1. 1 Message In Message (MIM): A Case for Reordering Transmissions in Wireless Networks Naveen Santhapuri, Srihari Nelakuditi University of South Carolina Justin Manweiler, Souvik Sen, Romit Roy Choudhury, Kamesh Munagala Duke University

2. 2 Outline Motivation Understanding MIM How transmission order affects spatial reuse? Contributions Validate benefits of ordering Design MIM aware scheduling framework Future work on MIM and SIC

3. 3 Collision Signal of Interest (SoI) successful When SINR is substantially higher And, SoI arrives earlier than Interference Else, collision Collision SoI Interference R2 R1

4. 4 Collisions affect Spatial Reuse MAC protocols designed to avoid collisions 802.11 physical carrier sensing RTS/CTS Greatly limits spatial reuse

5. 5 PHY Capture [Kochut:ICNP04] Interference may not always cause collisions Possible to decode SoI with higher SINR If SoI arrives within preamble of Interference Interference Preamble Signal of Interest Preamble time small (20 us in 802.11a) Benefits small as well 20 us

6. 6 Message In Message (MIM) While receiving a message Receiver looks for new message preamble Stronger Message extracted while receiving ongoing Message Requires higher SINR than when SOI arrives earlier Preamble Signal of InterestInterference Capability exists in Atheros chipsets

7. 7 802.11, Capture, and MIM Preamble SoI Interference SoI Interference SoI Interference SoI Interference 802.11: PHY Capture: MIM: a) b)c) d) Yes (10 dB) Yes (10 dB) No No Yes (10 dB) Yes (10 dB) Yes ( 10 dB ) No Yes (10 dB) Yes (10 dB) Yes ( 10 dB ) Yes ( 20 dB ) Successful Reception Yes/No (SINR Threshold needed to avoid collision) Different thresholds based on frame ordering

8. 8 AP1 must start first Followed by staggered transmission from AP2 Allows weaker link R1 to lock on to signal at low SINR AP1 must start first Followed by staggered transmission from AP2 Allows weaker link R1 to lock on to signal at low SINR In general weaker transmissions must start first, stronger receiver can recover signal with MIM In general weaker transmissions must start first, stronger receiver can recover signal with MIM Link Ordering Matters R1 R2 10 dB 20 dB Data AP1 R1 AP2 R2 Controller AP1 AP2 Data AP1 R1 AP2 R2

9. Measurements Rx Tx Interferer 1 2 3 4 5 Order doesnt affect delivery Order affects delivery ratio Observe that 802.11 does not enforce the order and thus fails to exploit concurrency Observe that 802.11 does not enforce the order and thus fails to exploit concurrency

10. 10 MIM Capable vs MIM Aware MIM Capable Network card can recover MIM Current MAC does not exploit MIM Appropriate ordering happens by chance MIM Aware MAC MAC layer harnesses MIM capability Enforces appropriate ordering of transmissions

11. 11 Goal Design MIM aware scheduling that reorders transmissions for improving concurrency Research Questions Does MIM awareness yield significant benefits? What is the bound on improvement? How to effect the appropriate order?

12. 12 Optimality Analysis Integer Programming Formulation in CPLEX Optimal benefits from MIM significant

13. 13 MIM Aware MAC Shuffle Centralized MIM-aware scheduling protocol For Enterprise Wireless LANs (EWLAN) Why EWLAN? Increasingly popular architecture Realizes potential of MIM Controller AP1 AP2 AP3

14. 14 Shuffle: Assumptions Dominant downward traffic Powerful controller, Gigabit Ethernet Low latency for scheduling/communication Additive Interference Total = sum of individual interferences

15. 15 Shuffle: Components Rehearsal: Measuring interference relations Packet Scheduler Use rehearsal and MIM- constraints Output transmission schedules (ordered) Schedule Executer Rehearsal Interference Relations MIM Constraints Packet Queue Scheduler Ordered Transmissions

16. 16 Measuring Interference Relationships Periodic Rehearsals APs transmit probes at base rate Each client replies with RSSI values APs too record RSSI values from clients Controller derives interference map Opportunistic rehearsal Piggyback RSSI values in data transmissions Continually refine interference map

17. 17 MIM-Aware Scheduler Objective Maximize concurrency Avoid starvation With MIM, conflicts are asymmetric Conflict graph methods unsuitable Optimal link scheduling is NP-hard Least conflict greedy heuristic Score links based on asymmetric conflicts Links that prevent other links assigned higher score Compute link order based on lower score first

18. 18 Batch Selection & Dispatch P 31 P 22 P 31 P 13 P 12 R 11 R 12 R 13 R 21 R 22 R 31 R 32 P 13 P 32 AP1 AP2 AP3 Controller P 21 Batch

19. 19 Schedule Execution P 31 P 22 P 31 P 13 P 12 R 11 R 12 R 13 R 21 R 22 R 31 R 32 P 13 P 32 AP1 AP2 AP3 Controller P 21 APs transmit at specified time DATA Staggering order: AP1-AP3-AP2

20. 20 Evaluation Qualnet simulations Throughput and the effect of Fading Parameters 802.11a physical model with MIM PLCP: 20 us Fading: Ricean, varying K factor Wired backbone: 1 Gbps ethernet Controller processing latency: 50 us

21. 21 Duke EWLAN Topologies Client, AP placement traces used to derive topologies (topo1, topo2, etc.)

22. 22 Throughput Comparision Gain with scheduling Higher gains with Shuffle

23. 23 Increasing AP Density AP density yields higher benefit from Shuffle

24. 24 Impact of Channel Fading Better throughput gain at lower (Ricean) fading

25. 25 Related Work Location aware 802.11: Infocom 05 Takes advantage of MIM but not ordering CMAP: NSDI08 Partially benefits from MIM capable hardware Speculative Scheduling for EWLANs: Mobicom 07 Doesnt consider MIM

26. 26 Looking Forward MIM helps recover if SoI is stronger What if SoI is weaker than interference? MIM cannot help Successive Interference Cancellation

27. 27 Successive Interference Cancellation SIC can be used to recover weaker SoI First extract stronger frame Subtract it from the combination Recover weaker frame from residue Feasibility depends on Strengths of SoI and Interference

28. 28 Interplay of MIM and SIC Ordering helps SIC too If Interference (I) moderately stronger than SoI Initiate I first to take advantage of MIM Decode I If I much stronger than SoI Initiate SoI before I SoI characterized better for decoding later

29. 29 Characterization & Cancellation S2 first, S1, S2 decodable Both S1 and S2 lost S2 later S1, S2 decodable RSS of S1 RSS of S2 S1 Later S1, S2 decodable S1 first, S1, S2 decodable S2 not decodable S1 not decodable Both lost S2 too weak to satisfy SNR Rx Tx1 Tx2 S1 S2

30. 30 Future Work Shuffle Implementing and deploying on a test-bed Integrating upload traffic Comparing with other schemes SIC vs MIM Explore Characterization vs. Cancellation Advantage of reordering transmissions

31. 31 Thank you

32. 32 Coping with Fading Loss Immediate corrective rehearsal Controller identifies links suspected of fading Schedules a packet batch only for these APs This is a partial rehearsal Packets are transmitted in serial order APs and clients unaware, send Data and ACKs Controller updates Interference map from ACK RSSIs

33. 33 Idea to explore with SIC Power Control to enable SIC Suppose SINR threshold is 10dB SINR is 1 dB -60 dBm -59 dBm Tx1: tx power = 100mW Tx2

34. 34 Idea to explore with SIC Power control to enable SIC Suppose SINR threshold is 10dB SINR is 10dB after Tx1 reduced transmit power -60 dBm -70 dBm Tx1: tx power = 10mW Tx2

35. 35 20 us Limitations of Capture Capture does not help when SoI arrives after the preamble of interference i.e. Receiver locks on to interference Preamble Signal of InterestInterference Preamble time small (20 us in 802.11a) Benefits small as well