Shweta Jain

Collision avoidance in a dense RFID network

In this work, we develop a CSMA-based MAC protocol to avoid reader-reader and reader-tag collisions in a dense RFID network. The network is implemented using mote-based RFID readers. To implement the MAC protocol,we develop an appropriate carrier sensing circuit using an RFID tag as an antenna and the mote as an apparatus to sample received signal strength. We have augmented a commercially available OEM RFID module with such carrier sensing capability and interfaced it with motes. Performance evaluation shows much superior performance relative to a naive and a randomized protocol in dense deployment environments both in regards to accuracy and time per tag read.

Exploiting path diversity in the link layer in wireless ad hoc networks

We develop an anycast mechanism at the link layer for wireless ad hoc networks. The goal is to exploit path diversity in the link layer by choosing the best next hop to forward packets when multiple next hop choices are available. Such choices can come from a multipath routing protocol, for example. This technique can reduce transmission retries and packet drop probabilities in the face of channel fading. We develop an anycast extension of the IEEE 802.11 MAC layer based on this idea. We implement the protocol in an experimental proof-of-concept testbed using the Berkeley motes platform and S-MAC protocol stack. We also implement it in the popular ns-2 simulator and experiment with the AOMDV multipath routing protocol and Ricean fading channels. We show that anycast performs significantly better than 802.11 in terms of packet delivery, particularly when the path length or effect of fading is large. Further we experiment with anycast in networks that use multiple channels and those that use directional antennas for transmission. In these networks, deafness and hidden terminal problems are the main source of packet loss. We implemented anycast as extension of 802.11 like protocols that were proposed for these special networks. We are able to show that anycast is capable of enhancing the performance of these protocols by simply making use of the path diversity whenever it is available.

MAC Layer Multicast in Wireless Multihop Networks

Many applications in wireless ad-hoc networks require multicast communication. In order to provide efficient multicast, various multicast routing protocols have been designed in recent years to facilitate formation of routes between multicast senders and receivers. There has also been some work to develop a suitable MAC protocol to improve efficiency of multicast communication. In this work we explore some approaches for reliable multicast at the MAC layer. We develop a multicast extension of IEEE 802.11 protocol and evaluate its performance. We have implemented our protocol in the popular ns-2 simulator and have performed experiments with multicast routing protocol. Our approach demonstrates superior performance in terms of packet delivery fraction as well as delay compared to the IEEE 802.11 protocol

On estimating joint interference for concurrent packet transmissions in low power wireless networks

In a wireless network it is important to understand the nature of the joint interference generated at a receiver by multiple concurrent transmitters. This understanding helps developing packet scheduling algorithms. Prior experimental work using older generation mote-class radios (CC1000) have showed systematic deviations between estimation and direct measurement of the joint interference power, thus questioning whether the standard assumption that received signal powers are additive is applicable in practice. We, however, show via extensive experimentation that on newer generation radios (CC2420), the additive assumption is valid, particularly at the low power end.

An experimental study of the Cache-and-Forward network architecture in multi-hop wireless scenarios

The Cache-and-Forward (CNF) protocol architecture was proposed to support efficient mobile content delivery services in the future Internet. In contrast to the TCP/IP protocol stack which is based upon the assumption of reliable end-to-end path through the network, the CNF architecture considers varying access link speed/quality and periods of disconnection as inherent properties of the network. Routers in a CNF network are built with large memory space for in-network caching and temporary storage to support transient disconnections due to mobility or link quality variation. Content delivery through the network follows a hop-by-hop transport method in which files move as single entities from one router to the next rather than as end-to-end packet streams. A novel storage aware routing protocol (STAR) is proposed to efficiently support mobile and wireless end-users through the use of a two-dimensional metric that takes into account both short-term and long-term path quality in making forwarding and storage decisions. A reliable link layer provides per hop file transfer reliability. This paper provides an outline of the three basic protocol components of CNF i.e., transport, routing and link layers and describes a proof-of-concept implementation of the protocol stack on the ORBIT testbed. Performance evaluation results in multi-hop wireless scenarios with lossy link conditions show 66% improvement in wireless network throughput compared to TCP and 60% lower packet loss rate when compared to UDP.

Experimental Evaluation of OpenVZ from a Testbed Deployment Perspective

A scalable approach to building large scale experimentation testbeds involves multiplexing the system resources for better utilization. Virtualization provides a convenient means of sharing testbed resources among experimenters. The degree of programmability and isolation achieved with such a setup is largely dependent on the type of technology used for virtualization. We consider OpenVZ and User Mode Linux (UML) for virtualization of the ORBIT wireless testbed and evaluate their relative merit. Our results show that OpenVZ, an operating system level virtualization mechanism significantly outperforms UML in terms of system overheads and performance isolation. We discuss both qualitative and quantitative performance features which could serve as guidelines for selection of a virtualization scheme for similar testbeds.

Network caching strategies for intermittently connected mobile users

This paper presents an evaluation of in-network caching strategies for efficient delivery of content to mobile devices that are intermittently connected to the network. Placement of content into in-network caches is formulated as an optimization problem that minimizes access latency under certain cost constraints. Several heuristic solutions (longest lifetime, split & longest lifetime and proportional probability to lifetime) are investigated via numerical examples and simulations. The results show that the proposed methods offer significant performance improvement over random caching and can approach the performance of exhaustive caching at every node with reduced storage cost.

Design of link and routing protocols for Cache-and-Forward networks

Cache-and-Forward (CNF) is a future Internet architecture designed for content delivery to mobile users over wireless networks with varying link quality and intermittent connectivity. The CNF protocol is based on strict hop-by-hop transport of media files with in-network storage at each router or wireless access point. The protocol also incorporates content caching capabilities for efficient delivery of popular media files. In this paper, we briefly describe the CNF architecture, present a survey of prior work, and describe new CNF link and routing protocols. Throughput results comparing CNF with TCP/IP are summarized for an example wide-area Internet scenario with wireless access networks. The design of a reliable CNF link layer protocol is discussed and performance results are given for multihop wireless scenarios. The paper concludes with an outline of dynamic CNF routing algorithms which consider both short-term and long-term path quality along with available in-network storage.

Distributed Protocols for Scheduling and Rate Control to Achieve Max-Min Fairness in Wireless Mesh Networks

The goal in this paper is to develop comprehensive protocol support in all layers to provide max-min fairness for multihop flows in a wireless mesh network. Our approach has three parts. First, we estimate the max-min fair rate of all multihop flows in the network using a distributed protocol. This estimation uses the knowledge of the flow contention graph that the network nodes learn by exchanging local information. Second, the nodes enforce this rate by controlling the rate at which a flow is scheduled to the link layer. Third, a back pressure flow control is used to reduce the transmission rate of a flow if it has been exceeding its fair rate. Finally, we argue that the fair rate estimation can at best be approximated in an 802.11 based MAC protocol. Thus, to complement our fair rate estimation and scheduling procedures, we develop a virtual time based MAC protocol. We demonstrate via extensive simulations the benefit of all these approaches for ensuring fairness relative to the base case that uses 802.11 MAC and FIFO scheduling.

STAR: Storage aware routing protocol for generalized Delay Tolerant Networks

This paper presents a novel storage aware routing (STAR) protocol designed to provide a general networking solution over a broad range of mobile and wireless scenarios. STAR enables routing policies which adapt seamlessly from a well-connected wired network to a disconnected wireless network using a 2-Dl routing metric composed of a short and a long term route cost and storage availability on downstream routers. Temporary in-network storage is preferred over forwarding along a path that is slower than average and opportunistic transmission is encouraged when a faster route becomes available. Results from ns2 based simulations show that STAR achieves 20% higher throughput compared to OLSR under varying link conditions and delivers 20% more files in DTN scenarios. Experimental evaluation of STAR on the ORBIT testbed demonstrates significant performance improvements with 25% higher peak throughput compared to OLSR in a wireless mesh network.