval,and thus they will gain access to the channel in an orderly manner.The rate selection is performed at each RTS-CTS exchange. Next, theselected modulation rate and burst size are communicated from thereceiving node using a CTS frame with additional fields for data. Then,the transmitting node can additionally modify the modulation rateand, if necessary, burst size according to conditions described in asubsection.6.10 Simulation ResultsThe NS-2 simulator was used for evaluating the proposed rate adaptationprotocols. The one-hop, two-hop, and random topologies are used toevaluate the proposed schemes. However, because of the analytical resultsobtained for the proposed technique, any network topology can be utilized,and similar results can be seen. The one-hop topology is used toevaluate performance of the protocols in the presence channel fading. Thetwo-hop topology sets up two flows between two pairs of source–destinationnodes, where a common relay node is forwarding data for both flows.The relay node becomes a bottleneck for communication and presents anexcellent benchmarking case for rate adaptation. The last topology uses50 nodes randomly located in the area of 1000 × 1000 m with 25 flows setup through the network. The proposed heuristic protocol is comparedwith the RBAR (Holland et al. 2001).In case of the DP-based scheme, usage of the burst-mode transmissionsresults in increased data throughput because an overhead of backing offand transmitting RTS/CTS is reduced. Consequently, a comparisonbetween protocols with and without burst mode support will be difficult.