Elsevier

Computer Networks

Volume 51, Issue 15, 24 October 2007, Pages 4338-4355
Computer Networks

Association management for data dissemination over wireless mesh networks

https://doi.org/10.1016/j.comnet.2007.05.009Get rights and content

Abstract

To enable multimedia broadcasting services in mesh networks, it is critical to optimize the broadcast traffic load. Traditionally, users associate with access points (APs) with the strongest signal strength. We explore the concept of dual-association, where the AP for unicast traffic and the AP for broadcast traffic are independently chosen by exploiting overlapping coverages that are typical in mesh networks. The goal of our proposed solution is to optimize the overall network load by exploiting the flexibility provided by independent selection of unicast and broadcast APs. We propose a novel cost metric based on ETT (Expected Transmission Time) and the number of nodes in range of the APs, that are advertised in the beacons from the APs. Users periodically scan and associate with the AP which has the lowest cost metric. The proposed approach reduces the number of APs that handle the broadcast traffic resulting in a heavy reduction in control and data packet overhead. This leads to higher packet delivery rate and enhanced video quality measured in terms of PSNR. Our approach allows the freed up resources at APs to increase the unicast throughput. We compare the performance of our approach with traditional signal strength based association using extensive simulations and real experiments on an indoor testbed of 180 IEEE 802.11b based devices.

Introduction

Mesh Networking is emerging as a promising technology that brings Wireless LANs to the masses at a reduced deployment cost. Mesh networks are either flat or hierarchical in terms of the architecture [1], [2]. Our study is focused on hierarchical architectures, where the client–client communication is always via the APs (access points) and the APs are connected through a multi-hop wireless backbone. Public deployments of mesh networks are already operational in several cities including Philadelphia, Las Vegas, and Urbana-Champaign. Various types of WLANs, such as city-wide WLANs1, in-building WLANs, and temporary WLANs, can all benefit from the Mesh network technology.

While unicast services are essential for providing Internet access to individual users, emerging broadcast services are needed to deliver local news, visitor’s information, TV channels, or other multimedia content. In order to support efficient multimedia services with minimal impact to unicast services, it is critical to optimize the multicast load on the network.

In this paper we study the problem of optimizing the broadcast traffic load in the mesh using the concept of dual-association, where users maintain distinct associations for unicast and broadcast traffic. We use the AP with the best signal strength for unicast traffic, but other metrics such as unicast traffic load [3] can also be used to select the unicast AP. For reducing the broadcast traffic load, users select the AP for broadcast services independent of the AP selected for unicast traffic. The selected broadcast APs can be connected to the AP with the backbone access (Main AP or MAP) using any ad-hoc multicast routing protocol. As the multicast structure construction is not the focus of the paper, we choose to connect the selected APs to the MAP using a tree, only for purposes of simplicity. The tree construction and maintenance mechanisms are based on MAODV [4], but any other multicast routing protocol can be used as well. The concept of dual-association was first introduced in our prior work [5] where a simple multicast metric was also proposed. In contrast, this paper proposes a cost metric that captures the global cost on the network (in addition to the cost on the last hop between the user and the AP), and an efficient local synchronization algorithm to make it practicable to implement association based on the cost-metric. Moreover, in this paper we present data from real experiments and extensive simulations.

Our contributions in this paper are as follows. (1) We formalize the problem of efficient association for data dissemination over Mesh networks. (2) We prove that the problem is NP-hard by showing a reduction from the Steiner tree problem. (3) We propose the dual-association concept and a local synchronization method for ease of deployment. (4) We introduce a novel metric that optimizes the broadcast traffic load in the mesh and present a heuristic based distributed protocol, called COST, based on our metric. (5) Using simulations in ns2 we evaluate the performance of our approach and compare it with the traditional approach that uses signal strength based association. The key metrics studied are the size of the tree, and the quality of received MPEG video measured using PSNR (Peak Signal-to-Noise Ratio). (6) We have implemented the distributed approach and compared its performance with the traditional signal strength based approach, on an indoor testbed of 180 nodes with 802.11b radio.

The rest of the paper is organized as follows. Section 2 summarizes related work. Section 3 defines the problem, the notations, and the terminology used in the paper. Dual-AP management framework is described in Section 4. The distributed solution is presented in Section 5. Section 6 presents a detailed evaluation of our approach and comparison with signal strength based approach using simulations. The results from the testbed experiments are presented in Section 7. Finally, Section 8 concludes the paper.

Section snippets

Related work

In this section, we outline related work in the areas of mesh networking, controlled association in 802.11 networks, and sub-structure computation in ad-hoc and mesh networks.

Mesh networking: Providing connectivity to large communities using wireless back-haul networks, also known as mesh networks, has lately received an increased attention [6], [7]. Several companies including Mesh networks, Firetide, Strix, and BelAir Networks have various commercial products and large-scale public

Terminology and problem definition

We represent the connectivity between the users and the access points using a graph G = (V, E), where V (same as V(G)) is the set of nodes (users and access points) and E (same as E(G)) is the set of edges. E consists of edges connecting users to access points in range, and between access points that are in range of each other. E does not include user-to-user edges as we do not consider ad-hoc communication between the users. V can be partitioned into the set of users, Vu, and the set of access

AP management framework

We propose an AP management framework for simultaneous support of higher quality broadcast and unicast services. The framework is characterized by Dual-AP association, Dual-traffic cycles, and Synchronized AP switching.

  • Dual-AP association: Users requesting broadcast services maintain two independent associations with APs: one for unicast (unicast-AP) and the other for broadcast (broadcast-AP). In this paper the strongest signal strength is used for electing the unicast-AP. Other techniques such

Distributed association management

In this section, we present a distributed solution, called COST, for computing the broadcast association to optimize the broadcast load. We propose a metric that is advertised in the beacons from the APs. The AP with the lowest metric is selected as the broadcast-AP, in case the unicast-AP is not involved in broadcasting. If the unicast-AP is also involved in broadcasting, it is selected for both traffics. The following design guidelines form the basis of the solution:

  • Associate with an AP with

Simulations

We present performance evaluation of the proposed cost metric based distributed algorithm (COST) using simulations on Network Simulator ns-2 [26]. The default ns-2 channel model has been used in the simulations (two-ray ground propagation model). Packet losses may happen due to collisions and capture effects. The simulation parameters are summarized in Table 2. We use a grid topology of 10 × 10 APs. The distance between neighboring APs is 200 m and radio propagation range of AP is 250 m, unless

Testbed evaluation

In this section we present the results obtained by implementing and testing our protocol on Kansei [27], [28], which is an indoor testbed of 180 Stargate nodes. The Stargate (sold by Crossbow Inc.) is a 32-bit hardware platform running Linux, which has a PCMCIA wireless interface and an Ethernet interface. The nodes are arranged in a 15 × 12 grid, with an inter-node separation of about 3 ft. An IEEE 802.11b card is used in each node for testing our protocols. The Ethernet interface of all the

Conclusion

In this paper we studied a novel technique for association that reduces the load for broadcast traffic in mesh networks. We propose the concept of dual-association, where the AP for unicast traffic and the AP for broadcast traffic are independently chosen by exploiting multiple coverages that are typical in mesh networks. We propose a cost metric based on ETT and the number of nodes in range of the APs, that is advertised in the beacons from the APs. Users periodically scan and associate with

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grants CNS-0546630 (CAREER Award) and CNS-0403342, and IT Scholarship Program supervised by IITA (Institute for Information Technology Advancement) and MIC (Ministry of Information and Communication), Republic of Korea. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Dongwook Lee is currently a Senior Engineer at Samsung Electronics Company, Korea.

He was a post-doctoral researcher at Ohio State University from August 2004 to January 2007. He received his B.S. degree in Computer Engineering from Chungbuk National University, Chungju, Korea (1997), and M.S. (1999) and Ph.D. (2004) degrees in Information and Communication from Gwang-Ju Institute of Science and Technology (G-JIST), Gwangju, Korea. His research interests include Wireless Mesh networks, Wireless

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    Dongwook Lee is currently a Senior Engineer at Samsung Electronics Company, Korea.

    He was a post-doctoral researcher at Ohio State University from August 2004 to January 2007. He received his B.S. degree in Computer Engineering from Chungbuk National University, Chungju, Korea (1997), and M.S. (1999) and Ph.D. (2004) degrees in Information and Communication from Gwang-Ju Institute of Science and Technology (G-JIST), Gwangju, Korea. His research interests include Wireless Mesh networks, Wireless Sensor networks, multimedia delivery in mobile networks, network-adaptive media delivery, wired/wireless multicast, and end-to-end QoS provisioning. He received the Best Paper Award at APCC in 2001 and the Best Paper Award at JCCI in 2003.

    Gayathri Chandrasekaran is a Ph.D. Student in Computer Science at Rutgers University, New Brunswick. She works at the Wireless Information Networking Lab (Winlab). Her research interests include Experimental Evaluation of Networking protocols, Location Based Systems and Mesh Networks. She received her M.Sc.(Tech.), Information Systems from BITS (Pilani), India. She is a recipient of the University Fellowship at the Ohio State University and was a graduate student at Ohio State University between 2004 and 2006.

    Mukundan Sridharan is currently a research assistant for the Dependable Distributed and Networked Systems Lab and is pursuing his Ph.D. degree in The Ohio State University under the guidance of Prof. Anish Arora. He received his Bachelor of Engineering degree from the University of Madras in June 2000 and M.S. from The Ohio State University in June 2003. His research interest includes wireless and sensor networks, Internet measurements, video conferencing and congestion control. He has worked previously as a research assistant for the Network Measurement and Monitoring group of Oarnet, OSC, Ohio.

    Prasun Sinha received his Ph.D. from University of Illinois, Urbana-Champaign in 2001, MS from Michigan State University in 1997, and B.Tech. from IIT Delhi in 1995. He worked at Bell Labs, Lucent Technologies as a Member of Technical Staff from 2001 to 2003. Since 2003 he is an Assistant Professor in Department of Computer Science and Engineering at Ohio State University. His research focuses on design of network protocols for sensor networks and mesh networks. He served on the program committees of various conferences including INFOCOM (2004–2007) and MOBICOM (2004–2005). He has won several awards including Ray Ozzie Fellowship (UIUC, 2000), Mavis Memorial Scholarship (UIUC, 1999), and Distinguished Academic Achievement Award (MSU, 1997). He received the prestigious NSF CAREER award in 2006.

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