Span

1
Span
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Goals

• Minimize energy consumption
• Wireless interface is largest power drain
• Maximize OFF time
• Minimize end-to-end delay
• No centralized controller
• Capacity should not be diminished
• Generalizable to different link layers
• Interoperable with routing system

3
Span Design

• Randomized,distributed algorithm
• Nodes decide when to
• Sleep
• Receive packets (idle)
• Forward packets (coordinate)
• Forms backbone connected dominating set of
  coordinators
• Geographic routing
• Periodic HELLO messages
• Nodes status, coordinators, neighbors
• Operates under the routing layer, and above the
  MAC and physical layers
• Affects routing process
• Utilizes link-layer power saving features of MAC

4
Coordinator announcement

• Algorithm
• If 2 neighbors cant reach each other
• Then
• calculate randomized backoff delay
• Delay
• If no one else has become a coordinator, become
  coordinator
• Sensitivity
• Number of nodes that could become connected
• Current energy level
• Round trip delay over wireless link

5
Coordinator withdrawal

• If
• all pairs of neighbors are inter-reachable
• OR have been a coordinator for a long time
• Then
• Mark self as tentative
• Delay
• Withdraw if another node announces
• Sensitivity
• Energy level
• Round trip delay

6
Integration with 802.11

• Power Saving Mode
• Beacons synchronize nodes
• Traffic can be advertised during ATIM window
• Spans Enhancements
• No advertisements between coordinators
• Packets routed via non-coordinator nodes during
  the Advertised Traffic Window

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Results Capacity Preservation

• Slight increase in latency over 802.11
• Large decrease in loss over 802.11 PSM
• ATIM window in 802.11 PSM not big enough
• 802.11 without PSM has lower loss
• Span uses a smaller ATIM window
• Span decreases utilization due to backoff
• Span reduces number of voids in geographic
  routing
• Coordinators are unlikely to occur at edge of void

8
Results Coordinator Election

• Compared to hexagonal grid layout
• Span elects more nodes than this
• Variation of node density
• Energy constraints forces rotation

9
Results Energy Consumption

• Span saves considerable energy over 802.11 PSM
• 802.11 PSM loses efficiency due to broadcasts in
  geographic forwarding
• Energy savings increase sublinearly with the node
  density
• Node lifetime increases by about 2x