Joseph Camp

Measurement driven deployment of a two-tier urban mesh access network

Multihop wireless mesh networks can provide Internet access over a wide area with minimal infrastructure expenditure. In this work, we present a measurement driven deployment strategy and a data-driven model to study the impact of design and topology decisions on network-wide performance and cost. We perform extensive measurements in a two-tier urban scenario to characterize the propagation environment and correlate received signal strength with application layer throughput. We find that well-known estimates for pathloss produce either heavily overprovisioned networks resulting in an order of magnitude increase in cost for high pathloss estimates or completely disconnected networks for low pathloss estimates. Modeling throughput with wireless interface manufacturer specifications similarly results in severely underprovisioned networks. Further, we measure competing, multihop flow traffic matrices to empirically define achievable throughputs of fully backlogged, rate limited, and web-emulated traffic. We find that while fully backlogged flows produce starving nodes, rate-controlling flows to a fixed value yields fairness and high aggregate throughput. Likewise, transmission gaps occurring in statistically multiplexed web traffic, even under high offered load, remove starvation and yield high performance. In comparison, we find that well-known noncompeting flow models for mesh networks over-estimate network-wide throughput by a factor of 2. Finally, our placement study shows that a regular grid topology achieves up to 50 percent greater throughput than random node placement.

The IEEE 802.11s Extended Service Set Mesh Networking Standard

Today, municipalities are planning to deploy metro-scale two-tier wireless mesh networks at a rapid pace. Fittingly, the IEEE 802.11s standard is being developed to allow interoperability between heterogeneous mesh network devices. In this article we describe and discuss how the initial standard addresses key factors for standardization of these networks: Efficient allocation of mesh resources at the routing and MAC layers. Protection and conservation of the network resources via security and energy efficiency. Assurance of fairness and elimination of spatial bias via mesh congestion control. We draw on examples from existing two-tier deployments, simulations, and analytical models to motivate these enhancements within the standard.

WARP: a flexible platform for clean-slate wireless medium access protocol design

The flexible interface between the medium access layer and the custom physical layer of the Rice University Wireless Open-Access Research Platform (WARP) provides a high performance research tool for clean-slate cross layer designs. As we target a community platform, we have implemented various basic PHY and MAC technologies over WARP. Moreover, we are implementing cross-layer schemes such as rate adaptation and crosslayer MIMO MAC protocols. In this demo, we demonstrate the flexibility of the interaction between the the WARP PHY and MAC layers by showing the capability to instantaneously change the modulation scheme, disabling/enabling MAC features such as carrier sensing or RTS/CTS 4-way handshake, and different multi-rate schemes.

Measurement and Modeling of the Origins of Starvation in Congestion Controlled Mesh Networks

Significant progress has been made in understanding the behavior of TCP and congestion-controlled traffic over multi- hop wireless networks. Despite these advances, however, no prior work identified severe throughput imbalances in the basic scenario of mesh networks, in which one-hop flows contend with two-hop flows for gateway access. In this paper, we demonstrate via real network measurements, test-bed experiments, and an analytical model that starvation exists in such a scenario, i.e., the one-hop flow receives most of the bandwidth while the two- hop flow starves. Our analytical model yields a solution consisting of a simple contention window policy that can be implemented via mechanisms in IEEE 802.11e. Despite its simplicity, we demonstrate through analysis, experiments, and simulations, that the policy has a powerful effect on network-wide behavior, shifting the networks queuing points, mitigating problematic MAC behavior, and ensuring that TCP flows obtain a fair share of the gateway bandwidth, irrespective of their spatial locations.

A Measurement Study of Multiplicative Overhead Effects in Wireless Networks

In this paper, we perform an extensive measurement study on a multi-tier mesh network serving 4,000 users. Such dense mesh deployments have high levels of interaction across heterogeneous wireless links. We find that this heterogeneous backhaul consisting of data-carrying (forwarding) links and non- data-carrying (non-forwarding) links creates two key effects on performance. First, we show that low-rate management and control packets can produce a disproportionally large degradation in data throughput. We define a metric for this effect called Wireless Overhead Multiplier and use it to quantify the impact of MAC and PHY mechanisms on the the throughput degradation. Surprisingly, we show that these multiplicative effects are primarily driven by the non-forwarding links where, in the worst case, data packets lose physical layer capture to the overhead, yielding disproportionate throughput degradation. Finally, we show that when data flows contend in this worst-case scenario, the loss-based autorate policy is unnecessarily triggered, causing throughput imbalance and poor network utilization.

Developing and Deploying Multihop Wireless Networks for Low-Income Communities

We are architecting and deploying a multi-hop wireless network in one of Houston’s most economically disadvantaged neighborhoods. The objective of this network is to empower under-resourced communities with access to technology and educational and work-at-home tools. Here, we present the societal objectives of the network, describe the joint technical and economic objectives that drive its architecture, and discuss future deployment, performance, and research challenges. I. SOCIETAL OBJECTIVES In many homes across the United States, children, youth, and their families have access to the world’s information-technology resources at their fingertips, while in low-income communities, access to technology and the opportunities it provides are often limited to brief periods of computer use and Internet access at school or at the public library [1]. Technology for All (TFA)1 addresses the disparity of opportunity that exists in our cities’ low-income neighborhoods through the tools of technology. By working with local community-based organizations, corporations, foundations, technology providers and public entities, TFA creates educational, economic and personal opportunities for low-income persons and the communities in which they

A Flexible Framework for Wireless Medium Access Protocols

In this paper, we present a framework for Medium Access Control (MAC) protocol development and performance evaluation. The framework, developed for the Rice University Wireless Open-Access Research Platform (WARP), allows us to interface a large class of medium access protocols with custom physical layer (PHY) implementations, thereby providing a flexible and high-performance research tool. MAC protocols for our framework are written in C and targeted to embedded PowerPC cores within the Xilinx Virtex II-Pro class of FPGAs. A key innovation is a flexible interface between the PHY and the MAC capable of exposing user-defined parameters to either layer, thus enabling cross-layer research.

iBeam: Intelligent client-side multi-user beamforming in wireless networks

Frequently, client-side wireless devices have a view of multiple WiFi access points, whether from open residential and commercial networks, corporate networks, or mesh networks. Given the increasing number of radios and antennas in todays wireless devices, residual capacity from these multiple APs could be leveraged if client devices communicate with multiple APs simultaneously. In this paper, we exploit multi-user multi-input multi-output (MU-MIMO) technology to improve throughput and reliability in both directions of a wireless connection. For uplink, we use multi-user beamforming to enable the client devices to send multiple data streams to multiple APs simultaneously. For downlink, we leverage interference nulling technology to allow the client devices to decode parallel packets from multiple APs. This iBeam system requires no changes to existing APs or backhaul networks and is compatible with the IEEE 802.11 standards. We experimentally evaluate iBeam and show significant throughput improvements over both single-AP connections and multi-AP connections in a time division mode. The clients reliability and stability are also significantly improved due to the multi-AP diversity gain.

SAMU: Design and implementation of selectivity-aware MU-MIMO for wideband WiFi

In anticipation of the increasing demand of wireless traffic, WiFi standardization efforts have recently focused on two key technologies for capacity improvement: multi-user MIMO and wider bandwidth. However, users experience heterogeneous channel orthogonality characteristics across sub-carriers in the same channel bandwidth, which prevents ideal multi-user gain. Moreover, frequency selectivity increases as bandwidth scales and correspondingly severely deteriorates multi-user MIMO performance. In this work, we consider the frequency selectivity of current and emerging WiFi channel bandwidths to optimize multi-user MIMO by dividing the occupied channel bandwidth into equally-sized sub-channels according to the level of frequency selectivity. In our selectivity-aware multi-user MIMO design, SAMU, each sub-channel is allocated according to the largest bandwidth that can be considered frequency-flat, and an optimal subset of users is chosen to serve in each sub-channel according to spatial orthogonality, achieving a significant performance improvement for all users in the network. Additionally, we propose a selectivity-aware very high throughput (SA-VHT) mode, which is based on and an extension to the existing IEEE 802.11ac standard. Over emulated and real indoor channels, even with minimal mobility, SAMU achieves as much as 80 percent throughput improvement compared to existing multi-user MIMO schemes, which could serve as a lower bound as bandwidth scales.

Coupled 802.11 flows in urban channels: model and experimental evaluation

Contending flows in multi-hop 802.11 wireless networks compete with two fundamental asymmetries: (i) channel asymmetry, in which one flow has a stronger signal, potentially yielding physical layer capture, and (ii) topological asymmetry, in which one flow has increased channel state information, potentially yielding an advantage in winning access to the channel. Prior work has considered these asymmetries independently with a highly simplified view of the other. However, in this work, we perform thousands of measurements on coupled flows in urban environments and build a simple, yet accurate model that jointly considers information and channel asymmetries. We show that if these two asymmetries are not considered jointly, throughput predictions of even two coupled flows are vastly distorted from reality when traffic characteristics are only slightly altered (e.g., changes to modulation rate, packet size, or access mechanism). These performance modes are sensitive not only to small changes in system properties, but also small-scale link fluctuations that are common in an urban mesh network. We analyze all possible capture relationships for two-flow sub-topologies and show that capture of the reverse traffic can allow a previously starving flow to compete fairly. Finally, we show how to extend and apply the model in domains such as modulation rate adaptation and understanding the interaction of control and data traffic.