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Network Conditions (network + condition)
Selected AbstractsApplication of model-free LQG subspace predictive control to TCP congestion controlINTERNATIONAL JOURNAL OF ADAPTIVE CONTROL AND SIGNAL PROCESSING, Issue 6 2008Belinda A. Chiera Abstract We investigate the application of a model-free linear quadratic Gaussian (LQG) subspace-based predictive controller to Internet congestion control. Specifically, we consider a classically designed LQG linear congestion controller with a non-standard performance index and determine whether a model-free controller is a viable alternative in this instance. We employ the model-free subspace predictive controller methodology which we customize for end-to-end transmission control protocol (TCP) congestion control. A series of network simulations support the use of the more easily implementable model-free controller over its classical analogue. We further demonstrate that the model-free controller provides increased stability under transient network conditions when compared with the first feedback congestion controller, TCP Vegas. Copyright © 2007 John Wiley & Sons, Ltd. [source] An opportunistic cross-layer architecture for voice in multi-hop wireless LANsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 4 2009Suhaib A. Obeidat Abstract We propose an opportunistic cross-layer architecture for adaptive support of Voice over IP in multi-hop wireless LANs. As opposed to providing high call quality, we target emergencies where it is important to communicate, even if at low quality, no matter the harshness of the network conditions. With the importance of delay on voice quality in mind, we select adaptation parameters that control the ratio of real-time traffic load to available bandwidth. This is achieved in two ways: minimizing the load and maximizing the bandwidth. The PHY/MAC interaction improves the use of the spectral resources by opportunistically exploiting rate-control and packet bursts, while the MAC/application interaction controls the demand per source through voice compression. The objective is to maximize the number of calls admitted that satisfy the end-to-end delay budget. The performance of the protocol is studied extensively in the ns-2 network simulator. Results indicate that call quality degrades as load increases and overlonger paths, and a larger packet size improves performance. For long paths having low-quality channels, forward error correction, header compression, and relaxing the delay budget of the system are required to maintain call admission and quality. The proposed adaptive protocol achieves high performance improvements over the traditional, non-adaptive approach. Copyright © 2008 John Wiley & Sons, Ltd. [source] The distribution of file transmission duration in the webINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 5 2004R. Nossenson Abstract It is well known that the distribution of files transmission duration in the Web is heavy-tailed (A practical guide to Heavy Tails: Statistical Techniques and Application. Birkhauser: Boston, 1998; 3,26). This paper attempts to understand the reasons for this phenomenon by isolating the three major factors influencing the transmission duration: file size, network conditions and server load. We present evidence that the transmission-duration distribution (TDD) of the same file from the same server to the same client in the Web is Pareto and therefore heavy tailed. Furthermore, text files transmission delay for a specific client/server pair is not significantly affected by the file sizes: all files transmitted from the same server to the same client have very similar transmission duration distributions, regardless of their size. We use simulations to estimate the impact of network conditions and server load on the TDD. When the server and the client are on the same local network, the TDD of each file is usually Pareto as well (for server files and client requests that are distributed in a realistic way). By examining a wide-area network situation, we conclude that the network conditions do not have a major influence on the heavy-tailed behaviour of TDD. In contrast, the server load is shown to have a significant impact on the high variability of this distribution. Copyright © 2004 John Wiley & Sons, Ltd. [source] EVA: a better TCP version for resource-insufficient networksINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 7 2002Jung-Shian Li Abstract TCP Vegas exhibits unfair congestion avoidance mechanism, which aggravates when there are insufficient network resources to accommodate buffer space of a pipe (bandwidth delay product). To remedy this shortcoming, we propose an Enhanced VegAs (EVA) that employs three auxiliary mechanisms: , revision, congestion detection and congestion tendency detection. A 2k factorial design with replications is used to study the effect of the three mechanisms. Our results show that TCP EVA achieves better performance than Vegas under various network conditions. Furthermore, congestion avoidance schemes, such as TCP EVA, perform much better than congestion control schemes, such as TCP Reno, in resource-insufficient networks. Copyright © 2002 John Wiley & Sons, Ltd. [source] Improving TCP performance over networks with wireless components using ,probing devices'INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 6 2002A. Lahanas Abstract TCP error control mechanism lacks the ability to detect with precision the nature of potential errors during communication. It is only capable of detecting the results of the errors, namely that segments are dropped. As a result, the protocol lacks the ability to implement an appropriate error recovery strategy cognizant of current network conditions and responsive to the distinctive error characteristics of the communication channel. TCP sender always calls for the sending window to shrink. We show that probing mechanisms could enhance the error detection capabilities of the protocol. TCP could then flexibly adjust its window in a manner that permits the available bandwidth to be exploited without violating the requirements of stability, efficiency and fairness that need to be guaranteed during congestion. Our experiments have three distinct goals: First, to demonstrate the potential contribution of probing mechanisms. A simple probing mechanism and an immediate recovery strategy are grafted into TCP-Tahoe and TCP-Reno. We show that, this way, standard TCP can improve its performance without requiring any further change. Second, to study the performance of adaptive strategies. An adaptive TCP with probing is used, that is responsive to the detected error conditions by alternating slow start, fast recovery and immediate recovery. An adaptive error recovery strategy can yield better performance. Third, to study the design limitations of the probing device itself. The aggressive or conservative nature of the probing mechanisms themselves can determine the aggressive or conservative behaviour of the protocol and exploit accordingly the energy/throughput trade-off. Copyright © 2002 John Wiley & Sons, Ltd. [source] Efficiency and fairness of system-optimal routing with user constraints,NETWORKS: AN INTERNATIONAL JOURNAL, Issue 4 2006Andreas S. Schulz Abstract We study the route-guidance system proposed by Jahn, Möhring, Schulz, and Stier-Moses Operations Research 53 (2005), 600,616 from a theoretical perspective. As system-optimal guidance is known to be problematic, this approach computes a traffic pattern that minimizes the total travel time subject to user constraints. These constraints are designed to ensure that routes suggested to users are not much longer than shortest paths for the prevailing network conditions. To calibrate the system, a certain measure,called normal length,must be selected. We show that when this length is defined as the travel time at equilibrium, the resulting traffic assignment is provably efficient and close to fair. To measure efficiency, we compare the output to the best solution without guidance and to user equilibria. To measure unfairness, we compare travel times of different users, and show that they do not differ too much. Inefficient or unfair traffic assignments cause users to travel too long or discourage people from accepting the system; either consequence would jeopardize the potential impact of a route-guidance system. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 48(4), 223,234 2006 [source] | ||||||||||