Rabin K. Patra

Routing in a delay tolerant network

We formulate the delay-tolerant networking routing problem, where messages are to be moved end-to-end across a connectivity graph that is time-varying but whose dynamics may be known in advance. The problem has the added constraints of finite buffers at each node and the general property that no contemporaneous end-to-end path may ever exist. This situation limits the applicability of traditional routing approaches that tend to treat outages as failures and seek to find an existing end-to-end path. We propose a framework for evaluating routing algorithms in such environments. We then develop several algorithms and use simulations to compare their performance with respect to the amount of knowledge they require about network topology. We find that, as expected, the algorithms using the least knowledge tend to perform poorly. We also find that with limited additional knowledge, far less than complete global knowledge, efficient algorithms can be constructed for routing in such environments. To the best of our knowledge this is the first such investigation of routing issues in DTNs.

Packet Loss Characterization in WiFi-Based Long Distance Networks

Despite the increasing number of WiFi-based Long Distance (WiLD) network deployments, there is a lack of understanding of how WiLD networks perform in practice. In this paper, we perform a systematic study to investigate the commonly cited sources of packet loss induced by the wireless channel and by the 802.11 MAC protocol. The channel induced losses that we study are external WiFi, non-WiFi and multipath interference. The protocol induced losses that we study are protocol timeouts and the breakdown of CSMA over WiLD links. Our results are based on measurements performed on two real-world WiLD deployments and a wireless channel emulator. The two deployments allow us to compare measurements across rural and urban settings. The channel emulator allows us to study each source of packet loss in isolation in a controlled environment. Based on our experiments we observe that the presence of external WiFi interference leads to significant amount of packet loss in WiLD links. In addition to identifying the sources of packet loss, we analyze the loss variability across time. We also explore the solution space and propose a range of MAC and network layer adaptation algorithms to mitigate the channel and protocol induced losses. The key lessons from this study were also used in the design of a TDMA based MAC protocol for high performance long distance multihop wireless networks [12].

Deploying a Rural Wireless Telemedicine System: Experiences in Sustainability

A primary concern for projects supported by aid programs is sustainability. Although there is a growing belief that information and communication technologies (ICT) can significantly impact development, in practice creating sustainable ICT projects is extremely difficult. A typical project consists of a pilot stage that aims to demonstrate the basic goals, followed by a deployment stage that aims for both scale and sustainability. A novel ICT project in rural India uses long-distance Wi-Fi networking to enable high-quality videoconferencing between eye hospitals and remote village clinics. The project highlights the importance of sustainability as a first-class goal for systems research.

Hardware speech recognition for user interfaces in low cost, low power devices

We propose a system architecture for real-time hardware speech recognition on low-cost, power-constrained devices. The system is intended to support real-time speech-based user interfaces as part of an effort to bring information and communication technologies (ICTs) to underdeveloped regions of the world. Our system architecture exploits a shared infrastructure model. The computationally intensive task of speech model training and retraining is performed offline by shared servers, while the actual recognition of speech is conducted on low-cost hand-held devices using custom hardware. The recognizer is extremely flexible and can support multiple languages or dialects with speaker-independent recognition. Dynamic loading of speech models is used for changing language grammar and retraining, while reprogramming is used to support evolution of recognition algorithms. The focus on small sets of words (at one time) reduces the complexity, cost and power consumption. We design the speech decoder, the central component of the recognizer, and we validate it via a prototype FPGA implementation. We then use ASIC synthesis to estimate power and size for the design. Our evaluations demonstrate an order of magnitude improvement in power compared with optimized recognition software running on a low-power embedded general-purpose processor of the same technology and of similar capabilities. The synthesis also estimates the area of the design to be about 2.5mm, showing potential for lower cost. In designing and testing our recognizer we use datasets in both English and Tamil languages.

An adaptive, high performance mac for long-distance multihop wireless networks

We consider the problem of efficientMAC design for long-distance WiFi-based mesh networks. In such networks it is common to find long propagation delays, the use of directional antennas, and the presence of inter-link interference. Prior work has shown that these characteristics make traditional CSMA-based MACs a poor choice for long-distance mesh networks and this finding has led to several recent research efforts exploring the use of TDMA-based approaches to media access. In this paper we first identify, and then address, several shortcomings of current TDMA-based proposals. First, because they use fixed-length transmission slots, current TDMA-based solutions do not adapt to dynamic variations in traffic load leading to inefficiencies in both throughput and delay. As we show in this paper, the throughput achieved by existing solutions falls far short of the optimal achievable network throughput. Finally, due to the scheduling constraints imposed by inter-link interference, current TDMA-based solutions only apply to bipartite network topologies. In this paper, we present JazzyMac, a simple, practical and efficient MAC protocol that addresses the above limitations. JazzyMac achieves efficiency by allowing variable-length link transmissions slots and then defining a distributed protocol by which nodes adapt the length of their transmission slots to changing traffic demands. JazzyMac is practical in that the adaptation at each node uses purely local information and that our protocol applies to arbitrary network topologies. Finally, the use of dynamic slot sizes allows JazzyMac to achieve better tradeoffs between throughput and delay. We evaluate JazzyMac using detailed simulation over a range of traffic patterns and realistic topologies. Our results show that JazzyMac improves throughput in all considered scenarios. This improvement is often substantial (e.g.,in 50% of our scenarios, throughput improves by over 40%) and is particularly pronounced for the common case of asymmetric traffic (e.g.,leading to almost 100% improvements). Furthermore, compared to current solutions, JazzyMac can achieve much better average delay for the same throughput.

Usage models of classroom computing in developing regions

This paper examines low-cost computing projects for education in developing regions, and presents some of the common entrepreneurial and technical problems faced by past and current initiatives. In particular, we look at various models of computer usage, and evaluate their appropriateness according to their effectiveness in education, their socio-cultural suitability, and economic feasibility. Based on detailed field studies and interviews conducted in rural Indian classrooms and economic analysis, we show that shared rather than single-user devices constitute a more realistic and sustainable approach for low-cost computing projects targeting childrens education.

Potential of CDMA450 for rural network connectivity

This article evaluates CDMA450 as a potential solution for rural data and voice connectivity. We begin by analyzing the main strengths of CDMA450, but also some of the potential limitations for rural coverage, from both a technical and an economic standpoint. We argue that CDMA450 is a promising technology, competitive in both capacity-centric urban environments and in coverage-centric rural environments. Consequently, we discuss the opportunities of providing universal coverage by adopting a business model targeting both urban and rural deployments, and utilizing urban to rural cross-subsidization. We then explore the advantages of deploying CDMA450 using a fixed wireless (WLL) model. To this end, we explore the effective range of WLL CDMA450, and the impact of using directional antennas and receive antenna diversity in real-world commercial CDMA450 deployment in Romania. If used properly, these techniques can aid us in increasing cell radii and thus lead to substantial cost benefits

Hierarchical identity based cryptography for end-to-end security in DTNs

Delay Tolerant Networks (DTN) arise whenever traditional assumptions about todaypsilas Internet such as continuous end-to-end connectivity, low latencies and low error rates are not applicable. These challenges impose constraints on the choice and implementation of possible security mechanisms in DTNs. The key requirements for a security architecture in DTNs include ensuring the protection of DTN infrastructure from unauthorized use as well as application protection by providing confidentiality, integrity and authentication services for end-to-end communication. In this paper, we examine the issues in providing application protection in DTNs and look at various possible mechanisms. We then propose an architecture based on Hierarchical Identity Based Encryption (HIBE) that provides end-to-end security services along with the ability to have fine-grained revocation and access control while at the same time ensuring efficient key management and distribution. We believe that a HIBE based mechanism would be much more efficient in dealing with the unique constraints of DTNs compared to standard public key mechanisms (PKI).

Simplifying fault diagnosis in locally managed rural WiFi networks

The last three years have seen a lot of work in making WiFi-enabled Long Distance (WiLD) networking a reality in rural areas. Generally these networks are managed by non-local users who cannot guarantee long term support beyond a pilot. For long term operational sustainability, it is essential that maintenance duties be transferred to local administrators. In this paper, we argue that the research agenda should expand into areas of simplified diagnosis solutions as an en-abler for locally managed WiLD networks. Motivated by real faults we have seen in our own deployment at the Aravind Eye Hospital, we propose a framework to simplify diagnosis and show some initial results towards this direction.

Optimal scheduling and power control for tdma based point to multipoint wireless networks

In TDMA-based point-to-multipoint rural wireless deployments, co-located base station radios and sector antennas are used to increase base station capacity. To achieve maximum capacity with limited availability of non-overlapping wireless channels, we need to operate as many radios as possible from different sectors on the same channel. However, operating co-located radios on the same channel can result in substantial interference especially with the current practice of operating all radios at maximum power. We investigate techniques that increase network throughput by eliminating this interference. To this end we formulate an LP optimization problem that maximizes throughput by computing optimal transmit schedules, optimal allocation of clients to base station radios, and optimal radio power levels. Our results suggest that there is a large gap between currently-used and optimal strategies, creating opportunities for simple, practical algorithms to address these issues. Our techniques are equally applicable to both WiFi based networks as well as other point-to-multipoint technologies such as WiMax.