Scheduled WiFi using distributed contention in WLANs: algorithms,experiments, and case-studies

The ubiquitous adoption of WiFi introduces large diversity in types of application requirements and topological characteristics. Consequently, considerable attention is being devoted to making WiFi networks controllable without compromising their scalability. However, the main MAC protocol of WiFi, distributed coordination function (DCF), is a contention-based protocol using random backoff. Thus, operating under DCF, the access of channel is hard to control and nonpredictable. In order to provide controllability of channel access in WiFi, we propose Rhythm, a MAC protocol that achieves scheduled WiFi efficiently using distributed contention. By achieving scheduled WiFi, channel access can be controlled by manipulating the schedule decision. We evaluate the performance of Rhythm through analysis, experiments, and case-studies.     

Cue-based networking

In this paper we present a new approach called cue-based networking that uses hints or cues about the physical environment to optimize networked application behavior. We define the notion of cues and describe how cues can be obtained using wireless sensor networks as the underlying platform. We identify both the research and system challenges that need to be addressed to realize benefits of the approach under a target application of video delivery over IP networks. In the process, we identify key challenges of wireless sensor networks, namely timeliness and robustness. We design an adaptive algorithm that balances the tradeoff between them satisfying both timeliness and robustness requirements. Through an implementation of the video delivery application using the proposed algorithm in a real home environment, we highlight the practical benefits of the proposed approach.     

Multi-gateway association in wireless mesh networks

Most traditional models of wireless mesh networks involve a mobile device connecting to the backbone through one of the available gateways in a wireless mesh network. In this paper, we present an alternate model, in which mobile devices are allowed to connect through more than one of the available gateways. We call the model multi-gateway association (MGA). We present arguments for why such a model can result in better capacity, fairness, diversity and security when compared to the default single-association model. We also identify the primary challenges that need to be addressed when using multiple-gateway associations, and propose solutions to handle these challenges. Using simulations, we show that throughput benefits ranging from 10% to 125% can be obtained by the proposed model as compared to a default single association model with just two gateways and more importantly, benefits linear in the number of gateways are obtainable. Through simulations and analysis, we establish why only intelligent multi-gateway association and neither single or simple multi-gateway association strategies can yield significant benefits.     

A Receiver-Centric Transport Protocol for Mobile Hosts with Heterogeneous Wireless Interfaces

Numerous transport protocols have been proposed in related work for use by mobile hosts over wireless environments. A common theme among the design of such protocols is that they specifically address the distinct characteristics of the last-hop wireless link, such as random wireless errors, round-trip time variations, blackouts, handoffs, etc. In this paper, we argue that due to the defining role played by the wireless link on a connection’s performance, locating the intelligence of a transport protocol at the mobile host that is adjacent to the wireless link can result in distinct performance advantages. To this end, we present a receiver-centric transport protocol called RCP (Reception Control Protocol) that is a TCP clone in its general behavior, but allows for better congestion control, loss recovery, and power management mechanisms compared to sender-centric approaches. More importantly, in the context of recent trends where mobile hosts are increasingly being equipped with multiple interfaces providing access to heterogeneous wireless networks, we show that a receiver-centric protocol such as RCP can enable a powerful and comprehensive transport layer solution for such multi-homed hosts. Specifically, we describe how RCP can be used to provide: (i) a scalable solution to support interface specific congestion control for a single active connection; (ii) seamless server migration capability during handoffs; and (iii) effective bandwidth aggregation when receiving data through multiple interfaces, either from one server, or from multiple replicated servers. We use both packet level simulations, and real Internet experiments to evaluate the proposed protocol. In consideration of the typically prevalent server-client nature of traffic in the Internet, we term the protocol ‘receiver-centric’ although precisely it is the mobile host that drives the protocol operation. Note that in this paper, we define the sender and the receiver of a connection with respect to the direction of the data flow. Hung-Yun Hsieh received the B.S. and M.S. degrees in electrical engineering from National Taiwan University, Taipei, Taiwan, ROC, and the Ph.D. degree in electrical and computer engineering from Georgia Institute of Technology, Atlanta, Georgia, USA. He joined the Department of Electrical Engineering and the Graduate Institute of Communication Engineering at National Taiwan University as an Assistant Professor in August 2004. His research interests include wireless networks, mobile computing, and Internet protocols. E-mail: hungyun@ntu.edu.tw Kyu-Han Kim is currently a Ph.D student in the Department of Electrical Engineering and Computer Sicence at Univeristy of Michigan at Ann Arbor. He received his M.S. degree in computer science from Georgia Institute of Technology, where he worked in the GNAN Research Group under the guidance of Prof. Raghupathy Sivakumar. His main research interests are mobile computing, wireless networks, and network performance evaluation. E-mail: kyuhkim@eecs.umich.edu Yujie Zhu received her bachelor’s degree and master’s degree from the Electrical Engineering Department of Beijing University of Aeronautics and Astronautics in 1994 and 1997 respectively. After that she worked as a Network Engineer in ATM Network Management Center of China Telecom, Guangzhou Co. She is currently a Ph.D. student in the ECE department of Georgia Tech. Her research interest includes transport layer protocols, sensor networks and mobile ad-hoc networks. E-mail: zhuyujie@ece.gatech.edu Raghupathy Sivakumar received the BE degree in Computer Science from Anna University, India, in 1996 and master’s and doctoral degrees in Computer Science from the University of Illinois at Urbana-Champaign in 1998 and 2000 respectively. He joined the School of Electrical and Computer Engineering at the Georgia Institute of Technology as an Assistant Professor in August 2000. His research interests are in wireless network protocols, mobile computing, and network quality of service. E-mail: siva@ece.gatech.edu An erratum to this article is available at .     

Hughes syndrome: a common problem in Kuwait hospitals

The objective was to study antiphospholipid antibody syndrome (APS or Hughes syndrome) in two major teaching hospitals in Kuwait. patients with suspected Hughes syndrome were investigated with tests for anticardiolipin antibodies (aCL) and lupus anticoagulants (LAC) over 1 yr. Diagnosis was considered confirmed if significant levels of either or both antibodies with no obvious cause (primary), or with systemic lupus erythematosus (SLE) or SLE-like illness (including SLE serology) (secondary) were present. Twelve (37.5%; seven females, 58%) primary and 20 (62.5%; 18 females, 90%) secondary Hughes syndrome patients were seen during this period. patients were Kuwaiti, Middle-Eastern and North-African Arabs (29). Filipinos (2) and White (1). None were from the Indian subcontinent. The main presentation was thrombosis in 75% (arterial in 25% and venous in 50%), and recurrent abortions in 50% of married women. Haematological and dermatological manifestations were limited entirely to the secondary variety, seen in 25% and 19%, respectively. Clinical manifestations were severe, leading to death in one, intensive-care management in 31% and with partial or complete warfarin resistance or brittleness in 25%. Neurological/eye and cardiac manifestations were not seen, as these patients may be attending separate speciality hospitals for these diseases in Kuwait. The approximate prevalence of this syndrome was 2.66/1000 admissions in medical wards. Projected to the total referral areas of the two hospitals, an approximate figure of 52 patients/million population/year was obtained. Hughes syndrome was a common problem among Arabs, Filipinos and possibly Whites in Kuwait. Its manifestations were severe, often requiring intensive-care management, and in one case it was fatal. Patients from the Indian subcontinent were conspicuous by their absence, despite the fact that they were well represented in all other rheumatic disease groups. Ethnic and/or geographical factors could be important in this syndrome. To the best of our knowledge, this is the first report of Hughes syndrome from the Middle East.     

WTCP: A Reliable Transport Protocol for Wireless Wide-Area Networks

Wireless wide-area networks (WWANs) are characterized by very low and variable bandwidths, very high and variable delays, significant non-congestion related losses, asymmetric uplink and downlink channels, and occasional blackouts. Additionally, the majority of the latency in a WWAN connection is incurred over the wireless link. Under such operating conditions, most contemporary wireless TCP algorithms do not perform very well. In this paper, we present WTCP, a reliable transport protocol that addresses rate control and reliability over commercial WWAN networks such as CDPD. WTCP is rate-based, uses only end-to-end mechanisms, performs rate control at the receiver, and uses inter-packet delays as the primary metric for rate control. We have implemented and evaluated WTCP over the CDPD network, and also simulated it in the ns-2 simulator. Our results indicate that WTCP can improve on the performance of comparable algorithms such as TCP-NewReno, TCP-Vegas, and Snoop-TCP by between 20% to 200% for typical operating conditions.     

The effect of sulphide inclusions on fracture toughness and fatigue crack growth in 12 wt% Cr steels

The variation of plane strain fracture toughness and fatigue crack growth rates with sulphide inclusion content has been examined for four different sulphur levels in a 12 wt% Cr stainless steel. Charpy impact tests have also been conducted and the experimental results are correlated with the volume fraction of inclusions and the inter-inclusion spacing. The availability of the experimental results for the case of steam turbine blading is discussed with a hypothetical example problem.