Wireless Ad Hoc and Sensor Networks

346 Wireless Ad Hoc and Sensor NetworksAdditionally, handshake between other nodes can exist at the same timeinstance. As a result, the interference can increase due to multiple nodescommunicating with their destinations by accessing the channel. The estimatedtransmitter power has to overcome such an interference, which isnormally not known. In addition, the packet transmission as well as arrivaltimes will vary between frames due to the following reasons. First, theframes within the 4-way handshake have different size, ranging from fewbytes (ACK) to over 2500 bytes (DATA) due to the frame type. Second, thedelay between two consecutive handshakes can vary because of channelcontention. As the result, the estimation error will also vary depending onthese time gaps. Therefore, the target signal-to-interference ratio (SIR) hasto be chosen to overcome the worst-case scenario due to these uncertainties.Thus in our implementation, the target SIR value is calculated by multiplyingthe minimum SIR with a safety factor.In the real scenario, it is possible that channel conditions can changetoo quickly, which will prevent any algorithm to accurately estimate thepower value. Then, the frame losses will occur. Two mechanisms areintroduced to overcome or mitigate such problems: increase of transmissionpower in the case of retransmission and reset of power in the caseof a long idle connection.A retransmission indicates that the received signal has been attenuated.A simple approach would be to retransmit the frame with the same poweras the first transmission by hoping that the channel conditions are better.However, the channel attenuation or interferences could also be worse thanbefore. To overcome this problem, an active approach can be used. In thisapproach, for each retransmission, the transmission power is increased by acertain safety margin. Unfortunately, this will increase interference as wellas power consumption. However, experiments indicated that active adaptationof power with the DPC yielded higher throughput and resulted in lessretransmissions when compared to the passive method where no safety marginwas utilized. As the result, safety margin is used with the DPC scheme.Additionally, the estimated power value for a given destination canbecome inaccurate when significant delays are present between any twoconsecutive frames or delayed feedback. To overcome this, after certainidle interval, the DPC algorithm will reset the transmission power to themaximum value defined for the network. The DPC process described inChapter 6 is then restarted from the beginning.7.6.3 Energy-Aware MAC ProtocolFor this DPC implementation, the original MAC protocol for 802.11 hasbeen modified. These modifications occur at different layers. Furthermore,an idea, which is presented in Zawodniok and Jagannathan (2004), for adhoc networks, is used in the energy-aware protocol.