Kameswari Chebrolu

Brimon: a sensor network system for railway bridge monitoring

Railway systems are critical in many regions, and can consist of several tens of thousands of bridges, being used over several decades. It is critical to have a system to monitor the health of these bridges and report when and where maintenance operations are needed. This paper presents BriMon, a wireless sensor network based system for such monitoring. The design of BriMon is driven by two important factors: application requirements, and detailed measurement studies of several pieces of the architecture. In comparison with prior bridge monitoring systems and sensor network prototypes, our contributions are three-fold. First, we have designed a novel event detection mechanism that triggers data collection in response to an oncoming train. Next, BriMon employs a simple yet effective multi-channel data transfer mechanism to transfer the collected data onto a sink located on the moving train. Third, the BriMon architecture is designed with careful consideration of the interaction between the multiple requisite functionalities such as time synchronization, event detection, routing, and data transfer. Based on a prototype implementation, this paper also presents several measurement studies to show that our design choices are indeed quite effective.

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.

PIP: a connection-oriented, multi-hop, multi-channel TDMA-based MAC for high throughput bulk transfer

In this paper, we consider the goal of achieving high throughput in a wireless sensor network. Our work is set in the context of those wireless sensor network applications which collect and transfer bulk data. We present PIP (Packets in Pipe), a MAC primitive for use by the transport module to achieve high throughput. PIP has a unique set of features: (a) it is a multi-hop connection oriented primitive, (b) it is TDMA-based, (c) it uses multiple radio channels, and (d) it is centrally controlled. This represents a significant shift from prior MAC protocols for bulk data transfer. PIP has several desirable properties: (a) its throughput degrades only slightly with increasing number of hops, (b) it is robust to variable wireless error rates, (c) it performs well even without any flow control, and (d) requires only small queue sizes to operate well. We substantiate these properties with a prototype implementation of PIP on the Tmote-Sky CC2420-based platform. PIP achieves about twelve times better throughput than the state-of-the-art prior work, over a network depth of ten nodes.

LiT MAC: addressing the challenges of effective voice communication in a low cost, low power wireless mesh network

In this work, we consider the goal of enabling a local voice communication system, within a village, using a low cost and low power wireless mesh network. The design of an appropriate MAC is a major challenge in this context. Towards this goal, we present LiT: a full-fledged TDMA-based MAC protocol for real-time applications over such networks. We showcase the practicality of such a system through implementation-based evaluation of LiT on an inexpensive, low power 802.15.4 platform. While there is plentiful literature on the use of TDMA for wireless mesh networks, a practical multi-hop TDMA system remains elusive. In this regard, LiT addresses several practical concerns. It has built-in support for time-synchronization, has a flexible interface with routing, and has a dynamic TDMA schedule dissemination mechanism. LiT is multi-channel capable and is centrally controlled. It achieves robustness in the face of wireless packet errors by making extensive use of soft-state mechanisms. With appropriate duty cycling, LiT can make nodes run for several weeks without power off the grid. Evaluation of LiT on outdoor testbed shows quick flow setup (latency < 1s), low packet delay (< 240ms) and negligible data path jitter (median 0ms), essential for real-time applications.

PIP: A multichannel, TDMA-based MAC for efficient and scalable bulk transfer in sensor networks

In this article, we consider the goal of achieving high throughput in a wireless sensor network. Our work is set in the context of those wireless sensor network applications which collect and transfer bulk data. We present PIP (Packets in Pipe), a MAC primitive for use by the transport module to achieve high throughput. PIP has a unique set of features: (a) it is a multihop connection-oriented primitive, (b) it is TDMA based, (c) it uses multiple radio channels, and (d) it is centrally controlled. This represents a significant shift from prior MAC protocols for bulk data transfer. PIP has several desirable properties: (a) its throughput degrades only slightly with increasing number of hops, (b) it is robust to variable wireless error rates, (c) it performs well even without any flow control, and (d) requires only small queue sizes to operate well. We substantiate these properties with a prototype implementation of PIP on the Tmote-Sky CC2420-based platform. PIP achieves about eleven times better throughput than the state-of-the-art prior work, over a network depth of 24 hops. We also show that PIP can be interagted with duty cycling, and that PIP can support streaming data from/to flash at little overhead.

Loss behavior analysis and its application in design of link quality metrics

Wireless losses play a significant role in determining the performance of many higher-layer protocols. Various measurement studies have presented results that explore the many facets of link behaviour. However, the results have often been contradictory. In the first half of the paper, we delve into understanding the underlying causes of losses and resolve two contradictions that appear in literature. One of the key insight we gain from this study is how packet loss manifests as a function of the packet count over which the averaging is done. This observation has wide applicability in many research domains that routinely employ loss rate measurements to dictate protocol behavior. To illustrate this point, we focus on a specific research domain: link quality metrics as employed in routing protocols. In the second half of the paper, we show that the popular link quality metric ETX [1] and variants thereof employ incorrect averaging which results in unstable and degraded performance. We modify the averaging mechanism based on our insight and term this modified version SLIQ. SLIQ stands for stability based link quality metric. The modification is simple yet subtle, and the performance improvement it provides is substantial. We carry out our evaluation of these metrics using realistic traces obtained from a wireless 802.11a testbed. We consider the impact the modified link metric has at both the routing layer and the application layer by defining appropriate performance metrics. When compared with ETX and another previously proposed metric ROMA [2], SLIQ provides stable and persistent routes (cuts the number of route flaps by a factor of 6) and can support twice as many high quality voice calls as ETX.

SAFE: Smart Authenticated Fast Exams for Student Evaluation in Classrooms

Considerable experimentation is happening in todays classrooms to handle large classes. In this paper, we present SAFE (Smart Authenticated Fast Exams), a tool that enables continuous assessment in the form of regular quizzes in classes. SAFE is based on a BYOD (bring your own device) model that leverages student smart-phones to conduct auto-graded, cheating-free exams in a proctored class room setting. SAFE has 3 components: a smart-phone app, a web server and WiFi infrastructure to enable app-server communication. SAFE support a rich set of features to handle various types of questions as well as instructor preferences. In the design of SAFE, we set to achieve 4 goals: easy setup, cheating-free operation, robustness and scale. Easy setup is achieved predominantly due to the BYOD model and online mode of exams. Cheating is prevented via locking the app for the intended purpose along with reporting to the server any user attempts at cheating. Robustness is handled via periodic syncing of messages and careful consideration of corner cases. Scale is handled by carefully tuning the WiFi as well as via application level scheduling. SAFE has been used so far to conduct 90+ in-class quizzes in 9 courses in the last year. It was also used to conduct a high stake admission test for a Masters program in Computer Science. The feedback from end-users has been very positive and we continue to get new requests for trials in other courses. This paper presents the design of SAFE and evaluation based on our experience thus far.

Cutting Internet Access Costs Through HTTPS Caching: A Measurement Study

In this paper, we look at web caching as a means to cut Internet access costs. We specifically look at caching of HTTPS traffic which has thus far not received much attention. We first look at client side caching on smartphones in mobile web scenarios to evaluate the potential for bandwidth savings. Our analysis based on user logs reveals that app traffic dominates browser traffic at 82.7% and HTTPS traffic dominates HTTP traffic at 82.3%. There is around 15% redundancy in this traffic, however much of this redundancy does not lend itself to practical savings since app logic or server cache configurations cannot be controlled.