1. SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in Ad Hoc Networks Victor Bahl (Microsoft Research) Ranveer Chandra (Cornell University) John Dunagan (Microsoft Research)

2. 2 Motivation: Improving Capacity Traffic on orthogonal channels do not interfere e.g. Channels 1, 6 and 11 for IEEE 802.11b Channel 11 Channel 1 Channel 6 Example: An IEEE 802.11b network with 3 Access Points Can we get the benefits of multiple channels in ad hoc networks? Channel 6

3. 3 Channel Hopping: Prior Work Using multiple radios: –DCA (ISPAN’00): a control and a data channel –MUP (Broadnets’04): multiple data channels Consumes more power, expensive Using non-commodity radios: –HRMA (Infocom’99): high speed FHSS networks –Nasipuri et al, Jain et al: listen on many channels Expensive, not easily available Using a single commodity radio: –Multi-channel MAC (MMAC) (Mobihoc’04)

4. 4 Channel Hopping: MMAC MMAC Basic idea: Periodically rendezvous on a fixed channel to decide the next channel Issues Packets to multiple destinations  high delays Control channel congestion Does not handle broadcasts Channel 1 Channel 6 Channel 11 Data ControlData Control

5. 5 Our Contributions SSCH: a new channel hopping protocol that –Increases network capacity using multiple channels –Overcomes limitations of dedicated control channel –No control channel congestion –Handles multiple destinations without high delays –Handles broadcasts for MANET routing

6. 6 Outline of the Talk Problem Overview Related Work SSCH: The Main Idea SSCH: A Few Details Performance of SSCH Conclusion

7. 7 SSCH: Slots and Seeds Divide time into slots: switch channels at beginning of a slot 3 channels E.g. for 802.11b Ch 1 maps to 0 Ch 6 maps to 1 Ch 11 maps to 2 1 02102 1 0 01201201 New Channel = (Old Channel + seed) mod (Number of Channels) seed is from 1 to (Number of Channels - 1) Seed = 2 Seed = 1 (1 + 2) mod 3 = 0 (0 + 1) mod 3 = 1 A B Enables bandwidth utilization across all channels Does not need control channel rendezvous

8. 8 SSCH: Syncing Seeds Each node broadcasts (channel, seed) once every slot If B has to send packets to A, it adjusts its (channel, seed) 3 channels 1 02102 1 0 0121021 0 Seed Follow A: Change next (channel, seed) to (2, 2) A B 22 222 2 22 12 2 22222 2 2 1 Stale (channel, seed) info simply results in delayed syncing B wants to start a flow with A 2

9. 9 Outline of the Talk Problem Overview Related Work SSCH: The Main Idea SSCH: A Few Details –Parity Slots: Ensuring overlap –Partial Sync: Sending to multiple destinations –Handling broadcasts Performance of SSCH Conclusion

10. 10 Nodes might not overlap! If seeds are same and channels are different in a slot: 3 channels Seed = 2 Nodes are off by a slot  Nodes will not overlap 1 02102 1 0 1 02102 12 A B

11. 11 SSCH: Parity Slots 3 channels Seed = 1 Every (Number of Channels+1) slot is a Parity Slot In the parity slot, the channel number is the seed Parity Slot Guarantee: If nodes change their seeds only after the parity slot, then they will overlap 012 0122 2 0111 111 0 A B

12. 12 SSCH: Partial Synchronization Syncing to multiple nodes, e.g., A sends packets to B & C Each node has multiple seeds Each seed can be synced to a different node Parity Slot Still Works Parity slot: (Number of Channels)*(Number of Seeds) + 1 In parity slot, channel is the first seed First seed can be changed only at parity slot If the number of channels is 3, and a node has 2 seeds: 1 and 2 2210110221100 (1 + 1) mod 3 = 2 (2 + 2) mod 3 = 1 Parity Slot = seed 1

13. 13 Illustration of the SSCH Protocol 2210110221100 Node A 2012120221100 Node B Seeds B wants to start a flow with A Complete Sync (sync 1 st seed) Seeds (1, 2) Channels: (1, 2) Partial Sync (only 2 nd seed) Seeds: (2, 2) Channels: (2, 1) 121212121212 Seeds212222121212 Suppose each node has 2 seeds, and hops through 3 channels.

14. 14 SSCH: Handling Broadcasts A single broadcast attempt will not work with SSCH since packets are not received by neighbors on other channels 21010 01220 B’s broadcast Node A Node B Seeds 1212 2222 SSCH Approach Rebroadcast the packet over ‘X’ consecutive slots  a greater number of nodes receive the broadcast B’s broadcast in SSCH

15. 15 Outline of the Talk Problem Overview Related Work SSCH: The Main Idea SSCH: A Few Details Performance of SSCH –Improvement in throughput –Handling broadcast packets –Performance in multi-hop mobile networks Conclusions

16. 16 Simulation Environment QualNet simulator: IEEE 802.11a at 54 Mbps, 13 channels Slot Time of 10 ms and 4 seeds per node –a parity slot comes after 4*13+1 = 53 slots, –53 slots is: 53*10 ms = 530 ms Channel Switch Time: 80 µs –Chipset specs [Maxim04], –EE literature [J. Solid State Circuits 03] CBR flows of 512 byte packets per 50 µs

17. 17 SSCH: Stationary Throughput Per-Flow throughput for disjoint flows IEEE 802.11a SSCH SSCH significantly outperforms single channel IEEE 802.11a

18. 18 SSCH Handles Broadcasts 10 Flows in a 100 node network using DSR For DSR, 6 broadcasts works well (also true for AODV) Average discovery time for IEEE 802.11a Average route length for IEEE 802.11a

19. 19 SSCH in Multihop Mobile Networks Random waypoint mobility: Speeds min: 0.01 m/s max: rand(0.2, 1) m/s Average flow throughput for IEEE 802.11a Average route length for IEEE 802.11a SSCH achieves much better throughput although it forces DSR to discover slightly longer routes

20. 20 Conclusions SSCH is a new channel hopping protocol that: Improves capacity using a single radio Does not require a dedicated control channel Works in multi-hop mobile networks –Handles broadcasts –Supports multiple destinations (partial sync)

21. 21 Future Work Analyze TCP performance over SSCH Study interoperability with non-SSCH nodes Study interaction with 802.11 auto-rate Implement and deploy SSCH (MultiNet)

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