Performance evaluation of multi-hop ad-hoc wireless LANs

Continuing advances in computing devices and mobile communications and the ever-increasing demand for network services have been fueling the interest in wireless packet services. Mobility, installation speed, simplicity, scalability, and less vulnerability to point-of-break problem, make wireless local area networks (WLAN) a good candidate for providing wireless packet services. Among the introduced WLAN standards, IEEE 802.11 due to its compatibility with the IEEE 802 protocol suite has become the dominating standard. The need for easy and spontaneous communications between proliferated mobile devices has attracted the attention of researchers and industry toward ad-hoc networks over the past few years. Ad-hoc networks can be used in situations where either there is no wired or wireless infrastructure present or, if present, it can not be used because of security, cost, or safety reasons. Due to the employed random back-off algorithm and system complexity, most of the works toward system performance evaluation resort to simulation. In order to optimally select system parameters to fulfill specific needs, a mathematical description of the system turns to be a lot helpful in observing the trend of any parameter changes made. In this thesis we mathematically model the distributed coordination function (DCF) mode of operation of IEEE 802.1I WLAN. Our model is based on the presentation of the system with a pair of one-dimensional state diagrams which accommodate variations of many input parameters. The model enables us to make optimum choices for the above parameters in different wireless channel qualities represented by the average bit error rate (BER). System performance is evaluated through three following measures: throughput, delay, and probability of fail to deliver of which the last two are crucial for real-time applications. Following that and aiming at multi-hop WLAN communications, we develop a detailed simulation environment spanning both medium access control (MAC) and physical (PHY) layers to investigate a greater number of phenomena such as hidden terminal, mobility, channel fading and path loss, etc. Results show that optimal selection of the system parameters can lead to considerable improvement in performance especially for time-sensitive applications