by Zakhia (Zak) AbicharA dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY
Major: Computer Engineering
Dr. Morris Chang, Co-major Professor
Dr. Ahmed Kamal, Co-major Professor
Dr. Daji Qiao
Dr. Yong Guan
Dr. Lu Ruan
Wireless Local Area Networks (WLANs) and Wireless Metropolitan Area Networks (WMANs) are two of the main technologies in wireless data networks. WLANs have a short range and aim at providing connectivity to end users. On the other hand, WMANs have a long range and aim at serving as a backbone network and also at serving end users. In this dissertation, we consider the problem of Medium Access Control (MAC) in WLANs and the placement of Relay Stations (RSs) in WMANs. We propose a MAC scheme for WLANs in which stations contend by using jams on the channel. We present analytic and simulation results to find the optimal parameters of the scheme and measure its performance. Our scheme has a low collision rate and delay and a high throughput and fairness performance. Secondly, we present a MAC scheme for the latest generation of WLANs which have very high data rates. In this scheme, we divide the stations into groups and only one station from each group contends to the channel. We also use frame aggregation to reduce the overhead. We present analytic and simulation results which show that our scheme provides a small collision rate and, hence, achieves a high throughput. The results also show that our scheme provides a delay performance that is suitable for real-time applications and also has a high level of fairness. Finally, we consider the problem of placing Relay Stations (RSs) in WMANs. We consider the Worldwide Interoperability for Microwave Access (WIMAX) technology. The RSs are used to increase the capacity of the network and to extend its range. We present an optimization formulation that places RSs in the WiMAX network to serve a number of customers with a pre-defined bit rate. Our solution also provides fault-tolerance by allowing one RS to fail at a given time so that the performance to the users remains at a predictable level. The goal of our solution is to meet the demands of the users, provide fault-tolerance and minimize the number of RSs used.
This chapter provides an introduction to Wireless LANs and broadband wireless networks, the two areas in which this dissertation provides contribution. It also introduces the three main chapters of the dissertation.
In today's wireless networks, stations using the IEEE 802.11 Standard contend for the channel using the Distributed Coordination Function (DCF). Research has shown that DCF's performance degrades especially with the large number of stations. This becomes more concerning due to the increasing proliferation of wireless devices. In this chapter, we present a Medium Access Control (MAC) scheme for wireless LANs and compare its performance to DCF and to other efficient schemes. Our scheme, which attempts to resolve the contention in a constant number of slots (or constant time), is called CONTI. The contention resolution happens over a pre-defined number of slots. In a slot, the stations probabilistically send a jam signal on the channel. The stations listening retire if they hear a jam signal. The others continue to the next slot. Over several slots, we aim to have one station remaining in the contention, which will then transmit its data. We find the optimal parameters of CONTI and present an analysis on its performance. More comprehensive evaluation is presented in the simulation results where we compare CONTI, DCF and other efficient schemes from the literature. We consider the number of slots used, the collision rate, the throughput, the delay and fairness. The highest throughput was achieved by CONTI. Moreover, our results provide measurements from each of the schemes that we consider and provide the insight on each scheme's operation.
The latest generation of Wireless Local Area Networks (WLANs) was initiated by the IEEE 802.11n Standard. The standard uses the latest advances in the physical layer that provide high data rates. It also aims at improving the efficiency of the Medium Access Control (MAC) scheme to provide a throughput that is higher than 100 Mbps. In this chapter, we present a Group-based MAC (GMAC) scheme that aims at reducing the contention and the overhead. The stations are divided into groups and only one station from each group does the contention, thus reducing the probability of a collision. A station that gains access to the channel transmits a polling frame to allow the other stations in its group to transmit without requiring RTS/CTS frames, thus reducing the overhead. GMAC is interoperable with legacy 802.11 devices. To evaluate GMAC, we present analytic and simulation results. We also consider other schemes from the literature and compare them to GMAC. The results show that GMAC achieves a high throughput, high fairness, low delay and high scalability with respect to the data rates.
The IEEE 802.16j standard specifies the use of Relay Stations (RS) in WiMAX networks. The standard does not specify how to plan the locations of the RSs within the network. There have been several papers that aim at planning the locations of RSs in the WiMAX architecture. However, placement of RSs in WiMAX networks such that an RS failure won't interrupt the service, hence making the network fault-tolerant, is an important design and planning problem that has not been addressed in the literature. In this chapter, we address this problem, and present an Integer Linear Program (ILP) formulation that provides the planning of RS locations with fault-tolerance. We allow one RS to fail while keeping the provided service at a designated level (defined in throughput to users). We present numerical results that show how our model can be used to plan the positions of RSs. We also incorporate the existence of obstacles in the planning field, such as large structures or natural formations, that might happen in real life. To the best of our knowledge, this is the first work that addresses planning the RS locations in WiMAX in a fault-tolerant manner.
This chapter summarizes all the contribution of the dissertation.