Load Balanced Link Reversal Routing in Mobile Wireless Ad Hoc Networks

1. Load Balanced Link Reversal Routing in Mobile Wireless Ad Hoc Networks Costas Busch CSCI Department RPI Nabhendra Bisnik, Alhussein Abouzeid ECSE Department RPI

2. Mobile Wireless Networks • Wireless nodes are mostly battery driven ) limited transmission range • Nodes act as relays • Often involves many-to-one communication • Multihop wireless mesh networks • Mobile sensor networks • Link reversal routing (LRR) is a good choice • Loop free routes • Low overhead • However LRR may lead to unbalanced distribution of load (traffic forwarded)

3. Contributions • Identify the causes of load unbalance in LRR • Propose three heuristic mechanisms that attack different causes of load unbalance • Evaluate the performance of the heuristics using simulations

4. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

5. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

6. Link Reversal Routing • Properties • Distributed • Loop free at every instant • Low overhead • Offers both proactive and reactive modes • Multiple routes to destination • Two phases • Route creation phase • Route maintenance phase

7. Route Creation Phase 3 3 4 4 1 1 2 2 1 1 2 2 5 5 1 1 1 1 0 0 Destination Height = 0 Directed Acyclic Graph (DAG) • Route creation phase assigns height to each node and transforms connected network into a DAG • a ! b exists in the DAG only iff h(a) > h(b) • Thus DAG is loop free • In general h(a) = [h1(a), h0(a) ] where h1(a) = height assigned by LRR and h0(a) = node id of a • Lexicographical ordering used QRY UPD

8. Route Maintenance Phase Full Link Reversal Algorithm 3 3 3 3 4 4 4 4 2 2 4 2 2 6 2 2 5 5 5 5 3 3 1 1 0 0 0 0 7 7 • Brings network from a bad stateto a good state • Runs in (n2) time • Leads to increase in height of at least one node 4 8 4 8 6 6 5 5 7 7 0 0

9. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

10. Causes of Load Unbalance • Traffic flows from higher height to lower height • Each time a node looses route to the destination, its height increases • The nodes with stable routes to destination tend to have lower height • Thus stable nodes relay large amount of traffic leading to • Battery exhaustion • Congestion

11. Load Unbalance - Example 7 7 8 8 8 8 6 5 6 5 7 7 0 0 Although alternate path is now available, most of the traffic is still routed through the node with height 5

12. Unbalanced Network State • If there exist routes to the destination in the undirected network graph whose use may lead to a more uniform spread of load, but the routes are absent in DAG • Characteristics of unbalanced network state • Selfish nodes (nodes with no incoming links) • High height gradients (h(a) – h(b) > 2 and a ! b exists in the DAG) 6 Selfish Node L 5 K 4 G J 4 High Height Gradient 3 C F 2 3 I 2 2 H E B 1 D 1 A 0 Isolated RoutingComponents

13. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

14. Load Balancing Problem • Two Components of the problem • Maintaining a good DAG () • Use good forwarding strategy over the DAG (S) • Forwarding Strategy maps a link l of the DAG to traffic flowing over it, xS(l) • Total traffic forwarded by a nodewhere E(i) is the set of outgoing links of node i • Load balance metrics • Balance Factor (BF) • Squared Sum (SS)

15. Load Balancing Problem • From optimization point of view, the load balance problem is to find and s.t. • This problem is NP-hard, distributed solution is even more difficult Or,

16. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

17. Heuristic Mechanisms • Three heuristic mechanisms • Selfish Node Based Mechanism (SNBM) • Proactive Decrease in Height (PDH) • Reactive Increase in Height (RIH) • Height manipulation • Decrease height ) attract traffic • Increase height ) repel traffic

18. Selfish Node Based Mechanism • Aims to balance the size of isolated routing components • Periodically each node checks if it is selfish • If node selfish then • If hmax – hmin > 2 then • Sets height to minimum height that ensures path to the destination • Fix link directions • Update neighors

19. 5 5 5 5 3 5 H H L L H L 4 4 4 G G K K G K 4 4 6 C C C 2 2 3 J F 3 3 J F 3 7 3 J F 3 2 2 I I 2 I B E B E B E 2 2 2 2 2 2 D D D 1 1 1 A A A 0 0 0 5 3 H L 4 G K 4 C 2 3 J F 3 2 I B E 2 2 D 0 1 A Selfish Node Based Mechanism 5 8 H L 4 G K 6 C 7 3 J F 3 2 I B E 2 2 D 1 A 0

20. 9 M 5 8 H L 4 G K 6 C 3 J F 3 2 I B E 2 2 D 1 A 0 Selfish Node Based Mechanism • However every instances of load unbalance does not involve selfish nodes • Example ) • Solution – reduce heightwhenever it is possible inorder to balance DAG • This observation leads to PDH 7

21. Proactive Decrease in Height • Each node periodically compares its height with neighbors • If it is possible to decrease height without becoming a sink, then • Set height to minimum possible height that allows route to destination • Fix link directions • Update neighbors

22. 9 M 5 8 H L 4 G K 6 C 3 J F 3 2 I B E 2 2 D 1 A 0 Proactive Decrease in Height 9 4 9 M 5 M M 5 5 8 3 8 H L H H L L 4 7 4 G 4 K 2 2 7 G G K 6 K 4 4 C C C 3 J F 3 3 J F 3 3 J F 3 2 I 2 I 2 I B E B E B E 2 2 2 1 1 1 D D D 1 1 1 A A A 0 0 0

23. Reactive Increase in Height • Both SNBM and PDH are proactive in nature • RIH acts only when needed • Each node records the amount of traffic forwarded during an update window • If load served during an update window exceeds threshold then • Set height equal to hmax + 1 • Fix link directions • Update neighbors

24. Reactive Increase in Height 4 4 6 6 6 5 5 5 7 7 6 6 6 6 8 C C C C C D D D D D E E E E E 4 4 6 6 6 G G G G G B B B B B 3 5 5 5 5 F F F F F 5 5 5 5 5 A A A A A 0 0 0 0 0

25. Forwarding Strategies • Load distribution is also affected by the forwarding strategies • Two forwarding strategies considered • Multi-path routing • Distribute load equally among all downstream links • Requires maintenance of forwarding records • Shortest path routing • Forward packets to downstream neighbor that lies on the shortest path available in the DAG • Requires no state information

26. Talk Outline • Link Reversal Routing • Causes of load unbalance • Load balancing problem • Heuristic mechanisms • Simulations

27. Simulation Setting • N mobile nodes, initially deployed randomly over 1000m £ 1000m area • Communication radius is m • Random waypoint mobility model used with vmin = 2m/s, vmax = 5m/s, pause time = 5s • Each node generates traffic at rate 1Kbps, destined to a sink node • Sink node located at (500m, 500m) • Models mobile wireless sensor network, multi-hop wireless mesh networks

28. Performance Metrics • Balance factor and squared sum for both multi-path and shortest path forwarding • Network lifetime • Routing updates

29. Balance Factor Multi-path routing Shortest path routing • PDH has highest balance factor • As number of nodes increases, path length increases leading to lower balance factor • Multi-path routing has larger balance factor

30. Squared Sum Multi-path routing Shortest path routing • Again PDH has smaller squared sum • Multi-path routing leads to longer routes, hence larger squared sum

31. Network Lifetime • PDH leads to highest network lifetime • Lifetime decreases with increase in number of nodes

32. Height Update Rate • An update message is produced each time height of a node is updated • Thus routing overhead is proportional to the height update rate • RIH may cause a chain reaction of height updates, thus has much higheroverhead

33. Conclusion and Future Work • All the proposed schemes achieve better load balance than basic LRR • PDH is the best, since it is most aggressive • Future Work • NS-2 implementation of the proposed schemes • Approximate algorithms based on optimization framework

34. Thank You!