1 Span

2 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 Node’s 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 can’t 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 Power Saving Mode Beacons synchronize nodes Traffic can be advertised during ATIM window Span’s Enhancements: No advertisements between coordinators Packets routed via non-coordinator nodes during the Advertised Traffic Window *

7 Results: Capacity Preservation Slight increase in latency over Large decrease in loss over PSM* ATIM window in PSM not big enough 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 PSM PSM loses efficiency due to broadcasts in geographic forwarding Energy savings increase sublinearly with the node density* Node lifetime increases by about 2x