Chandramohan A. Thekkath

COMBINE: leveraging the power of wireless peers through collaborative downloading

Mobile devices are increasingly equipped with multiple network interfaces: Wireless Local Area Network (WLAN) interfaces for local connectivity and Wireless Wide Area Network (WWAN) interfaces for wide-area connectivity. The WWAN typically provides much wider coverage but much lower speeds than the WLAN. To address this dichotomy, we present COMBINE, a system for collaborative downloading wherein devices that are within WLAN range pool together their WWAN links, significantly increasing the effective speed available to them. COMBINE makes a number of novel contributions overprior work in this area, including: (a) a framework of incentives for collaboration that addresses several practical issues including the unification of monetary and energy costs, and on-the-fly estimation of the energy cost of communication in a system in operation; (b) a protocol for collaborative group formation and workload distribution that is energy efficient and adaptive to fluctuations in network conditions; and (c) an application-level striping procedure that eases deployment by avoiding the need for special-purpose proxies in the infrastructure. We present experimental results based on the prototype we have implemented that showen couraging speeds-ups with COMBINE.

StarTrack: a framework for enabling track-based applications

Mobile devices are increasingly equipped with hardware and software services allowing them to determine their locations, but support for building location-aware applications remains rudimentary. This paper proposes tracks of location coordinates as a high-level abstraction for a new class of mobile applications including ride sharing, location-based collaboration, and health monitoring. Each track is a sequence of entries recording a persons time, location, and application-specific data. StarTrack provides applications with a comprehensive set of operations for recording, comparing, clustering and querying tracks. StarTrack can efficiently operate on thousands of tracks.

SenSlide : a sensor network based landslide prediction system

Landslides are a serious geological hazard caused when masses of rock, earth, and debris flow down a steep slope during periods of intense rainfall and rapid snow melt. The western (Konkan) coast and the Himalayan region of India are subject to many such landslides every year. Landslides in these rocky regions are mainly caused by the increase in strain due to percolating rain water in rocks fissures, causing rocks to fracture and slide down the slope. According to government reports, from 1998 to 2001 alone, landslides have killed more than 500 people, disrupted the communication and transport for weeks and destroyed thousands of hectares of crop area. Existing solutions are restricted to landslide detection. A trip wire is installed along the landslide prone areas, and a break in the trip wire due to the falling rocks and debris triggers an alarm. Although this is an inexpensive solution for landslide detection, it is ineffectual in providing warning of the impending landslide. Typical sensors used for monitoring slope stability are multi-point bore hole extensometers, tilt sensors, displacement sensors, and volumetric soil water content sensors. These require drilling 20-30 meter holes into the surface making the installation very expensive (≈

Collaborative Downloading for Multi-homed Wireless Devices

50 per meter)

Niobe: A practical replication protocol

Mobile devices are increasingly equipped with multiple network interfaces: Wireless Local Area Network (WLAN) interfaces for local connectivity and Wireless Wide Area Network (WWAN) interfaces for wide-area connectivity. The WWAN typically provides much wider coverage but much lower speeds than the WLAN. To address this dichotomy, we consider collaborative downloading among mobile devices in close proximity. We demonstrate the potential benefits of such an approach and discuss the many challenges to realizing it in practice: incentivizing cooperation by adequately compensating nodes, effecting such cooperation via an efficient protocol, and facilitating it with a suitable user interface. We present our current thinking on these as we design a collaborative downloading system called COMBINE.

Senslide: a distributed landslide prediction system

The task of consistently and reliably replicating data is fundamental in distributed systems, and numerous existing protocols are able to achieve such replication efficiently. When called on to build a large-scale enterprise storage system with built-in replication, we were therefore surprised to discover that no existing protocols met our requirements. As a result, we designed and deployed a new replication protocol called Niobe. Niobe is in the primary-backup family of protocols, and shares many similarities with other protocols in this family. But we believe Niobe is significantly more practical for large-scale enterprise storage than previously published protocols. In particular, Niobe is simple, flexible, has rigorously proven yet simply stated consistency guarantees, and exhibits excellent performance. Niobe has been deployed as the backend for a commercial Internet service; its consistency properties have been proved formally from first principles, and further verified using the TLA + specification language. We describe the protocol itself, the system built to deploy it, and some of our experiences in doing so.

Graceful degradation via versions: specifications and implementations

We describe the design, implementation, and current status of Senslide, a distributed sensor system aimed at predicting landslides in the hilly regions of western India. Landslides in this region occur during the monsoon rains and cause significant damage to property and lives. Unlike existing solutions that detect landslides in this region, our goal is to predict them before they occur. Also, unlike previous efforts that use a few but expensive sensors to measure slope stability, our solution uses a large number of inexpensive sensor nodes inter-connected by a wireless network. Our system software is designed to tolerate the increased failures such inexpensive components may entail. We have implemented our design in the small on a laboratory testbed of 65 sensor nodes, and present results from that testbed as well as simulation results for larger systems up to 400 sensor nodes. Our results are sufficiently encouraging that we intend to do a field test of the system during the monsoon season in India.

Efficient implementation of PVM on the AN2 ATM network

Correctness of a fault-tolerant system hinges on the failure model, which typically constrains the number of concurrent failures in the system. These assumptions are sometimes violated in practice, inevitably leading to degraded system behavior that deviates from the systems specification and even causing complete unavailability of the system. This paper advocates the notion of graceful degradation as a complementary mechanism to fault tolerance in thedesign of highly available distributed systems. It provides three specifications for meaningful system behavior under degradation. The different specifications capture different tradeoffs between the gracefulness of degradation and the semantics preserved by a degraded view. The paper further demonstrates the practical relevance of the specifications by presenting three designs of versioned distributed storage systems that implement the specifications.

Chimera: data sharing flexibility, shared nothing simplicity

Advances in processor and network technology have increased the attractiveness of using a cluster of workstations connected by a highspeed network as a coarse-grained multicomputer. This paper describes our attempts at building such a system. Specifically, our system consists of a high-speed ATM network connecting a set of Alpha workstations that host the PVM environment. Using commercially available products, our system demonstrates end-to-end PVM communication performance that is very close to ATM link bandwidth.

Implementing an untrusted operating system on trusted hardware

The current database market is fairly evenly split between shared nothing and data sharing systems. While shared nothing systems are easier to build and scale, data sharing systems have advantages in load balancing. In this paper we explore adding data sharing functionality as an extension to a shared nothing database system. Our approach isolates the data sharing functionality from the rest of the system and relies on well-studied, robust techniques to provide the data sharing extension. This reduces the difficulty in providing data sharing functionality, yet provides much of the flexibility of a data sharing system. We present the design and implementation of Chimera -- a hybrid database system, targeted at load balancing for many workloads, and scale-out for read-mostly workloads. The results of our experiments demonstrate that we can achieve almost linear scalability and effective load balancing with less than 2% overhead during normal operation.