Achieving Weighted Fairness In WLAN-mesh Networks: Model Analysis And Protocol Design

WLAN-Mesh networks are meshing topology based multi-hop wireless networks. It can provideubiquitous wireless connectivity and broadband Internet access for the mobile stations. In September2011, the IEEE802.11Task Group “s” has approved the IEEE802.11s active standard forWLAN-mesh networking. The notable feature of the IEEE802.11s is the fact that it introduces theforwarding mechanism in the medium access control (MAC) layer, and employs the enhanceddistributed channel access (EDCA) defined in the IEEE802.11e as the medium access controlprotocol. However, since the EDCA protocol was originally designed for the purely single-hopindependent basic service set (IBSS) of WLAN, it exhibits a serious weighted fairness problem inmulti-hop WLAN-Mesh networks. Therefore, the main target of our work is developing novelmethods to effectively achieve the weighted fairness for mulit-hop WLAN-Mesh networks based onthe EDCA protocol, thus providing reliable weighted fairness guarantee for diverse priority traffic,such as Data flow, Video flow, Voice flow etc. According to this target, the innovation points of ourwork are summarized as follows.(1). According to the simulation and the analysis of the weighted fairness problem of the EDCAprotocol in multi-hop WLAN-Mesh networks, we define the instantaneous collision zone and thepersistent collision zone of receivers. Then, we confirm that collisions induced by the signalstransmitted by hidden jammer nodes located in the persistent collision zone of receivers are theprimary causes for the unfairness of multi-hop WLAN-Mesh networks. Afterwards, we propose athree-dimesional Markov chain model to accurately model and analyze the weighted fairness problemof the EDCA protocol in multi-hop WLAN-Mesh networks. In this model, we introduce pseudo statesto effectively distinguish the different backoff procedures induced by the RTS collisions and the datacollisions. Through analytical modeling, we find that a predefined weighted fairness goal can beeffectively achieved by properly tuning the contention parameters under different network topologyconditions. It establishes the theoretical foundation for effective protocol performance improvementsand optimizations. Comparions between the numerical results derived by the model and thesimulation results obtained by QualNet simulator validate the effectiveness of the model.(2). Based on the modeling and analyzing of the weighted fairness problem of the EDCAprotocol in multi-hop WLAN-Mesh networks, we take into account the cooperation of nodes in thedesign of the protocol stack, and then propose a distributed weighted fair channel access scheme based on the node cooperation for multi-hop WLAN-Mesh networks. Considering the primary causesfor the serious unfairness of multi-hop WLAN-Mesh networks presented above, this scheme adopts anadaptive power control for the RTS/CTS handshake according to the successful transmissionprobabilities of the RTS/CTS frames, thus ensuring that the nodes can transmit the data packetswithout collisions. Meanwhile, by piggybacking an extra field to declare the flow’s channeloccupation index (COI) in the head of the RTS/CTS frames, each transmitting node knows its wholeneighboring flows’ channel occupation ratio via overhearing the RTS/CTS frames transmitted by itsneighboring nodes. The transmitting nodes can synthetically judge whether the current flow yieldsexcess channel bandwidth according to the weights of the flows and the COIs of neighboring flows.Afterwards, the nodes cooperatively adjust the contention window (CW) sizes to achieve the weightedfair bandwidth allocation among diverse priority flows according to their weights. The simulationresults obtained by the QualNet simulator validate the effectiveness of this scheme.