<p>The Transmission Control Protocol (TCP) has been the dominant transport protocol in the Internet for many years. One of the reasons to this is that TCP employs congestion control mechanisms which prevent the Internet from being overloaded. Although TCP's congestion control has evolved during almost twenty years, the area is still an active research area since the environments where TCP are employed keep on changing. One of the congestion control mechanisms that TCP uses is fast retransmit, which allows for fast retransmission of data that has been lost in the network. Although this mechanism provides the most effective way of retransmitting lost data, it can not always be employed by TCP due to restrictions in the TCP specification.</p><p>The primary goal of this work was to investigate when fast retransmit inhibitions occur, and how much they affect the performance of a TCP flow. In order to achieve this goal a large series of practical experiments were conducted on a real TCP implementation.</p><p>The result showed that fast retransmit inhibitions existed, in the end of TCP flows, and that the increase in total transmission time could be as much as 301% when a loss were introduced at a fast retransmit inhibited position in the flow. Even though this increase was large for all of the experiments, ranging from 16-301%, the average performance loss, due to an arbitrary placed loss, was not that severe. Because fast retransmit was inhibited in fewer positions of a TCP flow than it was employed, the average increase of the transmission time due to these inhibitions was relatively small, ranging from 0,3-20,4%.</p>

The Transmission Control Protocol (TCP) has been the dominant transport protocol in the Internet for many years. One of the reasons to this is that TCP employs congestion control mechanisms which prevent the Internet from being overloaded. Although TCP's congestion control has evolved during almost twenty years, the area is still an active research area since the environments where TCP are employed keep on changing. One of the congestion control mechanisms that TCP uses is fast retransmit, which allows for fast retransmission of data that has been lost in the network. Although this mechanism provides the most effective way of retransmitting lost data, it can not always be employed by TCP due to restrictions in the TCP specification. The primary goal of this work was to investigate when fast retransmit inhibitions occur, and how much they affect the performance of a TCP flow. In order to achieve this goal a large series of practical experiments were conducted on a real TCP implementation. The result showed that fast retransmit inhibitions existed, in the end of TCP flows, and that the increase in total transmission time could be as much as 301% when a loss were introduced at a fast retransmit inhibited position in the flow. Even though this increase was large for all of the experiments, ranging from 16-301%, the average performance loss, due to an arbitrary placed loss, was not that severe. Because fast retransmit was inhibited in fewer positions of a TCP flow than it was employed, the average increase of the transmission time due to these inhibitions was relatively small, ranging from 0,3-20,4%.

Multi-hop wireless networks or ad-hoc networks face several limiting characteristics that make it difficult to support a multitude of applications. It is in this context that we find smart antennas to find significant applications in these networks, owing to their ability to alleviate most of these limitations. The focus of my research is thus to investigate the use of smart antennas in ad-hoc networks and hence efficiently design network protocols that best leverage their capabilities in communication. There are two parts to the proposed objective of designing efficient network protocols that pertain to the nature of the smart antenna network considered, namely, homogeneous and heterogeneous smart antenna networks. Unlike heterogeneous smart antenna networks, where different devices in the network employ different antenna technologies, homogeneous smart antenna networks consist of devices employing the same antenna technology. Further, in homogeneous smart antenna networks, different antenna technologies operating in different strategies tend to perform the best in different network architectures, conditions and application requirements. This motivates the need for developing a {em unified} framework for designing efficient communication (medium access control and routing) protocols for homogeneous smart antenna networks in general. With the objective of designing such a unified framework, we start by designing efficient MAC and routing protocols for the most sophisticated of the smart antenna technologies, namely multiple-input multiple-output (MIMO) links. The capabilities of MIMO links form a super-set of those possible with other antenna technologies. Hence, the insights gained from the design of communication protocols for MIMO links are then used to develop unified MAC and routing frameworks for smart antennas in general. For heterogeneous smart antenna networks, we develop theoretical performance bounds by studying the impact of increasing degree of heterogeneity on network throughput performance. Given that the antenna technologies are already unified in the network, unified solutions are not required. However, we do develop efficient MAC and routing protocols to best leverage the available heterogeneous capabilities present in the network. We also design efficient cooperation strategies that will further help the communication protocols in exploiting the available heterogeneous capabilities in the network to the best possible extent.

Content of master’s thesis is description field of Sliding window and it’s expansion algorithms, witch are Slow start, Congestion avoidance, Fast Retransmit and Fast Recovery algorithm. Thereinafter is described creation of model in Opnet Modeler’s simulation area. In this simulation area was analyzed reactions of average transfer speed onto variance of data size, lost ratio, latency in short and long time slot and variance of receiver’s buffer size. In last section of this document is method design witch makes it possible of transfer speed control through the use of receiver’s buffer size dynamic setting.