1. SPAN: An Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc Wireless Networks Benjie Chen, Kyle Jamieson, Hari Balakrishnan, Robert Morris Mobile Computing and Networking 2002 Speaker : hsiwei

2. Outline Introduction SPAN’s algorithm Simulation Summary

3. Introduction Minimizing energy consumption is essential in sensor networks Sensor networks have high level of redundancy nodes A dense sensor network can work with only part of its nodes being active It possible to prolong the network lifetime while maintain its functionality by carefully choosing the active nodes

4. Introduction (cont’d) A good power-saving coordination technique should have the following characteristics – It should allow as many nodes as possible to turn their radio receivers off – Enough nodes must stay awake to form a connected backbone

5. SPAN This paper presents Span, a power saving technique for multi-hop ad hoc wireless network Nodes make local decisions on whether to sleep, or join a forwarding backbone as a coordinator Rotates the role of coordinator among nodes

6. The Essential Ideas of SPAN Part of the nodes become coordinators to form the network backbone, only coordinators can forward messages Non-Coordinator nodes check periodically whether it should become a coordinator A node with higher utility and energy level is more likely become a coordinator A coordinator checks periodically whether it should become a non-coordinator A coordinator may withdraw if it is redundant or it can find a replacement Nodes makes announcements when it’s role changes

7. How span works OnWakeUp() { if( ! all neighbors can reach each other directly or via one or two coordinators) { backoff if( no announcements from other neighbors received during the backoff) { become coordinator sent out HELLO annoucement } else { update state table { become coordinator sent out HELLO annoucement }

8. HELLO Announcement Each node perdically broadcasts HELLO message. HELLO message contains node’s status, its current coordinators, and its current neighbors Each node maintains its coordinators, neighbors, coordinators of neighbors 是否是 coordinator node

9. Coordinator Eligibility Rule If two neighbors of a non-coordinator node cannot reach each other directly or via one or two coordinators, the node should become a coordinator 1 3 5 67 2 4 1 3 5 67 2 4

10. Coordinator Contention 1 3 5 67 2 4 1 3 5 67 2 4 Try to be a coordinator at the same time Initial configuration 1 3 5 67 2 4 All the nodes are eligible 1 3 5 67 2 4 Boo Announcement Contention

11. Resolving announcement contention using backoff Each node delays its announcement by a certain value – Assume all the nodes have roughly equal energy – Only topology should play a role in deciding which nodes become coordinators number of neighbors for node i number of additional pairs among the neighbors that can be connected Number of neighbor pairs R:random form the interval 0-1.

12. Energy concern Nodes with more energy should volunteer as a coordinator more quickly The energy level is normalized using the max energy level, which is Er/Em

13. – E r : the amount of energy at a node that still remains – E m : the maximum amount of energy available at the same node – N max : max possible number of neighbors – C i : number of additional pairs of nodes among these neighbor that would be connected if i were to become a coordinator. 0 <= C i <= N i *(N i -1)/2 <= N max *(N max-1 )/2 – T: round-trip Time for a small packet over the wireless link Putting everything together

14. Coordinator Withdrawal Rules – Every pair of its neighbors can reach other either directly or via some other coordinators – 為了達到公平. 在當 coordinator node 一段 時間之後. 如果 – Every pair of its neighbors can reach other either directly or via some other neighbors, 即使這些鄰居不是 coordinator node, 也做休息. – Grace period:To prevent temborary loss of conncetivity.

15. Capacity preservation

16. Network Lifetime

17. Summary & Discussion Span is a power saving technique for multi- hop ad hoc wireless network Nodes make local decisions on whether to sleep, or join a forwarding backbone as a coordinator Rotation can maintain the network topology longer