Simulation-based study of link-level hybrid FEC/ARQ-SR for wireless links and long-lived TCP traffic

International audienceSince the TCP protocol uses the loss of packets as an indication of network congestion, its performance degrades over wireless links, which are characterized by a high bit error rate. Different solutions have been proposed to improve the performance of TCP over wireless links, the most promising one being the use of a hybrid model at the link level combining FEC (Forward Error Correction), ARQ-SR (Automatic Repeat Request with Selective Repeat), and an in-order delivery of packets to IP. The drawback of FEC is that it consumes some extra bandwidth to transmit the redundant information. ARQ-SR does not consume much bandwidth, its drawback is that it increases the round-trip time (RTT), which may deteriorate the performance of TCP, if not done appropriately. We study in this paper the performance of TCP over a wireless link implementing hybrid FEC/ARQ-SQ. The study is done by simulating long-lived TCP transfers with ns-2 over wireless links showing Bernoulli errors. We are motivated by how to tune link-level error recovery, e.g. amount of FEC, persistency of ARQ, so as to maximize the performance of TCP. We provide simulation results for different physical characteristics of the wireless link (delay, error rate) and for different traffic loads

Analyze TCP Protocol Performance in Satellite Communications

This research is mainly about the influence of protocols on the performance of Satellite Communication. It is widely recognized that satellite communications are affected by some peculiar problems, which penalize heavily the performance and efficiency of the Transmission Control Protocol (TCP). In fact, wireless satellite channels are usually characterized by link-asymmetry and higher Round Trip Time and Bit Error Rate in comparison to wired links. It means that TCP, which was developed for wired channels, exhibits often poor performance in a satellite scenario (unfair bandwidth allocation, low throughput and long file-transfer delay).We have studied and compared many TCP variants recently proposed. In particular we have compared TCP-Reno standard implementation with TCP-SACK, TCP-Westwood, TCP-Vegas and TCP-Tibet. We tested the different protocols on a simulated satellite scenario, with the support of NS2, a well-know network simulator platform, annotating the advantages and drawbacks of various protocols in order to improve the transmission of IP data packets over satellite channels. Keywords: Satellite link, Performance analysis, TCP behaviors

Performance Modeling, Design and Analysis of Transport Mechanisms in Integrated Heterogeneous Wireless Networks

Recently, wireless access to Internet applications and services has attracted a lot of attention. However, there is no single wireless network that can meet all mobile users’ requirements. Con-sequently, integrated heterogeneous wireless networks are introduced to meet diverse wireless Internet applications and services requirements. On the other hand, integrated heterogeneous wireless networks pose new challenges to the design and development of reliable transport mechanisms. Wireless Application Protocol version 2 (WAP 2.0) is one of the promising trans-port mechanisms. It uses wireless profiled TCP (WP-TCP), which is fully compatible with TCP, as one of the reliable transport protocols to cope with the wireless link impairments. For WAP 2.0 to continue providing reliable and efficient transport services in the future, one of the key is-sues is to thoroughly study, understand, and improve its performance in integrated heterogeneous wireless networks. In this thesis, we develop analytical frameworks and propose a solution to respectively study and improve the performance of WP-TCP in integrated heterogeneous wireless networks. Spe-cifically, we consider WP-TCP short- and long-lived flows over integrated wireless local area network (WLAN) and wireless wide area network (WWAN), where WLAN can be static or mo-bile. In order to facilitate the analysis of WP-TCP performance in integrated WLAN and WWAN, we first construct a novel WLAN link model, which captures the impact of both uncor-related and correlated transmission errors, and derive mathematical expressions that describe packet loss probability and packet loss burst length over WWAN-WLAN link. Then, we develop analytical frameworks for studying the performance of WP-TCP short- and long-lived flows. Differently from those reported in the literature, our analytical framework for WP-TCP short-lived flows takes into account both correlated and uncorrelated packet losses. Furthermore, our analytical framework for long-lived flow can be used to study the short-term (during vertical handover) and long-term performances of WP-TCP and it captures the effects of vertical handover, such as excessive packet losses and sudden change in network characteristics, which are commonly experienced in integrated static WLAN and WWAN. By using the devel-oped analytical frameworks, we extensively analyze the performance of WP-TCP flows and in-vestigate the optimal protocol design parameters over a wide range of network conditions. Finally, based on our analytical studies, we propose a receiver-centric loosely coupled cross-layer design along with two proactive schemes, which significantly improve the vertical hand-over performance. The proposed solution is easy to implement and deploy, compatible with tra-ditional TCP, and robust in the absence of cross-layer information. Extensive simulations have been conducted to confirm the effectiveness and practicability of our schemes

Optimization of transmission control protocol and feedback control mechanisms for wireless internet

University of Technology, Sydney. Dept. of Computer Systems.All current versions of reliable Transmission Control Protocol (TCP) react to packet losses differently and adjust the TCP congestion window in various ways. These protocols assume congestion in the network to be the primary cause for packet losses and unusual delays. TCP performs well over wired networks by adapting to end-to-end delays and packet losses caused by congestion. The TCP sender uses the cumulative acknowledgements it receives to determine which packets have reached the receiver, and provides reliability by retransmitting lost packets. The sender identifies the loss of a packet either by the arrival of several duplicate cumulative acknowledgements (say, three ACKs) or the absence of an acknowledgement for the packet within a timeout. TCP reacts to packet losses by reducing its transmission (congestion) window size before retransmitting packets, initiating congestion window or avoidance mechanisms and backing off its retransmission timer. These measures result in a reduction in the load on the intermediate links, thereby controlling the congestion in the network. Unfortunately, when packets are lost in the networks for reasons other than congestion, these measures result in an unnecessary reduction in end-to-end throughput and sub-optimal performance. Wireless links typically have much higher bit error rates. This implies that packet loss would occur frequently. If no error correction is attempted at lower layer, TCP will exercise its congestion control procedure unnecessarily and the throughput will be reduced significantly. If the link layer performs error control by performing the retransmission itself, packet transmission time will vary greatly, sometime even exceeding TCP retransmission time out and again TCP slow start will occur. In wireless networks, “packet loss ’’ problem is also encountered during handover when a mobile device moves from the coverage of one cell to that of another. During the handover, if the mobile station decides to make a handover before the segments are transmitted over the air interface, it is likely that some TCP segments buffered in a base station may be forwarded to another base station. This results in excessive segment delay or loss. Thus, there is a clear demand for methods that can suppress the problems caused by the wireless environment. Recently, several techniques have been developed to improve end-to-end TCP performance over wireless links. They can be classified into three categories: end-to-end TCP, split TCP and link layer TCP. However, they have not addressed these problems successfully. In this thesis, we propose, design and implement several algorithms that are applicable to the wireless networks in order to solve outstanding problems. Firstly, the research investigates the relationship between packet loss and network congestion and introduces a feedback based end-to-end congestion control algorithm to the wireless network. This algorithm is a modification of a Fair Intelligent Congestion Control (FICC) proposed in [19]. The innovation of the algorithm is to modify the original FICC in such a way that the queue lengths can be effectively controlled when it is jointly employed with TCP in the wireless network. The next algorithm is the new design of Explicit Loss Notification (ELN) at base station in Wired-Cum-Wireless networks. With the combination of new ELN algorithm and Wireless FICC algorithm, the end-to-end performance and fairness are greatly improved by eliminating the misinterpretation of error related lost packets from congestion. Finally, the research investigates the effects of network congestion, which often happens over low bandwidth wireless link, and QoS performance (e.g. fairness, delay variation) of multiple sessions of TCP traffic in a hybrid network. We propose a framework, which consists of two main algorithms, feedback based congestion control and Explicit Window Adaptation (EWA)

Measuring NFS Performance in Wireless Networks

Technological trends suggest that soon communication networks will consist of a high speed wired backbone with numerous wireless Local Area Networks. Mobile computing and wireless subnetworks are increasingly in demand. Mobile routing solutions provide wireless LANs with seamless connectivity to backbone wired systems. However, these solutions do not provide acceptable performance. Wireless networks have distinct transmission characteristics which present challenges to achieving efficient performance. Performance over wireless links is limited by high error rates, mobility, and low bandwidth. We have studied the performance of TCP and NFS over a wireless network. The prevalence of these protocols means that mobile hosts will frequently use them when communicating with stationary hosts. Measurements have been collected to determine the response of these protocols in the presence of various error patterns. These measurements show that NFS and TCP performance suffer extreme degradation due to these wireless link characteristics. Unexpectedly, NFS performance is not better than an TCP FTP file transfer. NFS performance over wireless links is limited by large packet sizes, long retransmission timeouts, and slow response to losses. Our goal is to understand the effects of wireless communication on these protocols and improve performance without requiring changes to the current network Infrastructure. (Also cross-referenced as UMIACS-TR-95-125

Throughput and fairness of multiple TCP connections in wireless networks

TCP suffers from poor throughput performance in wireless networks. Furthermore, when multiple TCP connections compete at the base station, link errors and congestion lead to serious unfairness among the connections. Although the issue of TCP performance in wireless networks has attracted significant attention, most reports focus only on TCP throughput and assume that there is only a single connection in a congestion-free network. This paper studies the throughput and fairness of popular improvement mechanisms (the Snoop [8] and ELN [5]) and TCP variants with multiple TCP connections. Simulation results show that the improvement mechanisms under investigation are effective to improve TCP throughput in a wireless network. However, they cannot provide fairness among multiple TCP connections. From the studies presented, it is concluded that mechanisms to enhance TCP fairness are needed in wireless network