Improving fairness and throughput in wireless systems.

摘要

With the advancement of wireless technologies, wireless systems have been widely used in today's world. Especially, the IEEE 802.11 Wireless LANs (WLANs) have covered a large portion of the urban areas to provide anytime, anywhere Internet service. In addition, the Radio-Frequency Identification system (RFID) is another important wireless network which promises to revolutionize the inventory management in large warehouses, retail stores, hospitals, transportation systems, etc. In this dissertation, we first propose novel solutions for improving fairness and throughput in WLANs. We then introduce a new method to improve reading throughput in large RFID systems.;Our first work focuses on achieving MAC-layer time fairness among contending WLANs. The WLANs may overlap and contend with each other. We show that the contention among nearby WLANs is location-sensitive, which makes some hosts much more capable than others to obtain the channel for their transmissions. Another reality is that wireless hosts use different transmission rates to communicate with the access points due to attenuation of their signals. We show that location-sensitive contention aggravates the throughput anomaly caused by different transmission rates. It can cause throughput degradation and host starvation. Achieving time fairness across multiple WLANs is a very difficult problem because the hosts may perceive very different channel conditions and they may not be able to communicate and coordinate their operations due to the disparity between the interference range and the transmission range. In this work, we design a MAC-layer time fairness solution based on two novel techniques: channel occupancy adaptation, which applies AIMD on the channel occupancy of each flow, and Queue Spreading (QS), which ensures that all hosts and only those hosts in a saturated channel detect congestion and reduce their channel occupancies in response. The proposed solution is called AIMD/QS+k. We show that AIMD/QS+k approximates the generic adaptation algorithm for proportional fairness.;Our second work focuses on achieving transport-layer fairness among contending WLANs. TCP is the dominating transport-layer protocol used by many applications over WLANs. Contention among multiple nearby WLANs in urban areas may cause severe TCP unfairness, where some TCP flows can achieve very high throughput at the expense of starving others. This unfairness results from the fact that different physical nodes conveying TCP flows at a wireless bottleneck may have different channel observations and consequently they may provide inconsistent feedbacks to the TCP sources. Existing solutions to this problem try to synchronize channel observations of contending nodes by exchanging control messages among them. They rely on the assumption that these nodes are within each other's transmission range, which however may not always hold. In this work, we design a new protocol, called Wireless Probabilistic Drop (WPD), to improve TCP fairness without requiring direct communication among nodes. In WPD, when a node detects congestion, it probabilistically chooses to either drop some packets to resolve the congestion, or aggressively spread the congestion signal to other contending nodes. Each node makes the choice with a probability that is proportional to its flow rate. Henceforth, high-rate flows tend to perform rate reduction more often, and low-rate flows are more likely to increase their flow rates. Eventually, all flows passing the bottleneck are expected to get a fair share of the channel bandwidth.;Our third work focuses on improving the RFID reading throughput. In large RFID systems, periodically reading the IDs of the tags is an important function to guard against administration error, vendor fraud and employee theft. Given the low-speed communication channel in which a RFID system operates, the reading throughput is one of the most important performance metrics. The current protocols have reached the physical throughput limit that can possibly be achieved based on their design methods. To break that limit, we have to apply fundamentally different approaches. In this work, we investigate how much throughput improvement the analog network coding can bring when it is integrated into the RFID protocols. The idea is to extract useful information from collision slots when multiple tags transmit their IDs simultaneously. Traditionally, those slots are discarded. With analog network coding, we show that a collision slot is almost as useful as a non-collision slot in which exactly one tag transmits. We propose the Framed Collision-Aware Tag identification protocol (FCAT) that optimally applies analog network coding to maximize the reading throughput, which is 51.1% ∼ 70.6% higher than the best existing protocols. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)