We investigate the issue of deploying applications on a set of loosely coupled compute devices, connected through a wireless mesh, typical in community networks. Wireless mesh networks experience significant temporal and spatial variations in link bandwidth. When application components, modeled as a directed acyclic graph, need to be scheduled on such a mesh with bandwidth constraints (and variations), the problem of mapping components to specific compute nodes becomes an instance of bin packing with constraints of CPU, memory, and bandwidth limits within the mesh. To make the scheduling tractable, we propose BASS (Bandwidth Aware Scheduling System), and develop heuristics for scheduling, based on the directed graph topology of the application components. We evaluate BASS on an emulated mesh using bandwidth traces collected from an actual wireless testbed - CityLab. Detailed evaluations show that contemporary orchestration frameworks can plug in BASS to provide better end-to-end performance for the applications deployed on the mesh while reducing resource utilization.

A detailed examination of evolving traffic characteristics, operator requirements, and network technology trends suggests a move away from nonblocking interconnects in data center networks (DCNs). As a result, recent efforts have advocated oversubscribed networks with the capability to adapt to traffic requirements on-demand. In this paper, we present the design, implementation, and evaluation of OSA, a novel Optical Switching Architecture for DCNs. Leveraging runtime reconfigurable optical devices, OSA dynamically changes its topology and link capacities, thereby achieving unprecedented flexibility to adapt to dynamic traffic patterns. Extensive analytical simulations using both real and synthetic traffic patterns demonstrate that OSA can deliver high bisection bandwidth (60%-100% of the nonblocking architecture). Implementation and evaluation of a small-scale functional prototype further demonstrate the feasibility of OSA.

Full-bandwidth connectivity between all servers of a data center may be necessary for all-to-all traffic patterns, but such interconnects suffer from high cost, complexity, and energy consumption. Recent work has argued that if all-to-all traffic is uncommon, oversubscribed network architectures that can adapt the topology to meet traffic demands, are sufficient. In line with this work, we propose Proteus, an all-optical architecture targeting unprecedented topology-flexibility, lower complexity and higher energy efficiency.

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 substantial prior research, 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 three different cells to settle at 50%-94% lower transmit power than a per-cell power control solution; 2) increases network throughput by up to 50% across four 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 only causing a negligible degradation of voice quality.

We consider the problem of efficient link-layer multicasting in wireless networks with switched beamforming antennas. The inherent tradeoff between multicasting and beamforming -- the broadcast advantage of low-gain omni-directional transmissions versus the high-gain but spatially restricted transmissions of beamforming antennas -- makes this problem especially challenging.

We design, implement, and evaluate a vehicular communication system that improves uplink connectivity through multi-lobe beam pattern switching on a smart antenna. Directionality and base station-diversity are two well-known, independently developed mechanisms for improving the uplink connectivity of mobile clients. In this paper, we highlight that a system combining both mechanisms can achieve significant improvement in performance with multi-lobe beams that strike a tradeoff between directionality and diversity. This is in contrast to the mere steering of narrow beams used in conventional smart antenna systems. For tractability at vehicular speeds, our R2D2 system searches through a limited set of beam patterns with different numbers of lobes, and includes a two-stage algorithm that uses both runtime adaptation and cached candidate patterns. We design and evaluate several variants of run-time adaptation that tune the number and angle of lobes in the beam, and the bit rate. The design of these algorithms is guided by both analysis and real-world measurements with a smart antenna system mounted on a vehicle. These measurements with our prototype implementation show that R2D2 can achieve an uplink throughput increase of up to 154% over pure beamsteering and 45% over pure basestation diversity.

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 per-cell 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.

In this paper, we present a systematic experimental study of the effect of inter-cell interference on IEEE 802.11 performance. With increasing penetration of WiFi into residential areas and usage in ad hoc conference settings, chaotic unplanned deployments are becoming the norm rather than an exception. These networks often operate many nearby access points and stations on the same channel, either due to lack of coordination or insufficient available channels. Thus, inter-cell interference is common but not well-understood. According to conventional wisdom, the efficiency of an 802.11 network is determined by the number of active clients. Surprisingly, we find that with a typical TCP-dominant workload, cumulative system throughput is characterized by the number of interfering access points rather than the number of clients. We find that due to TCP flow control, the number of backlogged stations in such a network equals twice the number of access points. Thus, a single access point network proved very robust even with over one hundred clients. Multiple interfering access points, however, lead to an increase in collisions that reduces throughput and affects volume of traffic in the network.

Dedicated short range communications (DSRC)-based communications enable novel automotive safety applications such as an extended electronic brake light or intersection collision avoidance. These applications require reliable wireless communications even in scenarios with very high vehicle density, where these networks are primarily interference-limited. Given the uncertainties associated with current simulation models, particularly their interference models, it is critical to experimentally validate network performance for such scenarios. Towards this goal, we present a systematic, large-scale experimental study of packet delivery rates in a dense environment of 802.11 transmitters. We show that even with 100 transmitters in communication range with a frame size of 128 bytes and a bit-rate of 6Mbps, (a) most receivers can decode over 1500 pps in a saturated network, which corresponds to a packet delivery rate of 45% and (b) the mean packet delivery rate, for 10 pps per node workload that emulates vehicular safety applications, is about 95%. These results demonstrate that a COTS 802.11 implementation can correctly decode many packets under collision due to physical layer capture and can serve as a reference scenario for validation of network simulators.

Recent real-world measurements in dense congested radio environments have pointed out the inefficiency of frame error-based bit-rate adaptation mechanisms, which significantly reduce network capacity by misinterpreting frame errors due to collisions. These effects are likely to be amplified with the heavy use of media applications. Fortunately, traditional SNR-based rate adaptation, and the more recently proposed throughput-based, and collision-aware rate adaptation algorithms are expected to provide more robust performance in these scenarios. To our knowledge, however, their performance has never been experimentally validated in a congested environment. In this paper, we report our implementation experiences with rate adaptation in a dense, congested IEEE 802.11 network. We find that throughput-based adaptation, contrary to expectations, also suffers from poor bitrate selection. Due to an increase in physical layer capture, while using lower bitrates, nodes can increase their individual throughput at the expense of cumulative network throughput. SNR-based rate adaptation performs well in static environments but the lack of sufficient precision in RSSI measurements makes accurate rate selection in dynamic radio environments dfficult. The use of RTS/CTS, in the spirit of collision-aware rate adaptation, shows throughput improvements for frame error-based algorithms and, additionally, for throughput-based algorithms as well. However, results are below expectations likely due to RTS/CTS implementation issues on the Atheros 5212 platform.

To support real-time applications such as Voice-over-IP within an 802.11 Wireless LAN, efficient handoff mechanisms are required when a mobile client moves from one Access Point to another. In this paper, we present the design, implementation and performance results of MAC layer (layer-2) handoff algorithms which implement make-before-break mechanisms at the MAC layer unlike current algorithms that are based on break-before-make. As a baseline, we first present an algorithm that uses a single radio card on the client (as is done traditionally), but optimizes MAC layer handoff by periodically probing in the background for Access Points on other channels even when it is already associated with an Access Point and actively sending and receiving data. We then present two novel algorithms that use two radio cards on the client so that when one card is involved in communicating data, the other card can probe for neighboring Access Points. Of these multiple radio algorithms, the first one uses dedicated data and control cards at the client thereby implementing a soft version of make-before-break. The second algorithm uses two cards which can both perform control and data forwarding functions thereby implementing a strict version of make-before-break. Experimental results from a prototype implementation show that the make-before-brake algorithms lead to a significant reduction in MAC layer handoff overhead; the algorithm for the strict version of the make-before-break mechanism leads to sub 10 millisecond hand-off latency.

This paper presents an overview of the ORBIT (open access research testbed for next-generation wireless networks) radio grid testbed, that is currently being developed for scalable and reproducible evaluation of next-generation wireless network protocols. The ORBIT testbed consists of an indoor radio grid emulator for controlled experimentation and an outdoor field trial network for end-user evaluations in real-world settings. The radio grid system architecture is described in further detail, including an identification of key hardware and software components. Software design considerations are discussed for the open-access radio node, and for the system-level controller that handles management and control. The process of specifying and running experiments on the ORBIT testbed is explained using simple examples. Experimental scripts and sample results are also provided.

This paper generalizes the TCP Vegas congestion avoidance mechanism and uses accumulation , buffered packets of a flow inside network routers, as a congestion measure based on which a family of congestion control schemes can be derived. We call this model Accumulation-based Congestion Control (ACC), which fits into the nonlinear optimization framework proposed by Kelly. The ACC model serves as a reference for packet-switching network implementations. We show that TCP Vegas is one possible scheme under this model. It is well known that Vegas suffers from round trip propagation delay estimation error and reverse path congestion. We therefore design a new Monaco scheme that solves these problems by employing an out-of-band, receiver-based accumulation estimator, with the support of two FIFO priority queues from the (congested) routers. Comparisons between these two schemes demonstrate that Monaco does not suffer from the problems mentioned above and achieves better performance than Vegas. We use ns-2 simulations and Linux implementation experiments to show that the static and dynamic performance of Monaco matches the theoretic results. One key issue regarding the ACC model in general, i.e., the scalability of bottleneck buffer requirement, and a solution using a virtual queueing algorithm are discussed and evaluated.

This paper generalizes the TCP Vegas congestion avoidance mechanism and proposes a model to use accumulation, buffered packets of a flow inside network routers, as a congestion measure on which a family of congestion control schemes can be derived. We call this model accumulation-based congestion control (ACC). We use a bit-by-bit fluid model to define the accumulation concept and develop a general control algorithm, which includes a set of control policies. The new prove its proportional fairness and global stability. The ACC model serves as a reference for packet network implementations. We show that TCP Vegas is one possible scheme, which fits into the ACC model. It is well known that Vegas suffer from round trip propagation delay estimation error and reverse path queuing delay. We therefore design a new scheme called Monaco, which solves these problems by employing an out-of-band receiver-based accumulation estimator, with minimal support from network routers. Analysis and simulations comparisons between Vegas and Monaco demonstrate the effectiveness of the Monaco accumulation estimator. We use ns-2 simulations to show that the static and dynamic performance of Monaco matches the theoretic results. One key issue regarding the ACC model in general, i.e., the scalability of router buffer requirement, is discussed.