Scheme And Implementation Of Reliable Transmission In IEEE802.11WLANs

In recent years, wireless local area networks (WLANs) have been widely used due totheir convenient, flexible, and low-cost advantages. Compared with wired networks, the IEEE802.11based WLANs occupy different feathers, such as half-duplex communications betweenwireless devices (ie, the transceives embedded in the deviceds can not transmit and receive datasimultaneously), sharing wireless channel in transmitting, and so on, which makes WLANs facemore challenges than the wired networks. For instance, channel competition and interferencemake the links between nodes in the WLANs unreliable so that the reliability of datatransmission in the WLANs is worse than that in the wired network. Therefore, it becomes aproblem needed to be addressed urgently to improve the transmission reliability and enhance thethroughput of WLANs, which is a research focus in research community on the world.This dissertation maks effort on the schemes of reliable data delivery in the IEEE802.11based WLANs. The main contributions of the paper are as follows:1. The Two-step Binary Exponential Backoff (TBEB) algorithm is proposed. In a WLAN,collisions in data frames may occur if multiple nodes transmit simultaneously, which leads tofailure in data transmission. IEEE802.11standard adopts Binary Exponential Backoff (BEB) toreduce packet collision probability. When packet collision probability is large, the BEBalgorithm exhibits the shortcoming that it may bring with Contention Window (CW) oscillation.This shortcoming can be overcomed by the proposed TBEB. The TBEB is characterized byresetting CW to a suitable value rather than the minimum one set by the standard BEB, which isable to overcome CW oscillation and reduce packet collision probability.2. A GEB（Gathering Error-free Block）scheme is proposed. The IEEE802.11standardapplies positive acknowledgement (ACK) mechanism, in which the sender must perform BENand retransmit if it does not receive the ACK frame from the receiver. CW is enlarged each timethe sender performs BEB. In WLAN, environment changing, radio interference, multi-path,fading, etc. may lead to the failure in data transmissions. In the802.11based WLAN, thereceiver discards a received frame with bit error, which leads the sender to perform BEB and retransmit and brings longer delay. That is, the BEB introduced in IEEE802.11standard doesnot differentate the retransmission resulting from packet collision from that from bit error. Theproposed GEB remedies the problem. The scheme divides the payload of a frame into blockswith eachhaving a checksum. Moreover, the received frame with bit error is not discarded by thereceiver but the receiver to collects error-free blocks contained in the frame and sends NACK(Negative Acknowledgment) to the sender to inform it of not doubling CW. In addition, thereceiver sends ACK to the sender when the collected blocks can be assembled into the originalframe. The GEB scheme is able to reduce the number of retransmissions and delay, and it canimprove data delivery reliability and throughput of WLANs.