Sayandeep Sen

Long-distance 802.11b links: performance measurements and experience

The use of 802.11 long-distance links is a cost-effective means of providing wireless connectivity to rural areas. Although deployments in this setting are increasing, a systematic study of the performance of 802.11 in these settings is lacking. The contributions of this paper are two-fold: (a)we present a detailed performance study of a set of long-distance 802.11b links at various layers of the network stack, and (b)we document the various non-obvious experiences during our study.Our study includes eight long-distance links, ranging from 1km to 37km in length. Unlike prior studies of outdoor 802.11 links, we find that the error rate as a function of the received signal strength behaves close to theory. Time correlation of any packet errors is negligible across a range of time-scales. We have observed at least one of the link to be robust to rain and fog. But any interference on the long-distance links can be detrimental to performance. Apart from this however, such long-distance links can be planned to work well with predictable performance. During our measurements, we have observed a few hardware/driver quirks as well as system bottlenecks apart from the wireless link itself. We believe that our measurements and the documentation of our experience will help future network planning as well as protocol design for these networks.

Loss-aware network coding for unicast wireless sessions: design, implementation, and performance evaluation

Local network coding is growing in prominence as a technique to facilitate greater capacity utilization in multi-hop wireless networks. A specific objective of such local network coding techniques has been to explicitly minimize the total number of transmissions needed to carry packets across each wireless hop. While such a strategy is certainly useful, we argue that in lossy wireless environments, a better use of local network coding is to provide higher levels of redundancy even at the cost of increasing the number of transmissions required to communicate the same information. In this paper we show that the design space for effective redundancy in local network coding is quite large, which makes optimal formulations of the problem hard to realize in practice. We present a detailed exploration of this design space and propose a suite of algorithms, called CLONE, that can lead to further throughput gains in multi-hop wireless scenarios. Through careful analysis, simulations, and detailed implementation on a real testbed, we show that some of our simplest CLONE algorithms can be efficiently implemented in todays wireless hardware to provide a factor of two improvement in throughput for example scenarios, while other, more effective, CLONE algorithms require additional advances in hardware processing speeds to be deployable in practice.

Long distance wireless mesh network planning: problem formulation and solution

Several research efforts as well as deployments have chosen IEEE802.11 as a low-cost, long-distance access technology to bridge the digital divide. In this paper, we consider the important issue of planning such networks to the minimize system cost. This is a non-trivial task since it involves several sets of variables: the network topology, tower heights, antenna types to be used and the irorientations, and radio transmit powers. The task is further complicated due to the presence of network performance constraints, and the inter-dependence among the variables. Our first contribution in this paper is the formulation of this problem in terms of the variables, constraints and the optimization criterion. Our second contribution is in identifying the dependencies among the variables and breaking-down the problem into four tractable sub-parts. In this process, we extensively use domain knowledge to strike a balance between tractability and practicality. We have evaluated the proposed algorithms using random input sets as well as real-life instances with success. We have been able to show detailed planning of network topology, required tower heights, antenna types, and transmit powers for the Ashwini project, a long distance WiFi network under deployment in Andhra Pradesh, India, In this case, we are able to achieve within 2% additional cost of a lower bound estimate.

On the feasibility of the link abstraction in wireless mesh networks

Outdoor community mesh networks based on IEEE 802.11 have seen tremendous growth in the recent past. The current understanding is that wireless link performance in these settings is inherently unpredictable, due to multipath delay spread. Consequently, researchers have focused on developing intelligent routing techniques to achieve the best possible performance. In this paper, we are specifically interested in mesh networks in rural locations. We first present detailed measurements to show that the PHY layer in these settings is indeed stable and predictable. There is a strong correlation between the error rate and the received signal strength. We show that interference, and not multipath fading, is the primary cause of unpredictable performance. This is in sharp contrast with current widespread knowledge from prior studies. Furthermore, we corroborate our view with a fresh analysis of data presented in these prior studies. While our initial measurements focus on 802.11b, we then use two different PHY technologies as well, operating in the 2.4-GHz ISM band: 802.11g and 802.15.4. These show similar results too. Based on our results, we argue that outdoor rural mesh networks can indeed be built with the link abstraction being valid. This has several design implications, including at the MAC and routing layers, and opens up a fresh perspective on a wide range of technical issues in this domain.

A measurement study of a commercial-grade urban wifi mesh

We present a measurement study of a large-scale urban WiFi mesh network consisting of more than 250 Mesh Access Points (MAPs), with paying customers that use it for Internet access. Our study, involved collecting multi-modal data, e.g., through continuous gathering of SNMP logs, syslogs, passive traffic capture, and limited active measurements in different parts of the city. Our study is split into four components - planning and deployment of the mesh, success of mesh routing techniques, likely experience of users, and characterization of how the mesh is utilized. During our data collection process that spanned 8 months, the network changed many times due to hardware and software upgrades. Hence to present a consistent view of the network, the core dataset used in this paper comes from a two week excerpt of our dataset. This part of the dataset had more than 1.7 million SNMP log entries (from 224 MAPs) and more than 100 hours of active measurements. The scale of the study allowed us to make many important observations that are critical in planning and using WiFi meshes as an Internet access technology. For example, our study indicates that the last hop 2.4GHz wireless link between the mesh and the client is the major bottleneck in client performance. Further we observe that deploying the mesh access points on utility poles results in performance degradation for indoor clients that receive poor signal from the access points.

Can they hear me now?: a case for a client-assisted approach to monitoring wide-area wireless networks

We present WiScape, a framework for measuring and understanding the behavior of wide-area wireless networks, e.g., city-wide or nation-wide cellular data networks using active participation from clients. The goal of WiScape is to provide a coarse-grained view of a wide-area wireless landscape that allows operators and users to understand broad performance characteristics of the network. In this approach a centralized controller instructs clients to collect measurement samples over time and space in an opportunistic manner. To limit the overheads of this measurement framework, WiScape partitions the world into zones, contiguous areas with relatively similar user experiences, and partitions time into zone-specific epochs over which network statistics are relatively stable. For each epoch in each zone, WiScape takes a minimalistic view --- it attempts to collect a small number of measurement samples to adequately characterize the client experience in that zone and epoch, thereby limiting the bandwidth and energy overheads at client devices. For this effort, we have collected ground truth measurements for up to three different commercial cellular wireless networks across (i) an area of more than 155 square kilometer in and around Madison, WI, in the USA, (ii) a road stretch of more than 240 kilometers between Madison and Chicago, and (iii) locations in New Brunswick and Princeton, New Jersey, USA, for a period of more than 1 year. We justify various design choices of WiScape through this data, demonstrate that WiScape can provide an accurate performance characterization of these networks over a wide area (within 4% error for more than 70% of instances) with a low overhead on the clients, and illustrate multiple applications of this framework through a sustained and ongoing measurement study.

On the Feasibility of the Link Abstraction in (Rural) Mesh Networks

Outdoor community mesh networks based on 802.11 have seen tremendous growth in the recent past. The current understanding is that wireless link performance in these settings in inherently unpredictable, due to multipath delay spread. Consequently, researchers have focused on developing intelligent routing techniques to achieve the best possible performance. In this paper, we are specifically interested in mesh networks in rural locations. We first present detailed measurements to show that the PHY layer in these settings is indeed stable and predictable. There is a strong correlation between the error rate and the received signal strength. We show that interference, and not multipath fading, is the primary cause of unpredictable performance. This is in sharp contrast with current widespread knowledge from prior studies. Furthermore, we corroborate our view with a fresh analysis of data presented in these prior studies. Based on our results, we argue that outdoor rural mesh networks can indeed be built with the link abstraction being valid. This has several design implications, and opens up a fresh perspective on a wide range of technical issues in this domain.

A dual technology femto cell architecture for robust communication using whitespaces

We propose WhiteCell - a dual technology femto cell architecture for wide-area wireless users that addresses some of the critical challenges being faced by the cellular industry today in better meeting surging demands. WhiteCell extends the traditional notion of a femto cell in which small, low-power, short-range access points are placed in homes to improve coverage, connectivity and spatial re-use in the cellular operators own frequency and technology. In WhiteCell, each indoor access point is equipped with the ability to communicate over two separate blocks of spectrum - the cellular operators own spectrum using the same technology as before, and the recently released swath of TV whitespace spectrum that allows opportunistic use under specific FCC guidelines in the US. The two spectrum blocks and their corresponding technologies complement each other very well. While whitespace spectrum allows us to add significant capacity to the otherwise constrained cellular spectrum, the cellular frequencies allows the system to support some minimal expectation of performance guarantee that whitespace alone cannot, due to license exclusivity. While this extension is conceptually simple, it provides dramatic performance gains for both the cellular operator and the end users who are putting increasing demands on the limited cellular spectrum. This paper describes the overall WhiteCell architecture, a system implementation, and various challenges addressed in efficiently utilizing whitespace spectrum including a collaborative approach in spectrum sensing, as well as in efficiently transitioning traffic across this dual technology structure. In addition, the paper demonstrates the significant performance advantages of the architecture through detailed evaluation of our WhiteCell prototype.

Hybrid Window and Rate Based Congestion Control for Delay Sensitive Applications

There has been a dramatic increase in interactive cloud based software applications. Compared to classical real-time media applications (voice over IP (VoIP)/conferencing) and non real-time file delivery, these interactive software applications have unique characteristics: 1) they are delay sensitive yet demand in order and reliable data delivery, and 2) the traffic is usually bursty. Traditional window based congestion control does not work well for interactive applications because the bursty arrival of data leads to bursty network traffic, which causes additional queuing delay and packet loss in the network which reduces its delay performance. In this paper, we propose a new hybrid window plus rate based congestion control technique. This algorithm improves the delay performance of interactive applications by preventing congestion induced loss and minimizing queuing delay while still fully utilizing network capacity and maintaining fairness across multiple flows.

Rural Telephony: A Socio-Economic Case Study

Information and communication technology (ICT) has the potential to bring in development to rural areas in the third world. Any deployment of technology however should be backed by a positive economic activity to be sustainable. This paper reports on experience with the economics, as well as the social aspects of Voice-over-IP (VoIP) as a service. In Jan 2003, we established a WiFi link from a site with wired Internet connection to a remote village site. On learning the demand for a telephone facility at the village, we setup a VoIP service. The setup has been running with reasonable success since then. We have collected a wide variety of statistics on the usage of the service. Although we used the relatively low-cost WiFi technology, the expenses have been high. The capital expense is dominated by antenna tower (40m: U.S.