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Paper On An Energy-Efficient Coordination Method for Ad Hoc Wireless Networks

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An Energy-Efficient Coordination Method for Ad Hoc Wireless Networks

Abstract:

This paper presents a power saving technique for multi-hop ad hoc wireless networks that reduces energy consumption without significantly diminishing the capacity or connectivity of the network. It builds on the observation that when a region of a shared-channel wireless network has a sufficient density of nodes, only a small number of them need be on at any time to forward traffic for active connections.

The technique is a distributed, randomized algorithm where nodes make local decisions on whether or sleep, or to join a forwarding backbone as a coordinator. Each node bases its decision on an estimate of how many of its neighbors will benefit from it being awake and the amount of energy available to it. We give a randomized algorithm where coordinators rotate with time, demonstrating how localized node decisions lead to a connected, capacity-preserving global topology.

Introduction:

Minimizing energy consumption is an important challenge in mobile networking. Significant progress has been made on low-power hardware design for mobile devices that the wireless network interface is often a device’s single largest consumer of power. Since the network interface may often be idle, turning the radio off when not in use could save this power. In practice, however, this approach is not straightforward: a node must arrange to turn its radio on not just to receive packets addressed to it, but also to participate in any higher-level routing and control protocols. The requirement of cooperation between power saving and routing protocols is particularly acute in the case of multi-hop ad hoc wireless networks, where nodes must forward packets for each other. Coordination of power saving with routing in ad hoc wireless networks is the subject of this paper.

A good power-saving coordination technique for wireless ad-hoc networks ought to have the following characteristics.

  • It should allow as many nodes as possible to turn their radio receivers off mot of the time, since even an idle receive circuit can consume almost as much energy as an active transmitter.

  • On the other hand, it should forward packets between any source and destination with minimally more delays than if all nodes were awake. This implies that enough nodes must stay awake to form a connected backbone.

  • Furthermore, the backbone formed by the awake nodes should provide about as much total capacity as the original network, since otherwise congestion may increase. This means that paths that could operate without interference in the original network should be represented in the backbone.

Each node in the network makes periodic, local decisions on whether to sleep or stay awake as a coordinator and participate in the forwarding backbone topology. To preserve capacity, a node decides to volunteer to be a coordinator if it discovers that two of its neighbors cannot communicate with each other directly or through an existing coordinator. To keep the number of redundant coordinators low and rotate this role amongst all nodes, each node delays announcing its willingness with a random delay that takes two factors into account: the amount of remaining battery energy, and the number of pairs of neighbors it can connect together. This combination ensures, with high probability, a capacity-preserving connected backbone at any pint in time, where nodes tend to consume energy at about the same rate. The power saving technique does all this using only local information, consequently scaling well with the number of nodes.

Design:

The network adaptively elects “Coordinators” from all nodes and the coordinators stay awake continuously and perform multi-hop packet routing within the ad hoc network, while other nodes remain in power-saving mode and periodically check if they should wake up and become a coordinator.

It can be achieved by the four ways[1].
* It ensures that enough coordinators are elected so that every node is in radio range of at least one coordinator.
* It rotates the coordinators in order to ensure that all nodes share the task of providing global connectivity roughly equally.
* It attempts to minimize the number of nodes elected as coordinators, thereby increasing network lifetime, but without suffering a significant loss of capacity or an increase in latency.
* Finally, It elects coordinators using only local information in decentralized manner-each node only consults state stored in local routing tables during the election process.
Coordinator Announcement


Periodically, a non-coordinator node determines if it should become a coordinator or not. The following coordinator eligibility rule in Span ensures that the entire network is covered with enough coordinators:

 

Coordinator eligibility rule: if two neighbors of a non-coordinator node cannot reach each other either directly or via one or two coordinators, the node should become a coordinator.

While this election algorithm does not yield the minimum number of coordinators required to merely maintain connectedness, it forms a network that roughly contains a coordinator in every populated radio range in the entire network topology. Since packets will be routed through coordinators, this topology ought to yield good capacity.

Announcement contention occurs where multiple nodes discover the lack of a coordinator at the same time, and all decide to become a coordinator. We resolve contention by delaying coordinator announcements with a randomized back-off delay. Each node chooses a delay value, and delays the HELLO message that announces the node’s volunteering as a coordinator for that amount of time. If at the end of the delay, the node has not received any HELLO messages from other potential coordinators, it sends the HELLO message. Otherwise, it reevaluates its eligibility based on any HELLO messages received, and makes its announcement if and only if the eligibility rule still holds.


Coordinator Withdrawal:

Each coordinator periodically checks if it should withdraw as a coordinator. A node should withdraw if every pair of its neighbors can reach each other directly or via some other coordinators[3]. However, in order to also ensure fairness, after a node has been a coordinator for some period of time, it withdraws if every pair neighbor nodes can reach each other via some other neighbors, even if those neighbors are not currently coordinators. This rule gives other neighbors a chance to become coordinators.

To prevent temporary loss of connectivity between a coordinator’s withdrawal message and the announcement from a new coordinator, a node continues to serve as a coordinator for a short period of time even after announcing its withdrawal. This ‘grace period’ allows the routing protocol to continue to use the old coordinator until a new coordinator is elected.

Conclusion:

This paper presents a distributed coordination technique for multi-hop ad hoc networks that reduces energy consumption without significantly diminishing the capacity or connectivity of the network. In ad hoc network, coordinator is elected from all nodes and it rotates them in time. The coordinator may stay awake and perform multi-hop packet routing within the ad hoc network, while other nodes remain in power-saving mode and periodically check if they should awaken and become a coordinator.

Each node uses a random back off to decide whether to become a coordinator. This delay is a function of the number of other nodes in the neighborhood that can be bridged using this node, and the amount of energy it has remains the same. The implementation of the power saving technique periodically wakes up the nodes and makes to listen advertisements and increases the cost. This warrants investigation into a more robust and efficient power saving technique in MAC layer that minimizes the amount of time each node in power saving mode.

References: 1. Benjie chen, Kyle Jamieson, Hari Balakrishnan and Robert Morries. “Span: An energy-efficient coordination algorithm for topology maintenance in wireless networks”. Technical Report: Massachusetts Institute of Technology 8.5 (2002): 481-494.
2. Shepard. “Channel Access Scheme for Large Dense Packet Radio Networks”. Proceeding of ACM SIGCOMM. 1996 : 219-230.
3. C. Raghavendra, Singh, S. Pamas. “Power Aware Multi-Access Protocol with signaling for Ad Hoc Networks”. ACM Computer Communication Review (1988):5-26.

Contributing Writer  Dr. V. Mahesh has been teaching High Speed Networks, Multimedia Systems, Component Based Technologies, Data Mining, Artificial Intelligence and Management Information System for 11 Years. Working from 1999 - till date at Sathaybama University, Department of Computer Application, Jeppiaar Nagar, Chennai -600 119, Tamil Nadu, INDIA. He has published about 40 articles in various magazines like ‘The Hindu’, ‘The Information Technology’, etc and has presented and participated in about 20 various international and National conferences. He can be reached at freevenmah@gmail.com




 

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