Medium access control and network planning in wireless networks

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

Group-Based Medium Access Control for IEEE 802.11n Wireless LANs

Abstract-The latest generation of Wireless Local Area Networks (WLANs) is based on IEEE 802.11n-2009 Standard. The standard provides very high data rates at the physical layer and aims to achieve a throughput at the Medium Access Control (MAC) layer that is higher than 100 Mbps. To do that, the standard introduces several mechanisms to improve the MAC efficiency. The most notable ones are the use of frame aggregation and Block-ACK frames. The standard, however, doesn't introduce a mechanism to reduce the probability of collision. This issue is significant because, with a high data rate, an AP would be able to serve a large number of stations, which would result in a high collision rate. In this paper, we propose a Group-based MAC (GMAC) scheme that reduces the probability of collision and also uses frame aggregation to improve the efficiency. The contending stations are divided into groups. Each group has one station that is the group leader. Only the leader stations contend, hence, reducing the probability of a collision. We evaluate the performance of our scheme with analytic and simulation results. The results show that GMAC achieves a high throughput, high fairness, low delay and maintains a high performance with high data rates

Performance Evaluation of Varying Contention Window Size for Bandwidth Constrained Routing in Adhoc Networks

Providing bandwidth efficient routing in ad hoc networks is a challenging task. Available bandwidth of nodes is accurately evaluated before finding route from source to destination. Accuracy of available bandwidth is mainly affected by collision and overhead due to the execution of backoff scheme. Existing bandwidth constrained routing uses binary exponential backoff which follows serial transmission and causes unfair channel access. To overcome these, implicit pipelined backoff procedure is proposed to improve the available bandwidth and reduce the overhead associated with the backoff scheme employed in medium access control layer. In this, when two nodes are sharing the channel, the remaining nodes start the channel contention procedure in parallel to transmit next packet. Thus the channel waiting time is reduced. Each node maintains separate contention window for each phase in pipelined backoff. Proper choice of contention window size has great effect on performance of the network. This proposed algorithm is combined with a reactive link disjoint multipath routing protocol called AOMDV (Adhoc Ondemand Multipath Distance Vector) to find the best path based on bandwidth. Experimental results show that this algorithm outperforms existing approach in terms of QoS metrics such as delay, throughput, packet delivery ratio and energy consumption for different contention window sizes

Supporting Real-Time Communication in CSMA-Based Networks : the VTP-CSMA Virtual Token Passing Approach

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

Enhanced Collision Resolution for the IEEE 802.11 Distributed Coordination Function

The IEEE 802.11 standard relies on the Distributed Coordination Function (DCF) as the fundamental medium access control method. DCF uses the Binary Exponential Backoff (BEB) algorithm to regulate channel access. The backoff period determined by BEB depends on a contention window (CW) whose size is doubled if a station suffers a collision and reset to its minimum value after a successful transmission. BEB doubles the CW size upon collision to reduce the collision probability in retransmission. However, this CW increase reduces channel access time because stations will spend more time sensing the channel rather than accessing it. Although resetting the CW to its minimum value increases channel access, it negatively affects fairness because it favours successfully transmitting stations over stations suffering from collisions. Moreover, resetting CW leads to increasing the collision probability and therefore increases the number of collisions. % Quality control editor: Please ensure that the intended meaning has been maintained in the edits of the previous sentence. Since increasing channel access time and reducing the probability of collisions are important factors to improve the DCF performance, and they conflict with each other, improving one will have an adverse effect on the other and consequently will harm the DCF performance. We propose an algorithm, \gls{ECRA}, that solves collisions once they occur without instantly increasing the CW size. Our algorithm reduces the collision probability without affecting channel access time. We also propose an accurate analytical model that allows comparing the theoretical saturation and maximum throughputs of our algorithm with those of benchmark algorithms. Our model uses a collision probability that is dependent on the station transmission history and thus provides a precise estimation of the probability that a station transmits in a random timeslot, which results in a more accurate throughput analysis. We present extensive simulations for fixed and mobile scenarios. The results show that on average, our algorithm outperformed BEB in terms of throughput and fairness. Compared to other benchmark algorithms, our algorithm improved, on average, throughput and delay performance