Person:

Zegura, Ellen W.

Permanent Link

https://hdl.handle.net/1853/71879

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Organizational Unit

School of Computer Science

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Now showing 1 - 10 of 10

Message Ferries as Generalized Dominating Sets in Intermittently Connected Mobile Networks

(Georgia Institute of Technology, 2009) Ammar, Mostafa H. ; Polat, Bahadir K. ; Sachdeva, Pushkar ; Zegura, Ellen W.

Message ferrying is a technique for routing data in wireless and mobile networks in which one or more mobile nodes are tasked with storing and carrying data between sources and destinations. To achieve connectivity between all nodes, message ferries may need to relay data to each other. While useful as a routing technique for wireless mobile networks in general, message ferrying is particularly useful in intermittently connected networks where traditional MANET routing protocols are not usable. A wireless and mobile network is said to possess intrinsic message ferrying capability if a subset of the nodes can act as message ferries by virtue of their own mobility pattern, without introducing additional nodes or modifying existing node mobility. Our goal in this work is to provide a formalism by which one can characterize intrinsic message ferrying capability. We first observe that the use of message ferries is the mobile generalization of the well-known use of connected dominating set-based routing in wireless networks. We next consider the problem of identifying the set of nodes in a mobile network which can act as message ferries by virtue of their mobility pattern. To this end, we define the concept of a connected message ferry dominating set (CMFDS) in a manner that achieves data delivery within certain performance bounds. We then develop algorithms that can be used to find such a set within a mobile, wireless network. The general CMFDS algorithm is built around a core algorithm that determines whether a single node in the network can act as a ferry. We provide some illustrative examples to show the application of our algorithm to several mobility patterns.
Multicasting in Delay Tolerant Networks: Semantic Models and Routing Algorithms

(Georgia Institute of Technology, 2006) Zhao, Wenrui ; Ammar, Mostafa H. ; Zegura, Ellen W.

Delay tolerant networks (DTNs) are a class of emerging networks that experience frequent and long-duration partitions. These networks have a variety of applications in situations such as crisis environments and deep-space communication. In this paper, we study the problem of multicasting in DTNs. Multicast supports the distribution of data to a group of users, a service needed for many potential DTN applications. While multicasting in the Internet and mobile ad hoc networks has been studied extensively, due to the unique characteristic of frequent partitioning in DTNs, multicasting in DTNs is a considerably different and challenging problem. It not only requires new definitions of multicast semantics but also brings new issues to the design of routing algorithms. In this paper, we propose new semantic models for DTN multicast and develop several multicast routing algorithms with different routing strategies. We present a framework to evaluate these algorithms in DTNs. To the best of our knowledge, this is the first study of multicasting in DTNs. Our objectives are to understand how routing performance is affected by the availability of knowledge about network topology and group membership and to guide the design of DTN routing protocols. Using ns simulations, we find that efficient multicast routing for DTNs can be constructed using only partial knowledge. In addition, accurate topology information is generally more important in routing than up-to-date membership information. We also find that routing algorithms that forward data along multiple paths achieve better delivery ratios, especially when available knowledge is limited.
Hierarchical Power Management in Disruption Tolerant Networks with Traffic-Aware Optimization

(Georgia Institute of Technology, 2006) Jun, Hyewon ; Ammar, Mostafa H. ; Corner, Mark D. ; Zegura, Ellen W.

Disruption tolerant networks (DTNs) are wireless mobile networks that are characterized by frequent partitions and long delays. Such networks can be used in highly-challenged environment in which energy resources are limited. Efficient power management, therefore, is essential for their success. In this paper, we present a hierarchical power management in DTNs where nodes are equipped with two complementary radios: a long-range, high power radio and a short range, low-power radio. In this architecture, energy can be conserved by using the low-power radio to discover communication opportunities with other nodes and then wake up the high-power radio to undertake the data transmission. We develop a generalized power management framework and its variations around this idea and evaluate their relative performance. In addition, for the case in which traffic load can be predicted, we devise approximation algorithms to control the sleep/wake-up cycling to provide maximum energy conservation while discovering enough communication opportunities to handle a given traffic load. We evaluate our schemes and our choice of parameters through ns-2 simulations. Our simulation results show that the generalized power management mechanism could augment the usefulness of the low power radio and achieve better energy efficiency than mechanisms relying on one radio for discovery. In addition, our approximation algorithms reduce energy consumption from 73% to 93% compared with the case without power management. We also observe that while an additional low power radio does reduce the energy consumption needed for discovery, the improvement could be negligible in mobile DTNs due to the low density of nodes.
Capacity Enhancement Using Throw-Boxes in Mobile Delay Tolerant Networks

(Georgia Institute of Technology, 2006) Zhao, Wenrui ; Chen, Yang ; Ammar, Mostafa H. ; Corner, Mark D. ; Levine, Brian ; Zegura, Ellen W.

Delay tolerant networks (DTNs) are a class of emerging networks that are subject to frequent and long-duration partitions. Due to intermittent connectivity, DTNs might be significantly limited in supporting application needs, for example, leading to low throughput or high delay. To address this problem, we propose the use of throw-boxes to improve data delivery performance. Throw-boxes are small, inexpensive devices equipped with wireless interfaces and deployed to relay data between mobile nodes. Being small and inexpensive, throwboxes represent a flexible and cost-effective approach to enhance network capacity. In this paper, we systematically study two inter-related issues, namely deployment and routing, in using throw-boxes for throughput enhancement. Specifically, we develop algorithms for throw-box deployment and data forwarding under various routing strategies, including single path, multi-path and epidemic routing. Using extensive ns simulations, we evaluate the utility of throw-boxes and the impact of various routing and deployment strategies on network performance. Our objective is to guide the design and operations of throw-box-enhanced DTNs. We find that throw-boxes are very effective in improving both data delivery ratio and delay, especially for multi-path routing and environments with regular node movement.
Trading Latency for Energy in Wireless Ad Hoc Networks using Message Ferrying

(Georgia Institute of Technology, 2004) Zhao, Wenrui ; Ammar, Mostafa H. ; Zegura, Ellen W. ; Lee, Chungki ; Jun, Hyewon

Power management is a critical issue in wireless ad hoc networks where the energy supply is limited. In this paper, we investigate a routing paradigm, Message Ferrying (MF), to save energy while trading off data delivery delay. In MF, special nodes called ferries move around the deployment area to deliver messages for nodes. The reliance on the movement of ferries to deliver data increases the delivery delay. However, nodes can save energy by disabling their radios when ferries are far away. To exploit this feature, we present a power management framework, in which nodes switch their power management modes based on the knowledge of ferry location. We evaluate the performance of our scheme using ns-2 simulations and compare it with Dynamic Source Routing (DSR). Our simulation results show that MF achieves energy savings as high as 95% compared to DSR without power management and still delivers more than 98% of data. In contrast, a power-managed DSR delivers much less data than MF to achieve similar energy savings. In the scenario of heavy traffic load, the power-managed DSR delivers less than 20% of data. MF also shows robust performance for highly mobile nodes, while the performance of DSR suffers significantly. Thus, delay tolerant applications should use MF rather than a multihop routing protocol to save energy efficiently when both routing approaches are available.
The Energy-Limited Capacity of Wireless Networks

(Georgia Institute of Technology, 2004) Ammar, Mostafa H. ; Zegura, Ellen W. ; Zhao, Wenrui

The performance of large-scale wireless ad hoc networks is often limited by the broadcasting nature of the wireless medium and the inherent node energy constraints. While the impact of the former on network capacity has been studied extensively in the literature, the impact of energy constraints has not received as much attention. In this paper, we study the capacity limitations resulting from the energy supplies in wireless nodes. We define the energy-limited capacity of a wireless network as the maximum amount of data the network can deliver before the nodes run out of energy. This energy-limited capacity is an important parameter in networks where operating lifetime is critical, such as ad hoc networks deployed in hazardous environments and sensor networks. We study two types of static networks, networks without any infrastructure support and networks where base stations with unlimited energy are deployed to support data forwarding. We consider two kinds of traffic models motivated by ad hoc networks and sensor networks. We derive upper and lower bounds on the energy-limited capacity of these networks. While throughput has been shown to not scale with node density in static networks by previous studies, our results show that, depending on the energy consumption characteristics of wireless communication, the energy-limited capacity can scale well under both traffic models. In addition, we show that the deployment of base stations can improve the energy-limited capacity of the network, especially for networks with sensor traffic.
Routing in Space and Time in Networks with Predictable Mobility

(Georgia Institute of Technology, 2004) Ammar, Mostafa H. ; Zegura, Ellen W. ; Merugu, Shashidhar

We consider the problem of routing in emerging wireless networks where nodes move around explicitly carrying messages to facilitate communication in an otherwise partitioned network. The absence of a path at any instant of time between a source and destination makes the traditional mobile ad hoc routing protocols unsuitable for these networks. However, the explicit node movements create paths over time that include the possibility of a node carrying a message before forwarding to another suitable node. Identifying such paths over space and time is a key challenge in these store, carry and forward networks. In most of these networks, the mobility of nodes is predictable either over a finite time horizon or indefinitely due to periodicity in node motion. We propose a new space-time routing framework for these networks leveraging the predictability in node motion. Specifically, we construct space-time routing tables where the next hop node is selected from the current as well as the future neighbors. Unlike traditional routing tables, our space-time routing tables use both the destination and the arrival time of message to determine the next hop node. We devise an algorithm to compute these space-time routing tables to minimize the end-to-end message delivery delay. Our routing algorithm is based on a space-time graph model derived from the mobility of nodes. We empirically evaluate our approach using simulations and observe improved performance as compared to other approaches based on heuristics.
Bootstrapping in Gnutella: A Preliminary Measurement Study

(Georgia Institute of Technology, 2003) Ammar, Mostafa H. ; Dhamdhere, Amogh Dhananjay ; Raj, Himanshu ; Riley, George F. ; Zegura, Ellen W. ; Karbhari, Pradnya

To join an unstructured peer-to-peer network like Gnutella, peers have to execute a bootstrapping function in which they discover other on-line peers and connect to them. Until this bootstrapping step is complete, a peer cannot participate in file sharing activities. Once bootstrapping is complete, a peer’s experience is strongly influenced by the choice of neighbor peers resulting from the bootstrapping step. Despite its importance, there has been very little attention devoted to understanding the behavior of this bootstrapping function. In this paper, we study the bootstrapping process of a peer in the Gnutella network. This is a preliminary investigation, consisting of 1) an analysis and performance comparison of bootstrapping algorithms of four Gnutella servent implementations, 2) a measurement-based characterization of the global Gnutella Web Caching System (GWebCaches), a primary component of the current bootstrapping functions, and 3) a study of the behavior and experience of a single GWebCache that was setup locally and made part of the global caching infrastructure. Our study highlights the importance of understanding the performance of the bootstrapping function as an integral part of a peer-to-peer system. We find that 1) there is considerable variation among various servent implementations that correlates to their bootstrapping performance, 2) even though the GWebCache system is designed to operate as a truly distributed system in keeping with the peer-to-peer system philosophy, it actually operates more like a centralized infrastructure function, and 3) the GWebCache system is subject to misreporting of peer and cache availability due to stale data and absence of validity checks.
Selecting Among Replicated Adaptive Multicast Servers

(Georgia Institute of Technology, 2000) Ammar, Mostafa H. ; Zegura, Ellen W. ; Fei, Zongming

Server replication and multicasting are well-established techniques for increasing capacity of a networked service and improving client performance. In this paper, we consider the combination of these two techniques. Specifically, we investigate the problem of selecting amongst rate-adaptive multicast servers, which adjust their sending rate based on network conditions and/or feedback from clients. Effective server rate adaptation can lead to efficient utilization of network resources and performance improvement perceived by clients. In this initial study of adaptive multicast server selection, we explore some fundamental issues and study the implications of different selection strategies on the performance perceived by clients. We first define the Static Multicast Selection Problem, in which there are static sets of clients and servers, and one needs to establish a set of multicast trees with one tree for each server. We explore several optimization problems based on different performance measures. We prove that the general problem is NP-hard and then present two interesting special cases with an optimal polynomial-time solution in each case. We design a heuristic for the general case and show that it can improve the performance over some simple strategies. We also consider the Dynamic Multicast Selection Problem, in which clients may join and leave multicast trees already established. We design a heuristic for this dynamic case by which clients can select a tree to join. We investigate the performance of the heuristic through simulation.
Efficient Routing using NIx-Vectors

(Georgia Institute of Technology, 2000) Ammar, Mostafa H. ; Zegura, Ellen W. ; Riley, George F.

We introduce the concept if NIx-Vector routing which gives efficient and consistent routing of packets in Internet routers. A NIx-Vector is a compact representation of a routing path, which is small enough to be included in a packet header. We show how, by including the routing information in the packet header, routing decisions can be made in O(1) time at each router, with no caching or state in the routers (other than the existing routing tables). The creation of a NIx-Vector for a source and a destination pair requires one round trip time, but once the NIx-Vector is known, it can be retained by the source and reused indefinitely with no further setup time required (or until it becomes no longer valid, which is easily detected.) In addition to O(1) routing decisions, the use of NIx-Vectors to record and specify routes has other benefits. NIx-Vectors provide for route pinning, which is beneficial for reservation protocols and mitigates the effect of routing flaps on long lived flows. A variation of NIx-Vectors can insure symmetrical routes from a source to a destination and back to the source, which is also beneficial to some reservation protocols. Another variation of NIx-Vectors can insure that all packets from the same flow take the same path, even in the presence of redundant paths or links, which will reduce the frequency of pack misordering. Finally, the NIx-Vector concept can be combined with probabilistic packet market to allow for the reconstruction of a complete routing path for packets of unknown origin, which is useful in determining the source of denial of service security attacks. We give a complete description of how NIx-Vectors are created and used, and describe an end-to-end protocol for managing NIx-Vectors. We give empirical data showing the number of bits required to represent NIx-Vectors for typical Internet paths. We give a proposed IPv4 option which will allow for a simple implementation.