Aruna Balasubramanian

MAUI: making smartphones last longer with code offload

This paper presents MAUI, a system that enables fine-grained energy-aware offload of mobile code to the infrastructure. Previous approaches to these problems either relied heavily on programmer support to partition an application, or they were coarse-grained requiring full process (or full VM) migration. MAUI uses the benefits of a managed code environment to offer the best of both worlds: it supports fine-grained code offload to maximize energy savings with minimal burden on the programmer. MAUI decides at run-time which methods should be remotely executed, driven by an optimization engine that achieves the best energy savings possible under the mobile devices current connectivity constrains. In our evaluation, we show that MAUI enables: 1) a resource-intensive face recognition application that consumes an order of magnitude less energy, 2) a latency-sensitive arcade game application that doubles its refresh rate, and 3) a voice-based language translation application that bypasses the limitations of the smartphone environment by executing unsupported components remotely.

Energy consumption in mobile phones: a measurement study and implications for network applications

In this paper, we present a measurement study of the energy consumption characteristics of three widespread mobile networking technologies: 3G, GSM, and WiFi. We find that 3G and GSM incur a high tail energy overhead because of lingering in high power states after completing a transfer. Based on these measurements, we develop a model for the energy consumed by network activity for each technology. Using this model, we develop TailEnder, a protocol that reduces energy consumption of common mobile applications. For applications that can tolerate a small delay such as e-mail, TailEnder schedules transfers so as to minimize the cumulative energy consumed meeting user-specified deadlines. We show that the TailEnder scheduling algorithm is within a factor 2x of the optimal and show that any online algorithm can at best be within a factor 1.62x of the optimal. For applications like web search that can benefit from prefetching, TailEnder aggressively prefetches several times more data and improves user-specified response times while consuming less energy. We evaluate the benefits of TailEnder for three different case study applications - email, news feeds, and web search - based on real user logs and show significant reduction in energy consumption in each case. Experiments conducted on the mobile phone show that TailEnder can download 60% more news feed updates and download search results for more than 50% of web queries, compared to using the default policy.

DTN routing as a resource allocation problem

Many DTN routing protocols use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, so these approaches have only an incidental effect on such routing metrics as maximum or average delivery latency. In this paper, we present RAPID , an intentional DTN routing protocol that can optimize a specific routing metric such as worst-case delivery latency or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities which determine how packets should be replicated in the system. We evaluate RAPID rigorously through a prototype of RAPID deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real DTN at this scale. Our results suggest that RAPID significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads RAPID is within 10% of the optimal performance.

Augmenting mobile 3G using WiFi

We investigate if WiFi access can be used to augment 3G capacity in mobile environments. We rst conduct a detailed study of 3G and WiFi access from moving vehicles, in three different cities. We find that the average 3G and WiFi availability across the cities is 87% and 11%, respectively. WiFi throughput is lower than 3G through-put, and WiFi loss rates are higher. We then design a system, called Wiffler, to augments mobile 3G capacity. It uses two key ideas leveraging delay tolerance and fast switching -- to overcome the poor availability and performance of WiFi. For delay tolerant applications, Wiffler uses a simple model of the environment to predict WiFi connectivity. It uses these predictions to delays transfers to offload more data on WiFi, but only if delaying reduces 3G usage and the transfers can be completed within the applications tolerance threshold. For applications that are extremely sensitive to delay or loss (e.g., VoIP), Wiffler quickly switches to 3G if WiFi is unable to successfully transmit the packet within a small time window. We implement and deploy Wiffler in our vehicular testbed. Our experiments show that Wiffler significantly reduces 3G usage. For a realistic workload, the reduction is 45% for a delay tolerance of 60 seconds.

Interactive wifi connectivity for moving vehicles

We ask if the ubiquity of WiFi can be leveraged to provide cheap connectivity from moving vehicles for common applications such as Web browsing and VoIP. Driven by this question, we conduct a study of connection quality available to vehicular WiFi clients based on measurements from testbeds in two different cities. We find that current WiFi handoff methods, in which clients communicate with one basestation at a time, lead to frequent disruptions in connectivity. We also find that clients can overcome many disruptions by communicating with multiple basestations simultaneously. These findings lead us to develop ViFi, a protocol that opportunistically exploits basestation diversity to minimize disruptions and support interactive applications for mobile clients. ViFi uses a decentralized and lightweight probabilistic algorithm for coordination between participating basestations. Our evaluation using a two-month long deployment and trace-driven simulations shows that its link-layer performance comes close to an ideal diversity-based protocol. Using two applications, VoIP and short TCP transfers, we show that the link layer performance improvement translates to better application performance. In our deployment, ViFi doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing WiFi-style handoff protocol.

Replication routing in DTNs: a resource allocation approach

Routing protocols for disruption-tolerant networks (DTNs) use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, and so these approaches have only an incidental effect on such routing metrics as maximum or average delivery delay. In this paper, we present rapid, an intentional DTN routing protocol that can optimize a specific routing metric such as the worst-case delivery delay or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities that determine how packets should be replicated in the system. We evaluate rapid rigorously through a prototype deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real outdoor DTN. Our results suggest that rapid significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads, RAPID is within 10% of the optimal performance.

Web search from a bus

Opportunistic connections to the Internet from open wireless access points is now commonly possible in urban areas. Vehicular networks can opportunistically connect to the Internet for several seconds via open access points. In this paper, we adapt the interactive process of web search and retrieval to vehicular networks with intermittent Internet access. Our system, called Thedu, has mobile nodes use an Internet proxy to collect search engine results and prefetch result pages. The mobile nodes download the pre-fetched web pages from the proxy. Our contribution is a novel set of techniques to make aggressive but selective prefetching practical, resulting in a significantly greater number of relevant web results returned to mobile users. In particular, we prioritize responses in the order of the usefulness of the response to the query, that allows the mobile node to download the most useful response first. To evaluate our scheme, we deployed Thedu on DieselNet, our vehicular testbed operating in a micro-urban area around Amherst, MA. Using a simulated workload, we find that users can expect four times as many useful responses to web search queries compared to not using Thedus mechanisms. Moreover, the mean latency in receiving the first relevant response for a query is 2.7 minutes for our deployment; we expect Thedu to have even better performance in larger cities that have densely populated open APs.

Enhancing interactive web applications in hybrid networks

Mobile Internet users have several options today including high bandwidth cellular data services such as 3G, that may be the choice for many. However, the ubiquity and low cost of WiFi suggests an attractive alternative, namely, opportunistic use of open WiFi access points (APs) or planned municipal mesh networks. Unfortunately, for vehicular users, the intermittent nature of WiFi connectivity makes it challenging to support popular interactive applications such as Web search and browsing. Our work is driven by two questions. 1) How can we enable system support for interactive web applications to tolerate disruptions in WiFi connectivity from mobile nodes? 2) Can opportunistic mobile-to-mobile (m2m) transfers enhance application performance over only using APs, and if so, under what conditions and by how much? We present Thedu, a system that enables access to Web search from moving vehicles. The key idea is to use aggressive prefetching to transform the interactive Web search application into a one-shot request/response process. We deployed a prototype of Thedu on the DieselNet testbed in Amherst, MA, consisting of transit buses averaging 21 on the road at a time. Our deployment results show that Thedu can deliver 4 times as many relevant web pages than not using Thedu. A bus receives relevant web pages with a mean delay of 2.3 minutes and within 0.55 minutes in areas with high AP density. Thedu augments AP connectivity with m2m transfers using a utility-driven DTN routing algorithm and uses caching to exploit query locality. Our analytic model and trace-driven simulations suggest that m2m routing yields little benefit over using APs alone even under moderately dense AP deployment such as in Amherst. With sparsely deployed APs as may be the case in rural areas, our conclusions are more mixed: m2m routing with caching improves the number of relevant responses delivered per bus by up to 58%, but the mean delay is significantly high at 6.7 minutes, calling into question its practicality for interactive applications.

DOME: a diverse outdoor mobile testbed

A series of complex dependencies conspire to make it difficult to model mobile networks, including mobility, channel and radio characteristics, and power consumption. To address these challenges, we have designed and built a testbed for large-scale mobile experimentation, called the Diverse Outdoor Mobile Environment. DOME consists of computer-equipped buses, battery-powered nomadic nodes, organic WiFi APs, and a municipal WiFi mesh network. While the construction of a testbed such as DOME presents a significant engineering challenge, this paper describes a concrete set of scientific results derived from this experience. We argue that a broad range of mobility experiments could be performed in a testbed which provides the properties of temporal, technological, and spatial diversity. We demonstrate these properties in our testbed through analysis of data collected from DOME over a period of four years. Finally, we use DOME to provide insight into several open problems in mobile systems research.

R3: robust replication routing in wireless networks with diverse connectivity characteristics

Our work is motivated by a simple question: can we design a simple routing protocol that ensures robust performance across networks with diverse connectivity characteristics such as meshes, MANETs, and DTNs? We identify packet replication as a key structural difference between protocols designed for opposite ends of the connectivity spectrum---DTNs and meshes. We develop a model to quantify under what conditions and by how much replication improves packet delays, and use these insights to drive the design of R3, a routing protocol that self-adapts replication to the extent of uncertainty in network path delays. We implement and deploy R3 on a mesh testbed and a DTN testbed. To the best of our knowledge, R3 is the first routing protocol to be deployed and evaluated on both a DTN testbed and a mesh testbed. We evaluate its performance through deployment, trace-driven simulations, and emulation experiments. Our results show that R3 achieves significantly better delay and goodput over existing protocols in a variety of network connectivity and load conditions.