Due to the rapid growth of real-time applications and the ubiquity of IEEE 802.11 MAC as a layer-2 protocol for wireless local area networks (WLANs), it becomes increasingly important to support delay-based quality of service (QoS) in such WLANs. In this paper, we develop a simple and accurate enough analytical model for predicting the queueing delay of real-time multimedia traffic flows in non-homogeneous random access based WLANs. This leads to tractable analysis for meeting queueing delay specifications of a number of flows. In particular, we address the feasibility problem of whether the mean delays required by a set of User Datagram Protocol (UDP) flows supporting real-time multimedia traffic can be guaranteed in WLANs. Based on the model and feasibility analysis, we further develop an optimization technique to minimize the delays for the traffic flows. Moreover, we present a decentralized algorithm and report its implementation and present extensive simulation and experimental trace-based results to demonstrate the accuracy of our model and the performance of the algorithms.

ZigBee and other wireless technologies operating in the (2.4GHz) ISM band are being applied in Wireless Personal Area Networks (WPAN) for many medical applications. However, these low duty cycle, low power, and low data rate medical WPANs suffer from WiFi co-channel interferences. WiFi interference can lead to longer latency and higher packet losses in WPANs, which can be particularly harmful to safety-critical applications with stringent temporal requirements, such as ElectroCardioGraphy (ECG). This paper exploits the Clear Channel Assessment (CCA) mechanism in WiFi devices and proposes a novel policing framework, WiCop, that can effectively control the temporal white-spaces between WiFi transmissions. Such temporal white-spaces can be utilized for delivering low duty cycle WPAN traffic. We have implemented and validated WiCop on SORA, a software-defined radio platform. Experimental results show that with the assistance of the proposed WiCop policing schemes, the packet reception rate of a ZigBee-based WPAN can increase by up to 116% in the presence of a heavy WiFi interferer. A case study on the medical application of WPAN ECG monitoring demonstrates that WiCop can bound ECG signal distortion within 2% even under heavy WiFi interference. An analytical framework is devised to model the CCA behavior of WiFi interferers and the performance of WPANs under WiFi interference with or without WiCop protection. The analytical results are corroborated by experiments.

Internet-scale data centers (IDCs) have rapidly proliferated to such an extent that their energy consumption and GreenHouse Gas (GHG) emissions have become an important concern to society. As a result, many IDC operators have started using renewable energy, e.g., wind power, to power their data centers. Unfortunately, the utilization of wind energy has stayed at a low ratio due to the intermittent nature of wind. This paper makes the case that it is in fact possible for a distributed IDC system to exploit multiple uncorrelated wind energy sources to significantly reduce the effect of intermittency and nearly achieve “entirely green” cloud-scale services. This result is obtained based on the analysis of real-world wind power traces from 69 wind farms. The idea is to leverage the front-end load dispatching server to send work to the location where wind power is available. We propose a wind-power-aware (WPA) policy that routes jobs based only on the current states of workloads and wind power availabilities in the data centers. We show that with the WPA policy more than 95% of energy consumption in IDCs can in fact be satisfied by wind power, and, secondly, that achieving this does not require the delaying of processing of jobs due to wind availability. We also show that the locations where data centers are placed play an important role in achieving high wind power utilization. Our analysis shows that wind power utilization can generally lie in a range from 44% to 96%, depending on how the locations of wind farms are selected. We propose a method for location selection that uses the coefficient of variation instead of the correlation coefficient, and show that with this method the utilization can lie in the high end of the above range. Finally, we verify these results by simulations that are based on real-world traces for both workloads and wind power generations.

Mobile devices adopt the IEEE 802.11 PSM (Power-Saving Mode) scheme and its enhancements to reduce their energy consumption when using Wi-Fi interfaces. However, the capability of PSM to save energy is limited when the WLANs are highly congested by other Wi-Fi clients. In this paper, instead of further pursuing the trade-off between power saving and the incurred delay on the client side, we take a different approach and explore the energy saving potential by considering the scheduling policy on the Access Point (AP) side. We find that the traditional packet-level first-come-first-serve policy is not sleep optimal since it keeps the PSM clients awake unnecessarily. We propose SOFA, an AP-centric scheme, which helps PSM clients save energy by minimizing the time they are forced to stay awake while down link traffic is being transmitted to other clients. SOFA delivers down link packets to the PSM clients in an optimal sequence, such that several objective are simultaneously achieved: (i) system-sleep optimality, (ii) energy-fairness, (iii) attention fairness, and (iv) no unnecessary deferral of packets beyond a beacon period. First, it determines an attention quota for each client at the beginning of each beacon period, without requiring any knowledge of available wireless capacity. Then it takes the attention "quota'' and attention request as inputs to decide the down link packet scheduling. We prove the stability and optimality of SOFA. Simulation results shows SOFA dramatically decreases the energy consumption of PSM clients in a crowded WLAN, especially for those clients with small attention requests.

Due to the rapid growth of real-time applications and the ubiquity of IEEE 802.11 MAC as a layer-2 protocol for wireless local area networks (WLANs), it is of increasing interest to support quality of service (QoS) in such WLANs. In this paper, we develop a simple but accurate enough analytical model for predicting queueing delay in non-homogeneous random access based WLANs. This leads to tractable solutions for meeting queueing delay specifications of a number of flows. Using this model, we address the feasibility problem of whether the mean delays required by a set of inelastic flows can be guaranteed in WLANs. Based on the model and feasibility analysis, we further develop an optimization technique to minimize the delays for inelastic flows. We present extensive simulation results to demonstrate the accuracy of our model and the performance of the algorithms.

IEEE 802.11 DCF is the dominant protocol used in existing WLANs. However, the efficiency of DCF progressively degrades with the increase of contending clients in the network as well as the wireless link rate. To address this issue, in this paper, we present a distributed random media access protocol, named CHAIN, which significantly improves uplink performance of WLANs. CHAIN mainly uses overhearing to coordinate clients in a network, and thus introduces little control overhead. The key in CHAIN is a novel piggyback transmission opportunity. In CHAIN, clients maintain a precedence relation among one another, and a client can immediately transmit a new packet after it overhears a successful transmission of its predecessor, without going through the regular contending process. When the network load is low, CHAIN behaves similar to DCF; But when the network becomes congested, clients automatically start chains of transmissions to improve efficiency. CHAIN is derived from DCF and co-exists friendly with it. Moreover, it possesses all the advantages of the 802.11 DCF standard - simplicity, robustness, and scalability. We analytically prove the correctness and fairness of CHAIN. Our extensive simulations on J-SIM verify our analytical results, and demonstrate significant performance gain of CHAIN over DCF.

ZigBee and other wireless technologies operating in the (2.4GHz) ISM band are being applied in Wireless Body Area Networks (WBAN) for many medical applications. However, these low duty cycle, low power, and low data rate medical WBANs suffer from WiFi co-channel interferences. WiFi interference can lead to longer latency and higher packet losses in WBANs, which can be particularly harmful to safety-critical applications with stringent temporal requirements. Existing solutions to WiFi-WBAN coexistence either require modifications to WiFi or WBAN devices, or have limited applicability. In this paper, by exploiting the Clear Channel Assessment (CCA) mechanisms in WiFi devices, we propose a novel policing framework, WiCop, that can effectively control the temporal white-spaces between WiFi transmissions. Specifically, the WiCop Fake-PHY-Header policing strategy uses a fake WiFi PHY preamble-header broadcast to mute other WiFi interferers for the duration of WBAN active interval, while the WiCop DSSS-Nulling policing strategy uses repeated WiFi PHY preamble (with its spectrum side lobe nulled by a band-pass filter) to mute other WiFi interferers throughout the duration of WBAN active interval. The resulted WiFi temporal white-spaces can be utilized for delivering low duty cycle WBAN traffic. We have implemented and validated WiCop on SORA, a software defined radio platform. Experiments show that with the assistance of the proposed WiCop policing schemes, the packet reception rate of a ZigBee-based WBAN can increase by up to 43.8% in presence of a busy WiFi interferer.

In wireless networks, how to select transmit power that maximizes throughput is a challenging problem. On one hand, transmissions at a high power level could increase interference to others; on the other hand, transmissions at a low power level are prone to being interfered by others. Prior works consider this problem as a search for afixedoptimal power setting that maximizes communication spatial reuse. In this paper, we pursue a novel approach that combines power selection with a random medium access mechanism. For each transmission, a noderandomlyselects a transmit power from all available power levels to access the medium. In this way, all combinations of network power settings could be selected with some probability. Using a recently developed Markov chain model, we derive a distributed scheme that determines the access probabilities of each power setting, according to the arrival rate of traffic and the service rate achieved by the scheme. We show that this scheme always converges to the optimal solution. Moreover, we also show that the random scheme can attain the maximal throughput region that can be obtained by any time-sharing between power settings, and which is consequently larger than the region any fixed power setting can achieve.

Advances in networking, sensors, medical devices and smart phones have made it feasible to monitor and provide medical and other assistance to people either in their homes or outside. Aging populations will benefit from reduced costs and improved healthcare through assisted living based on these technologies. However, these systems challenge current state-of-the-art techniques for usability, reliability, and security. In this paper we present the PAS open architecture for assisted living, which allows independently developed third party components to collaborate. Furthermore, we incorporate cell phones in PAS as the local intelligence in order to enhance the robustness and ubiquity. We discuss key technological issues in assisted living systems, such as software architecture layout, power preserving, security and privacy; and results from our pilot study in a real assisted living facility are presented.

In recent years, wireless ad hoc networks have become increasingly popular in both military and civilian applications due to their capability of building networks without the need for a pre-existing infrastructure. While a number of studies have been carried out to study the performance of IEEE 802.11 DCF in single-cell wireless LANs, the analysis of IEEE 802.11 DCF in multi-hop wireless networks with consideration of the effects of physical carrier sense, SINR, and collision caused by accumulative interference has been lacking. In this paper, we substantially extend Cali's work and rigorously model, with all these effects considered, channel activities governed by IEEE 802.11 DCF in multi-hop wireless networks. We show that as in single-cell WLANs, the choice of the contention window size can impact the system throughput in multi-hop wireless networks. However, the optimal value of the contention window size is quite different. Moreover, the optimal attempt probability p (the optimal window size) derived for multi-hop wireless networks with respect to maximizing the system throughput is larger (smaller) than that for single-cell WLANs. We also show that, given the minimal SINR threshold beta, the optimal carrier sense range is smaller than the conventional value used (provided that the contention window size is tuned accordingly).

Advances in networking, sensors, and embedded devices have made it feasible to monitor and provide medical and other assistance to people in their homes. Aging populations will benefit from reduced costs and improved healthcare through assisted living based on these technologies. However, these systems challenge current state-of-the-art techniques for usability, reliability, and security. This is a particular challenge for open and extensible systems that combine software and hardware from many vendors and provide information to diverse clinicians. In this paper we present the I-Living architecture for assisted living that allows independent parties work together in a dependable, secure, and low-cost fashion with predictable properties. Our approach is based on an Assisted Living Service Provider (ALSP) who provides a server that collects and maintains encrypted assisted persons (APs)' records. Our ALSP can be a third party distinct from APs, communication providers, and clinicians; or it can be part of an ISP, hospital or similar enterprise. We have explored the architecture by developing a collection of applications and implementing them in a prototype system. Our system shows the feasibility and opportunity of an open approach to assisted living systems.