Post on 18-Dec-2015
Span: An Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc Wireless Networks
ACM Wireless Networks Journal, 2002BENJIE CHEN, KYLE JAMIESON, HARI BALAKRISHNAN and ROBERT MORRIS
Outline
Introduction Related works Span design Simulation Conclusion
Introduction
Important Considerations: 1. It should allow as many nodes as possible to turn their radio
receivers off most of the time 2. it should forward packets between any source and destination
with minimally more delay than if all nodes were awake 3. the backbone formed by the awake nodes should provide
about as much total capacity as the original network A good coordination technique should not make many
assumptions about the link layer’s facilities for sleeping
Without 5 being coordinator, 3->4 contend for bandwidth with 1->2
A connected dominating set
Span
Making periodic and local decisions on whether to sleep or stay awake as a coordinator
Use a delay announcement to prevent announcement contention
Use a withdrawal scheme to rotate the role of being a coordinator
Related Works (1/2)
Das and Bharghavan [6] approximate the minimum connected dominating set of an ad hoc network Span has the additional property of being capacity preserving
Wu and Li [27] propose a distributed algorithm for approximating connected dominating sets in an ad hoc network that also appears to preserve capacity Span, however, elects fewer coordinators because it actively pre
vents redundant coordinators by using randomized slotting and damping
Related Works (2/2)
The recent GAF [29] scheme of Xu et al. Span does not require location information Span integrates with 802.11 power saving mode nicely
In AFECA [28], A node switches between sleeping and listening, with randomized sleep times proportional to the number of nearby nodes. The net effect is that the of listening nodes is roughly constant, regardless of node number density Span never keeps a node awake unless it is absolutely essential
for connecting two of its neighbors
Span design
Span is proactive: each node periodically broadcasts HELLO messages
From these HELLO messages, each node constructs a list of the node’s neighbors and coordinators and for each neighbor, a list of its neighbors and coordinators
Data Structure
HELLO: <Id>,<isCoordinator>,<list of coordinators>,<list of neighbors>,<timeStamp>
Table:<13>,<True>,<27>,<3,23>,<11934><25>,<False>,<2,23>,<3,45>,<11933><27>,< True >,<8,12>,<10,34>,<12001><43>,<False>,<13,56>,<3,34>,<11912>
Coordinator announcement
Coordinator eligibility rule: A non-coordinator node should become a
coordinator if it discovers, using only information gathered from local broadcast messages, that two of its neighbors cannot reach each other either directly or via one or two coordinators
Examples (1/2)
Examples (2/2)
High Complexity
Announcement contention
Announcement contention occurs when multiple nodes discover the lack of a coordinator at the same time, and all decide to become a coordinator
Span resolves contention by delaying coordinator announcements with a randomized back-off delay
Randomized back-off delay
number of additional pairs of nodes among these neighbors that would be connected if i were to become a coordinator
R uniformly at random from the interval (0, 1]
Number of neighborsRound trip time
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 either directly or via one or two other coordinators
Example
Tentative
Coordinator
Parameters
A coordinator stays tentative for WT amount of time, where
the amount of time a node stays as a coordinator before turning on its tentative-bit is proportional to the amount of energy it has (Er/Em)
Simulator implementation
Span implementation uses a geographic forwarding algorithm [1] They piggyback Span HELLO information onto the broadcast updat
es required by geographic forwarding Upon receiving a packet for a node not in radio range:
find a neighbor coordinator is closest to the destination If no such coordinator exists, find a non-coordinator that is closer to the
destination Did not resolve void like GPSR[16] Do not use a location service
Use GOD module of ns to obtain location (hence, the result is better)
[16] GPSR: Greedy Perimeter Stateless Routing for Wireless Networks (2005 3-17 presented by cwlin)
Implemented detail (1/3)
Span election algorithm may not react fast enough to elect new coordinators
Because geographic forwarding falls back to using non-coordinators to route packets if coordinators do not exist, a non-coordinator node announces itself as a coordinator if it has received a large number of packets to route in the recent past
If this coordinator turns out to be redundant, the coordinator withdraw algorithm will force the node to withdraw itself as a coordinator soon after
Implemented detail (2/3)
When the 802.11 MAC layer is asked to send a packet, it may or may not be able to send it immediately
In Span implementation, they buffer packets for two beacon periods. Packets that have not been transmitted after two beacon periods are dropped
Implemented detail (3/3)
The beacon period and ATIM window size greatly affect routing performance [21]
They experimentally determined that a beacon period of 200 ms and an ATIM window size of 40 ms result in good throughput and low loss rate
Energy model
They took measurements of the Cabletron Roama bout 802.11 DS High Rate network interface card (NIC) operating at 2 Mbps in base station mode
And note that these closely match the results obtained by Feeney and Nilsson [7] for similar 802.11 network interface cards in the ad hoc mode
Simulation Environment
Ns-2 network simulator using CMU wireless extensions
Run on top of the 802.11 MAC layer with power saving support and some modification
120 nodes in different size of square regions 2 Mbps bandwidth 250m nominal radio range Define node density: number of nodes within a
radio range (without sources or destinations)
10 Source
Random
Always on
Never move
10 Destination
Random
Always on
Never move
Only 100 node participate in Span
Motion follows random waypoint model [2]
128 bytes packets CBR flow
Capacity
Effects of mobility
Conclusion
Span presents a distributed coordination technique that reduces energy consumption without significantly diminishing the capacity or connectivity of the network
Span adaptively elects coordinators from all nodes in the network, and rotates them in time