Enhancing Reliability in Ad Hoc Networks with Bidirectional Abstraction

1. SRL: A Bidirectional Abstraction for Unidirectional Ad Hoc Networks. Venugopalan Ramasubramanian Ranveer Chandra Daniel Mosse

2. Introduction • Links in an ad hoc network could be unidirectional. • Many Ad hoc network routing protocols are not designed to handle unidirectional links (TORA). • Some handle unidirectional links but are very inefficient (DSR).

3. C E A B D Noise: source of one-way link. • Transient unidirectional links. • Go away when noise subsides or nodes move.

4. C B A Asymmetry in Transmit Power • Topology Control Schemes: Sensor Network • Heterogeneity of hardware: Home Network C B A

5. A B C RTS MSG X CTS CTS MSG MSG Problems due to one-way links. • Collision avoidance (RTS/CTS) scheme is impaired • Even across bidirectional links!

6. Problems due to one-way links • Collision avoidance (RTS/CTS) scheme is impaired • Even across bidirectional links. • Unreliable transmissions through one-way link. • May need multi-hop Acks at Data Link Layer. • Link outage can be discovered only at downstream nodes.

7. Problems for Routing Protocols • Route discovery mechanism. • Cannot reply using inverse path of route request. • Need to identify unidirectional links. (AODV) • Route Maintenance. • Need explicit neighbor discovery mechanism. • Connectivity of the network. • Gets worse (partitions!) if only bidirectional links are used.

8. Average Bidirectional Connectivity

9. Distribution of Bidirectional Connectivity. 200 random topologies. Probablity of one-way link = 0.25

10. C B A Reverse route for one-way link • Let A  C be a one-way link. • C  B  A is a 2-hop reverse route.

11. Connectivity with reverse routes.

12. One-way links with reverse routes.

13. Average Reverse Route Length

14. Observations from analysis. • Topologies generated with asymmetric transmit power also produce similar graphs. • The connectivity follows a long tail distribution. • Reverse routes are short (2 or 3 hops) for most one-way links.

15. SRL: Sub Routing Layer • Short reverse routes for one-way links • Improve connectivity substantially. • Also decrease route lengths. • SRL discovers and maintains reverse routes for one-way links. • It provides a bidirectional abstraction to the routing protocols. • Provides services such as reliable transmission and link breakage detection.

16. Internals of SRL • Reverse Distributed Belmanford Algorithm • Distance vector based technique. • Each node maintains: • Shortest path from other nodes in its locality. • Periodically neighbor-casts this information. • Locality of node A: • Set of nodes that can reach A in r hops. • r: is the radius of locality.

17. C A; 1; C C; 2; B B; 1; A A B A; 2; C C; 1; B Reverse Distributed Belmanford Algorithm. Reverse Route: C  B  A Update Message Format: Source; #hops; First Hop

18. RDBA contd. • Periodic update messages are neighbor-cast: Source ID : Hop Count : First Hop • Sources restricted to locality of radius r. • r: called SRL radius is small (2 – 3). • Scalable to large networks. • No counting to infinity problem. • Ignore distances bigger than r. • No Route-loops. • Use first hop information to check for loops.

19. SRL: Periodic Updates • Incremental Updates • Most recent changes in hop count or first hop. • Sent periodically at same rate as hello messages. • Replaces hello messages. • Complete Updates • Contains entire data for locality. • Sent with much lower frequency. • Random distribution to avoid co-ordination. • Hello Packets • Sent when no incremental updates need to be sent.

20. Optimization 1: Dynamic SRL • The SRL radius of each node could be different. • Each node increases radius until it can find reverse routes. • Radius decreases if reverse routes are shorter than the radius. • Decreases the number of updates that is neighbor-cast: lower overhead.

21. Optimization 2: On-demand DSRL • Routing protocol requests DSRL to find reverse routes for certain one-way links. • Reverse routes maintained only for the chosen one-way links. • Routing strategy that uses one-way links only when route discovery along bidirectional links fail.

22. Services provided by SRL • Identification of one-way links (radius = 1): • Routing protocols can avoid them. • Reverse route forwarding: • Routing protocol uses reverse routes to send route replies and route errors. • Not good for data packets. • Link breakage detection: • Several protocols rely on lower layers to do this. • Reliable Transmission across unidirectional links: • Multi-hop Acks can be used if required by the protocol.

23. Simulation: AODV over SRL • AODV is adapted on top of SRL. • Use reverse routes for RREPs and RERRs. • Uses SRL’s link break discovery service. • Compared with traditional AODV. • Routes only along bidirectional links. • Uses black-list to identify unidirectional links. • Runs on top of IEEE 802.11

24. Simulation Setup • 80 nodes in 1300m x 1300m area. • 220m nominal radio range (WaveLan). • 360s total simulation time. • 300s of data origination. • 20 random src-dest pairs for each run. • 50 random topology for each experiment. • Packet Size: random between 64B – 1024B. • Average data rate: 1 packet per sec.

25. Static Experiments: Packet Delivery.

26. Static Experiments: Average Route Length.

27. Mobility Experiments: Packets Originated

28. Mobility Experiments: Packet Delivery.

29. SRL Overhead: Average Length of Update Packets.

30. Conclusions • SRL increases the packet delivery of AODV by 30%. • The overhead generated by SRL is not very significant and can be further reduced. • The effect of optimizations need to be studied. • RTS/CTS implementation with SRL would be interesting!