Analysis of a Campus-wide Wireless Network

1. Analysis of a Campus-wide Wireless Network David Kotz Kobby Essien Dartmouth College September 2002

2. Wi-Fi is becoming pervasive But how do people use it? How to design Wi-Fi networks? How to deploy Wi-Fi networks?

3. Dartmouth College

4. 5,500 students 3,330 live on campus 40% own laptops

5. Class of 2006 just arrived 88% own Wi-Fi laptops; 100% of MBA students do

6. The Dartmouth network 476 access points (now over 500) Cisco model 340 or 350

7. Dartmouth campus • Complete Wi-Fi coverage • 200 acre campus • 161 buildings • 82 Residential • 32 Academic • 22 Administrative • 6 Library • 19 Social

8. The largest Wi-Fi study • Fall 2001 • Wi-Fi at Dartmouth • 11 weeks • Over 1700 users • Diverse population • 161 buildings • 476 access points • Campus-wide coverage [Hutchins and Zegura] • Wi-Fi at GA Tech • 20 or 7 weeks • 444 users • Diverse population • 18 buildings • 109 access points • Partial coverage [Balachandran 2002] • Wi-Fi at SIGCOMM • 2.5 days • 195 users • Computer scientists • One room • 4 access points • Small sample [Tang and Baker 2000] • Wi-Fi at Stanford • 12 weeks • 74 users • Computer scientists • 1 building • 12 access points • Small sample

9. Data Collection

10. 1: Syslog data collection • AP reports interesting events • Authenticate • Associate • Deauthenticate • Disassociate • Record date, time, MAC, AP name • Sent by access point to syslog recorder

11. 2: SNMP data collection • Every 5 minutes, poll each AP • Record: • MAC of associated clients • Counter: inbound bytes • Counter: outbound bytes Outbound Inbound

12. 3: tcpdump data collection Hub AP Tiny Linux box promiscuously sniffed all packet headers

13. Results Traffic

14. Daily traffic (GB) Median: 53 GB/day

15. Average daily traffic, by weekday GB/day

16. Average hourly traffic, by hour To 11.4 GB (60% inbound) GB/hour

17. Average daily traffic per AP Median: 39 MB GB/day

18. Average daily traffic (GB)(by category) GB/day

19. Average GB/day per card(by category) GB/day/card

20. Average daily traffic (GB)(10 busiest buildings)

21. Results Sessions

22. Deauthenticated Reassociated Associated Session duration 30 mins Sessions time

23. Session length Median session length is 16.6 minutes Hours

24. Associated 10 seconds Flickering sessions Reassociated time

25. Roams per session Plot of the 18% of sessions that involve at least one roam Max: 19,902 Median: 2

26. Results Users

27. Activity per card (distribution) Maximum: 77 days, 64 buildings, 161 APs Median: 28 days, 5 buildings, 9 APs

28. Active cards per day

29. Active cards per hour (category)

30. Results Protocols

31. Brown (dormitory)

32. Berry Library

33. Collis Student Center

34. Sudikoff Lab for Computer Science

35. Common IP protocols 99.7% of all wireless frames contained IP packets

36. TCP and UDP traffic (GB)(by building) These top ten account for 85% of traffic

37. TCP and UDP traffic(by direction)

38. TCP connections(by building)

39. TCP connections(by direction)

40. Correction: Figures 27-28 Dartmouth College Computer Science Technical Report TR2002-432 http://www.cs.dartmouth.edu/~campus/

41. Summary • Largest trace-based study of a WLAN • Large, diverse population • Residential university campus • Mixture of academic and residential patterns • Results may be different for other populations

42. High variance in traffic and activity From day to day, hour to hour From place to place, user to user No clear dominance of inbound or outbound Varies by protocol and user Dormitories dominated traffic Especially the Tuck School of Business We need: Cards that avoid roaming across subnet boundary Support for roaming across subnets (Mobile IP, etc) Symmetric bandwidth Full-campus coverage: critical to acceptance Conclusions

43. Thank you

44. David Kotz Kobby Essien http://www.cs.dartmouth.edu/~campus/