ABSTRACT Over the past few years, wireless networking technologies have made vast forays into our daily lives. Today, one can find 802.11 hardware and other personal wireless technology employed at homes, shopping malls, coffee shops and airports. Present-day wireless network deployments bear two important properties: they are unplanned, with most access points (APs) deployed by users in a spontaneous manner, resulting in highly variable AP densities; and they are unmanaged, since manually configuring and managing a wireless network is very complicated. We refer to such wireless deployments as being chaotic.

Today, three di#erent physical (PHY) layers for the IEEE 802.11 WLAN are available (802.11a/b/g); they all provide multi-rate capabilities. To achieve high performance under varying conditions, these devices need to adapt their transmission rate dynamically. While this rate adaptation algorithm is a critical component of their performance, only very few algorithms such as Auto Rate Fallback (ARF) or Receiver Based Auto Rate (RBAR) have been published and the implementation challenges associated with these mechanisms have never been publicly discussed. In this paper, we first present the important characteristics of the 802.11 systems that must be taken into account when such algorithms are designed. Specifically, we emphasize the contrast between low latency and high latency systems, and we give examples of actual chipsets that fall in either of the different categories. We propose an Adaptive ARF (AARF) algorithm for low latency systems that improves upon ARF to provide both short-term and long-term adaptation. The new algorithm has very low complexity while obtaining a performance similar to RBAR, which requires incompatible changes to the 802.11 MAC and PHY protocol. Finally, we present a new rate adaptation algorithm designed for high latency systems that has been implemented and evaluated on an AR5212-based device. Experimentation results show a clear performance improvement over the algorithm previously implemented in the AR5212 driver we used.

Sleep modes of wireless network cards are used to switch these cards into low-power state when idle, but large timeout periods and frequent wake-ups can reduce the utility of this approach. Modern processors offer the ability to switch CPU voltages or clock frequencies and therefore reduce CPU energy consumption, however, that can reduce the sleep durations of a network device, adversely affecting the achievable energy savings. This paper describes an approach in which multiple resource managers cooperate to reduce a mobile device's energy consumption. This systemlevel approach is based on the integrated management of a real-time CPU scheduler, the frequency scaling capabilities of a modern processor, a QoS packet scheduler, and the low-power sleep mode of a wireless network card.

In distributed systems, transcoding techniques have been used to customize multimedia objects, utilizing trade-offs between the quality and sizes of these objects to provide differentiated services to clients. Our research uses transcoding techniques in wireless systems to customize video streams to the requirements of users, while minimizing the energy costs. We introduce an approach to dynamically determine which transcoders to execute and where to execute them (e.g., client or server). The goal is to select appropriate transcoders (a) to provide clients with the quality of service they desire while (b) minimizing the energy consumption of the end-hosts in accordance with application-specific global energy management directives. This paper investigates sample transcoder functions for video streaming on handheld devices and introduces a mechanism for selecting the most appropriate transcoders and transcoder parameters.

This paper presents an analytical model to predict energy consumption in saturated IEEE 802.11 single-hop ad hoc networks under ideal channel conditions. The model we introduce takes into account the different operational modes of the IEEE 802.11 DCF MAC, and is validated against packetlevel simulations. In contrast to previous works that attempted to characterize the energy consumption of IEEE 802.11 cards in isolated, contention-free channels (i.e., single sender/receiver pair), this paper investigates the extreme opposite case, i.e., when nodes need to contend for channel access under saturation conditions. In such scenarios, our main findings include: (1) contrary to what most previous results indicate, the radio's transmit mode has marginal impact on overall energy consumption, while other modes (receive, idle, etc.) are responsible for most of the energy consumed; (2) the energy cost to transmit useful data increases almost linearly with the network size; and (3) transmitting large payloads is more energy efficient under saturation conditions.

The relative power consumed in the WLAN interface of a mobile device is rising due to significant improvements in the energy efficiency of the other device components. The unpredictability of the incoming WLAN traffic limits the effectiveness of existing power saving techniques. This paper introduces a Power Aware Web Proxy (PAWP) architecture designed to schedule incoming web traffic into intervals of high and no communication. This traffic pattern allows WLAN interfaces to switch to a low power state after very short idle intervals. PAWP uses a collection of HTTP-level techniques to compensate any negative impact that traffic scheduling may have. PAWP does not require any client or web server modifications. In this paper, we describe our initial experiences with a PAWP implementation for 802.11b WLANs. Our experiments show savings of more than 50 % in the energy consumed by the WLAN interface. Finally, our experiences give us insights into possible browser improvements when power consumption is taken into account.

Asymmetric transmission ranges caused due to transmit power control have the undesirable effect of increasing the number of hidden terminals in the network as well as increasing the unfairness in channel access. In this paper we present a new reactive power controlled MAC protocol, SHUSH, which tackles the above problems. We compare the performance of SHUSH with four other transmit power controlled MAC protocols and demonstrate that SHUSH achieves superior aggregate goodput, spatial reuse, fairness, and minimal energy consumption.

The rapid deployment of wireless networks in various environments necessitates the development of new end-to-end tools that monitor and measure the properties of wireless paths well. In this paper, we implement AdHoc Probe, a recently proposed path capacity estimation tool specially designed for the multi-hop ad hoc wireless environment. We present an implementation of AdHoc Probe in Linux; discuss challenges in its deployment, and solutions to the various system issues encountered. We also evaluate the behavior/effectiveness of AdHoc Probe in various testbed setups; including an interfered setting that cannot be simulated. Experiment results validate the workings of AdHoc Probe and offer insights into how the capacity of a wireless path changes in real wireless environments. Our efforts provide a basis for realistic results that can be of assistance in activities such as capacity planning, protocol design, performance analysis, and etc. 1.

Abstract. Multi-hop ad hoc wireless networks generally use the IEEE 802.11 Distributed Coordination Function (DCF) MAC protocol, which utilizes the request-to-send/clear-to-send (RTS/CTS) mechanism to prevent the hidden terminal problem. It has been pointed out that the RTS/CTS mechanism cannot completely solve the hidden terminal problem in ad hoc networks because the interference range could exceed the basic rate transmission range. In this paper we provide a worst-case analysis of collision probability induced by the hidden terminal problem in ad hoc networks with multi-rate functionality. We show that the interference caused by the nodes in the area that is not covered by the RTS/CTS is bounded by C ′ R −4,whereC ′ is a constant and R is the distance between the two transmitting nodes. The analytic result showed that the interference could shorten the data transmission range up to 30 percent. We then propose a simple multi-rate MAC protocol that could prevent the hidden terminal problem when transmit power control (TPC) is employed.

Adaptive transmit power control in 802.11 Wireless LANs (WLANs) on a per-link basis helps increase network capacity and improves battery life of Wifi-enabled mobile devices. However, it faces the following challenges: (1) it can exacerbate receiver-side interference and asymmetric channel access, (2) it can incorrectly lead to lowering the data rate of a link, (3) mobility-induced channel variations at short timescales make detecting and avoiding these problems more complex. Despite significant research in rate and power control, state of the art solutions lack comprehensive techniques to address the above problems. In this paper, we design and implement Symphony—a Synchronous Two-phase Rate and Power control system, whose agility in adaptation enables us to systematically address the three problems, while maximizing the benefits of power control on a per-link basis. We implement Symphony in the Linux MadWifi driver, and show that it can be realized on hardware that supports transmit power control with no modifications to the 802.11 MAC, thereby fostering immediate deployability. Our extensive experimental evaluation on a real testbed in an office environment demonstrates that Symphony (1) enables up to 80 % of the clients in 3 different cells to settle at 50 % to 94 % lower transmit power than a percell power control solution, (2) increases network throughput by up to 50 % across realistic deployment scenarios, (3) improves the throughput of asymmetry-affected links by 300%, and (4) opportunistically reduces the transmit power of mobile clients running VOIP calls by up to 97%, while causing minimum impact on voice quality.