IEEE 802.11af 網路之增強版公平性

Abstract

The IEEE 802.11af, also called White-Fi, employs unused TV spectrum at frequencies between 470 MHz and 790 MHz, which provides bandwidth for sensors and monitors. The IoT technology revolution pushes IEEE 802.11af networks to handle a big number of stations and a wide transmission range. These constraints endanger one of the most important concerns of the protocol which rules these networks : providing fairness among stations. The most known causes to the fairness issue are the Capture Effect and the Hidden Terminal problem. The first one occurs when two stations of the same network, including the Access Point (AP), attempt a transmission at the same time and then instead of both failing, one of the two transmissions is successful. In the second one, two stations do not hear each other. Then a station can attempt a transmission while another one is already transmitting, inducing a collision and the failure of both transmissions. The original DCF has already been improved with the adding of RTS/CTS frames to prevent hidden nodes collisions but this mechanism just reduces the time of collision and does not completely treat the unfairness problem caused by this issue. In this thesis, we first design a fully distributed mechanism to detect capture effect relationships in mixed uplink and downlink transmissions. When the transmission of a station is captured, our new protocol forces this station to transmit again, directly after the reception of the Acknowledgement (ACK) addressed to the station which has just captured it. Then both stations are aware of the capture effect relationship and can adjust their Contention Window (CW) in order to have the same probability of successful transmission as the other stations of the network. In addition, we deal with the hidden node problem and establish a distributed way to inform the whole iii iv network about the hidden node relationships. As a result, every station of the same network has the same knowledge of its network topology and optimizes its contention window to have the same Successfull Count Transmission (STC) as each other . Our detection protocol uses the fact that when two hidden stations collide, there is a probability that one succeeds according to the arrival time of the frames. Then, the station which is the second to attempt a transmission will transmit again two times directly after receiving the ACK addressed to the first one. This double transmission will inform the whole network of a hidden node relationship. We compare through differents simulations our new mechanism with the original DCF of IEEE 802.11 af. In terms of fairness, our mechanism provides, for example to a network touched by only capture effect, a min-max fairness index of 96% and a Jains index value of 100%. In comparison, for the same network ruled by the original DCF, these indexes are respectively 35% and 86%. The STC fairness is achieved, instead of having twenty stations with a STC two times greater than twenty others, and every station has around the same STC. Moreover, for a network composed by twenty stations with six hidden nodes relationships, all these relationships are detected in less than 4 seconds by the whole network with our mechanism.