Vijay Gabale

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.

A Classification Framework for Scheduling Algorithms in Wireless Mesh Networks

Scheduling MAC-layer transmissions in multi-hop wireless networks is an active and stimulating area of research. There are several interesting algorithms proposed in the literature in the problem space of scheduling for multi-hop wireless networks, specifically for (a) WiMAX mesh networks, (b) long distance multi-hop WiFi networks, and (c) Vehicular Ad-hoc Networks (VANETs). In general, these algorithms have several dimensions in terms of the assumptions made, the input space considered and the solution space generated. In this context, the goal of this survey is three-fold. Firstly, we classify the scheduling algorithms proposed in the literature based on following parameters: problem setting, problem goal, type of inputs and solution technique. Secondly, we describe different scheduling algorithms based on this classification framework. We specifically cover the state-of-the-art scheduling mechanisms proposed for generic multi-channel, multi-radio wireless mesh networks and in particular scheduling algorithms for WiMAX mesh networks, long distance mesh networks and vehicular ad-hoc networks. We describe scheduling algorithms which consider scheduling data, voice as well as video traffic. Finally, we compare these algorithms based on our classification parameters. We also critique individual mechanisms and point out the practicality and the limitations, wherever applicable. We observe that, the literature in the domain of scheduling for wireless mesh network is quite extensive, in terms of depth as well as breadth. Our classification framework helps in understanding the pros and cons of various aspects of scheduling for wireless multi-hop (popularly known as wireless mesh) networks. We also list desirable properties of any scheduling mechanism and use our classification framework to point out the open research issues in the space of scheduling for wireless mesh networks.

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.

InSite: QoE-aware video delivery from cloud data centers

The Internet is witnessing a rapid increase in video traffic. Due to the scalability and the cost-savings offered by cloud-computing, Internet video service providers are increasingly delivering their content from multi-tenant cloud data centers. One of the major challenges faced by such a video service provider is the management of the Quality-of-Experience (QoE) of the end-users in the presence of Variable Bit Rate (VBR) video flows, time varying network conditions in the Internet, and the bounded egress bandwidth provided by the data center. To this end, we present InSite, a light-weight and easy-to-deploy solution for managing the QoE of a set of video flows of a service provider, which are served from a data center. InSite is deployed at the egress of a data center, between the video servers and the clients, and manages the video flows that are transmitted over TCP. The solution uses a novel generalized binary search technique to concurrently search for the appropriate flow rates for a set of flows, with the goal of maximizing the QoE-fairness across the flows, as opposed to TCP-fairness. The search takes into account the total egress bandwidth allocated for the set of video flows at the data center, the unknown and possibly time-varying capacities of any remote bottleneck links, and the playout buffer sizes of the video flows. The solution is also designed to operate with minimal modifications to the video servers and the clients. In our evaluations using extensive ns-3 simulations and a testbed implementation for serving videos over TCP, we observe that deploying InSite achieves several folds reduction in playout stalls over a system without InSite.

Deployments made easy: essentials of managing a (rural) wireless mesh network

In this work, we present our experiences of managing the deployment of a wireless mesh network to support real-time voice services in a village near Mumbai, India. We focus on three essential aspects of our deployment: (1) in-network mechanisms for ease of network planning, (2) network management and data collection in an operational network, and (3) fault-tolerance mechanisms for long-term network sustenance. Especially for rural deployment, where the amount of resources on the field are limited and frequent physical visits are costly, the consideration of these three aspects drastically simplified our deployment and measurement activities. Our in-network mechanisms constantly provided the desired network feedback to meet operational challenges while on the field. We carefully designed and implemented a low over-head and non-intrusive network management module over our TDMA based wireless mesh network. During our deployment, this module successfully diagnosed several network faults in a live network and collected required network statistics without affecting the primary application. We also implemented a set of fault-tolerance mechanisms in our prototype, and during our deployment, our network proved itself to be robust to various network failures. The villagers used our network for more than a month and availed more than hundred voice calls comprising of local calls within the village and remote calls to the phones in outside world.

DelayCheck: Scheduling voice over multi-hop multi-channel wireless mesh networks

In this work, we consider the goal of scheduling the maximum number of voice calls in a TDMA-based multi-radio, multi-channel mesh network. One of the main challenges to achieve this goal is the difficulty in providing strict (packet-level) delay guarantees for voice traffic in capacity limited multi-hop wireless networks. In this context, we propose DelayCheck, an online centralized scheduling and call-admission-control (CAC) algorithm which effectively schedules constant-bit-rate voice traffic in TDMA-based mesh networks. DelayCheck solves the joint routing, channel assignment and link scheduling problem while satisfying the constraint. We formulate an offline version of this scheduling problem as an Integer Linear Program (ILP), the LP relaxation of which gives us an optimality upper bound. We compare the output of DelayCheck with the LP-based upper bound as well as with two state-of-the-art prior scheduling algorithms. DelayCheck performs remarkably well, accepting about 93% of voice calls as compared to LP-based upper bound. As compared to state-of-the-art algorithms, DelayCheck improves scheduler efficiency by more than 34% and reduces call rejections by 2 fold. We also demonstrate that DelayCheck efficiently exploits the number of channels available for scheduling. With implementation optimizations, we show that DelayCheck has low memory and CPU requirements, thus making it practical.

The pilot deployment of a low cost, low power gateway to extend cellular coverage in developing regions

In spite of the tremendous growth in the number of cellular subscribers in the developing world, the teledensity in most of the rural areas remains low. For instance, in India, the rural teledensity is less than 30%. There are a large number of villages which do not have cellular coverage to this day. This lack of connectivity is a major hurdle in the development of these areas. However, many such villages are only a few kilometres away from the cellular coverage. Exploiting this, we present a low cost and low power gateway to extend cellular coverage into the rural areas. Specifically, we develop a low cost, low power 802.15.4 gateway which can connect a number of villages in its vicinity to the cellular backhaul. In prior work, we designed a low cost, low power 802.15.4 mesh network. In this work we show that, using the gateway node, 802.15.4 handsets and the mesh network, villagers can establish real-time voice calls from/to the village to/from the outside world. Such a network also allows villagers to make local voice calls (within the mesh network) using the 802.15.4 handsets. We deployed the prototype of our system for a day in Ahupe village, near Mumbai, India. Using our gateway node, we successfully established several phone calls from the village to phones in the outside world. We also established several local voice calls using the backbone mesh network, within the village. With our deployment experience, we believe that such a low cost and low power system can greatly benefit people in developing regions.

Async: De-congestion and yield management in cellular data networks

We design and implement a novel system called Async, which enables a mobile network operator (MNO) to efficiently manage the growth of mobile data by leveraging the delay-elastic nature of certain applications and the price-sensitive nature of certain users. Specifically, Async introduces an alternate “asynchronous” content-delivery paradigm for heavy content (e.g., videos), and facilitates an MNO to negotiate with users a delay in delivery in exchange for appropriate incentives. The MNO uses the negotiated delays to actively manage Async flows to reduce congestion and improve the quality-of-experience (QoE) of both delayed and regular flows. We show that in comparison to state-of-the-art, Asyncs network-based flow management enhances QoE for more than 30% of the regular flows, with up to 60% improvement in per-flow QoE metric, while still meeting the negotiated delivery times of 95% of the delayed flows. Async also lowers the delivery times of delayed flows by ∼67% and significantly increases robustness to traffic unpredictability. Our design is robust to disconnections and does not require any modifications to existing network infrastructure and protocols. Our prototype deployment (using Apaches mod_proxy and an Android app) on live networks confirms Asyncs efficacy in meeting EDTs for diverse deployment scenarios.

Importance-aware Bloom Filter for managing set membership queries on streaming data

In this work1, we consider a set of networking applications which generate or process a continuous stream of data items, for example, a web-cache which processes a stream of web-objects. These applications often require to answer membership queries for duplicate detection on an unbounded set of data items. Two key challenges to answer such membership queries are the limited space to store the entire stream and the different importance-levels associated with different data items. For instance, a web-caches are of finite sizes and the cost of fetching objects into the cache is proportional to the size of objects. Motivated by such examples, our work focuses on developing a time and space efficient indexing and membership query scheme which takes into account importance levels of objects in a data stream. We propose Importance-aware Bloom Filter (IBF) which provides a set of insertion and deletion algorithms to handle membership queries on a data stream. Our evaluation of IBF for a synthetic as well as a real data set of a stream of Youtube videos, shows that IBF has close to 0% error in answering membership queries on important data items, and it results in 4-fold better performance in comparison to other importance-agnostic Bloom filter-based schemes. Importantly, we also find properties of IBF analytically via a Markov model based analysis. Thus, we believe, IBF provides a practical framework to balance the application-specific requirements to index and query data streams based on the data semantics.

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.