<p>Internet has gained much popularity among the public since the mid 1990's and is now an integrated part of our society. A large range of high-speedbroadband providers and the development of new and more efficient Internet applications increase the possibilities to watch movies and live TV, use IP-telephony and share files over the Internet. Such applications demand high data transmission rates, which in turn consume network bandwidth. Since several users must share the common bandwidth capacity on the Internet, there will be locations in the network where the demand is higher than the capacity. This causes network congestion, which has negative impact on both the data transmission rate and transmission quality.</p><p>This thesis is about methods for measuring the available bandwidth of a network path between two computers. The available bandwidth can be interpreted as the maximum transfer rate possible without causing congestion. By deploying the methods studied in this thesis the available bandwidth can be measured without previous knowledge of the network topology. When an estimate of the available bandwidth is obtained, the transfer rate when sending messages between computers can be set to the measured value in order to avoid congestion.</p><p>In the thesis an active end-to-end available bandwidth measurement method called "Bandwidth Available in Real Time" (BART for short) is evaluated. BART measures the available bandwidth by injecting probe packets into the network at a given rate and then analysing how this rate has changed on the receiving side. A Kalman filter is used to update the current estimate of the available bandwidth using the new measurement sample.</p><p>The focus of the thesis is on how methods, such as BART, function in wireless 802.11 networks, which are very popular in work as well as in home environments. Wireless networks have a different construction compared to many other types of networks and this can affect the accuracy of the measurement methods discussed in this thesis. The effects must be analyzed and understood in order to obtain accurate available bandwidth estimates. Since wireless links are often parts of the network path between a sender and a receiver on the Internet, it is important to study how these links affect the estimates of the available bandwidth.</p>

Internet has gained much popularity among the public since the mid 1990's and is now an integrated part of our society. A large range of high-speedbroadband providers and the development of new and more efficient Internet applications increase the possibilities to watch movies and live TV, use IP-telephony and share files over the Internet. Such applications demand high data transmission rates, which in turn consume network bandwidth. Since several users must share the common bandwidth capacity on the Internet, there will be locations in the network where the demand is higher than the capacity. This causes network congestion, which has negative impact on both the data transmission rate and transmission quality. This thesis is about methods for measuring the available bandwidth of a network path between two computers. The available bandwidth can be interpreted as the maximum transfer rate possible without causing congestion. By deploying the methods studied in this thesis the available bandwidth can be measured without previous knowledge of the network topology. When an estimate of the available bandwidth is obtained, the transfer rate when sending messages between computers can be set to the measured value in order to avoid congestion. In the thesis an active end-to-end available bandwidth measurement method called "Bandwidth Available in Real Time" (BART for short) is evaluated. BART measures the available bandwidth by injecting probe packets into the network at a given rate and then analysing how this rate has changed on the receiving side. A Kalman filter is used to update the current estimate of the available bandwidth using the new measurement sample. The focus of the thesis is on how methods, such as BART, function in wireless 802.11 networks, which are very popular in work as well as in home environments. Wireless networks have a different construction compared to many other types of networks and this can affect the accuracy of the measurement methods discussed in this thesis. The effects must be analyzed and understood in order to obtain accurate available bandwidth estimates. Since wireless links are often parts of the network path between a sender and a receiver on the Internet, it is important to study how these links affect the estimates of the available bandwidth.

Available bandwidth is a time-dependant variable that defines the spare bandwidth in an end-to-end network path. Currently, there is significant focus in the research community on the design and development of Available Bandwidth Estimation Tools (ABETs), and a few tools have resulted from this research. However, there is no comprehensive evaluation of these tools and the research work in this thesis attempts to fill that gap. A performance evaluation of important ABETs like Pathload, IGI and pathChirp in terms of their accuracy, convergence time and intrusiveness is conducted in several scenarios. A 2k factorial design is carried out to analyze the importance of the size of probe packets, number of probe packets per train, number of trains, and frequency of runs in these performance metrics. ABETs are very important because of their potential in solving many network research problems. For example, ABETs can be used in congestion control in transport layer protocols, network management tools, route selection and configuration in overlay networks, SLA verification, topology building in peer to peer networks, call admission control, dynamic encoding rate modification in streaming applications, traffic engineering, capacity planning, intelligent routing systems, etc. This thesis looks at applying ABETs in the congestion control of transmission control protocol (TCP).Current implementations of TCP in the Internet perform reasonably well in terms of containing congestion, but their sending rate adjustment algorithm is unaware of the accurate network conditions and available resources. TCP's Additive Increase Multiplicative Decrease (AIMD) congestion control algorithm cannot efficiently utilize the available bandwidth to the full potential and this is especially true in high bandwidth networks.