1. IEEE 802.11 in the Large: Observations at the IETF Meeting Henning Schulzrinne, Andrea G. Forte, Sangho Shin Department of Computer Science Columbia University

2. Aug 5th 2006 2 Introduction 65 th IETF meeting Dallas, TX March 19 th ~ 24 th Hilton Anatole hotel 1200 attendee Data collection 21 st ~ 23 rd for three days 25GB data, 80 millions frames Goals Identify unusual behaviors due to the highly congested environment

3. Aug 5th 2006 3 Overview Introduction Related work Wireless network setup Measurement setup User behavior Load balancing Handoff behavior Overhead of multiple APs on a channel Conclusion

4. Aug 5th 2006 4 Related Work Jardosh et al.Rodrig et al.Balachandran et al. Target62 nd IETF (’05)SIGCOMM ’04SIGCOMM ’01 Attendee1100550200 Clients detected500377195 WLAN802.11b Channels1, 6, 11 (dynamic) 1, 8, 111, 4, 7, 11 Number of APs3854 Focused onTheir new metric for channel congestion, ‘Link reliability’ Overhead of 802.11, the data transmission rate Load balancing: “No correlation b/w num of clients and traffic load” * * The number does not mean the total number of wireless clients in the IETF meeting, but the number of clients they detected from their measurement.

5. Aug 5th 2006 5 Wireless network in IETF meeting Many hotel 802.11b APs on channel 6 91 additional APs in 802.11a/b by IETF Cisco Aironet 1200 AP Channel 1 and 11 in 802.11b One subnet with one ESSID ‘ietf65’ The largest wireless network measured so far No wireless security Hotel APs 802.11b channel 6 IETF APs 802.11a/b ESSID : ietf65

6. Aug 5th 2006 6 Four sniffers IBM T42 Think Pad Proxim ORiNOCO 11 a/b/g Sniffer software: Airopeek NX Channel assignment Three sniffers on channel 1, 6 and 11, respectively Fourth sniffer on all 8 channels in 802.11a Measurement setup

7. Aug 5th 2006 7 Measurement setup Measurement place Room Chantilly The biggest room - 142’ x 80’, 600 persons Two biggest IETF sessions, and the plenary session in the evening Six IETF APs + Six Hotel APs in the room Session 1 (AM)Session 1 (PM)Plenary Lunch Session 2 & break 9:00 11:3013:0015:0017:0019:30

8. Aug 5th 2006 8 The positions of the APs in the room Room Chantilly Lobby Gate screen AP8AP7 AP5 AP4 Seating area during IETF sessions and plenary Another seating area during plenary AP1 AP2 AP3 AP6 sniffers AP9 AP on channel 1 AP on channel 11 (estimated position) 142 feet 80 feet

9. Aug 5th 2006 9 Overview Introduction Related work Wireless network setup Measurement setup User behavior Load balancing Handoff behavior Overhead of multiple APs on a channel Conclusion

10. Aug 5th 2006 10 User behavior Overall traffic in 802.11b

11. Aug 5th 2006 11 User behavior Number of clients (all 8 channels)

12. Aug 5th 2006 12 User behavior Protocols

13. Aug 5th 2006 13 Overview Introduction Related work Wireless network setup Measurement setup User behavior Load balancing Handoff behavior Overhead of multiple APs on a channel Conclusion

14. Aug 5th 2006 14 Load balancing Distribution of clients No load balancing feature was used Client distribution is decided by the relative proximity from the APs. Ch 6 (ceiling) > Ch 1 > Ch 11 AP5 AP4 Seating area during IETF sessions and plenary Another seating area during plenary AP1 AP2 AP3 AP6 sniffers

15. Aug 5th 2006 15 Load balancing Number of clients in channel 1 Distribution of clients No load balancing feature was used Client distribution is decided by the relative proximity from the APs. Ch 6 (ceiling) > Ch 1 > Ch 11 AP1 > AP2 > AP3

16. Aug 5th 2006 16 Load balancing Throughput Average throughput in 802.11a/bAverage throughput in channel 1

17. Aug 5th 2006 17 Load balancing Average throughput per client in 802.11a/b Average throughput per client in channel 1 Throughput per client

18. Aug 5th 2006 18 Load balancing How? Number of clients? – Simple, but not accurate Throughput ? - Accurate, but too complex Study of Balanchandran et al. [1] “No correlation between the number of clients and traffic load” “Throughput per client needs to be considered” In the large scale wireless network like IETF meeting? [1] A. Balanchandran et al. “Characterizing user behavior and network performance in a public wireless LAN”, SIGMETRICS ‘02

19. Aug 5th 2006 19 Load balancing Number of clients vs. throughput in Ch. 6 Capacity in the channel

20. Aug 5th 2006 20 Load balancing Clear correlation between the number of clients and throughput The number of clients can be used for load balancing with low complexity of implementation, in large scale wireless networks Number of clients vs. Throughput Capacity in the channel

21. Aug 5th 2006 21 Effect of screen Small number of clients on AP3 Effect of a screen? Number of clients in channel 1

22. Aug 5th 2006 22 Effect of screen 30 ~ 100 feet An experiment

23. Aug 5th 2006 23 Overview Introduction Related work Wireless network setup Measurement setup User behavior Load balancing Handoff behavior Overhead of multiple APs on a channel Conclusion

24. Aug 5th 2006 24 Handoff behavior Handoff is triggered generally, by low signal strength in congested channel, by frame loss Effect of layer 2 handoff Increase of traffic Disruption of network (0.5 ~ 1.5 sec) The number of handoff per hour in each IETF session

25. Aug 5th 2006 25 Handoff behavior Handoff to the same channels : 72% Handoff to the same AP : 55% Handoffs between channels

26. Aug 5th 2006 26 Handoff behavior Too often handoff Disruption of network 0.5 ~1.5 sec per handoff Increase of traffic due to handoff related frames – probe request and response 10.4% of total Distribution of session time : time between handoffs

27. Aug 5th 2006 27 Handoff behavior Distribution of vendors Handoffs per vendor Distribution of handoffs per vendor (based on the number of handoffs during the whole day)

28. Aug 5th 2006 28 Handoff behavior Handoffs per vendor (session time) Apple Cisco

29. Aug 5th 2006 29 Overview Introduction Related work Wireless network setup Measurement setup User behavior Load balancing Handoff behavior Overhead of multiple APs on a channel Conclusion

30. Aug 5th 2006 30 Overhead of having multiple APs Overhead from replicated multicast and broadcast frames All broadcast and multicast frames are replicated by all APs  Increase traffic Router A channel

31. Aug 5th 2006 31 Overhead of having multiple APs Overhead from replicated multicast and broadcast frames All broadcast and multicast frames are replicated by all APs. DHCP request (broadcast) frames are replicated and sent back to each channel. Router A channel

32. Aug 5th 2006 32 Overhead of having multiple APs Overhead from replicated multicast and broadcast frames All broadcast and multicast frames are replicated by all APs. DHCP request (broadcast) frames are replicated and sent back to each channel. Multicast and broadcast frames : 10%

33. Aug 5th 2006 33 Overhead of having multiple APs Co-channel interference 14 APs around the room Chantilly in channel 6 Co-channel interference and too many clients are responsible for the very low throughput of channel 6.

34. Aug 5th 2006 34 Conclusions Uneven distribution of clients and throughput among channels and APs Correlation between the throughput and the number of clients The number of clients can be used for load balancing with low complexity in highly congested wireless networks Too many inefficient handoffs Disrupt network and increase traffic Need better handoff algorithms Having multiple APs on a channel Increases the traffic due to replication of multicast and broadcast frames. Decrease throughput due to co-channel interference

35. Aug 5th 2006 35 Thank you

36. Aug 5th 2006 36 Backup slides

37. Aug 5th 2006 37 Handoff behavior Total number of handoffs Number of handoffs per hour