Characterising the Use of a Campus Wireless Network 徐 志 賢 Paper From: D. Schwab and R.B. Bunt, "Characterising the Use of a Campus Wireless Network", Proc. IEEE Infocom 2004, Hong Kong, March 7-12, 2004.

2 Outline Overview Campus Characteristics Wireless Network Environment Trace Methodology Trace Results Conclusion

3 Overview Analysis of wireless usage at the University of Saskatchewan –Where –When –How much Trace allows evaluation of network design principles and plans for future development

4 Campus Characteristics 40 Buildings with over 363 acres of land 18,000 students attend the university

5 Wireless Network Environment Initial deployment in 2001 with 18 APs –Dispersed through various buildings –Not well advertised Wireless traffic is routed on a virtual private network with a unique subnet –Assign internal IP address by a DHCP Server Cicsco LEAP authentication is used to provide access to wireless

6 LEAP Authentication Process

7 Trace Methodology Re-programming campus router and Mirrored wireless packets to a computer port monitoring traffic Used EtherPeek to log packet data Used LEAP server to track authentication data Trace began Jan 22, 2003 and lasted one week Data analyzed with perl script

8 Trace Methodology (Cont) Anonymisation –authentication log was stripped of user identifications, leaving only machine addresses as identifiers. –IP addresses were only temporary addresses assigned by campus DHCP server. –No message bodies 、 No private information

9 Trace Results AttributeValue Total Packets24,431,794 Total Seconds603,054 Average Seconds 40.5 packets per second Total Time 6 days, 23 hours, 30 minutes, and 54 seconds Spurious wired multicast overhead recorded (38% of traffic)

10 Authentication Data Large number of authentications per user –Wireless cards store user/pass –Typical of low and fluctuating signal strength –Oscillations between two APs AttributeValue Total Authentications24973 Unique Users134 Mean Authentications per User186.4 Mode Authentications per User5 Median Authentications per User54.5 Access Points18 Mean Authentications per AP1387.4

11 Data Analysis Number of authentications not related to packet rate –Reasons stated previously Older access points are used more often –More well-known to campus Weekend traffic –Minimal at office locations –Average at social and study locations

12 Traffic vs. Authentications

13 Daily Traffic Patterns

14 Roaming Users connected to average 3 AP and max 8 Direct relation between roaming and proximity Large well clusters of well-known APs incur the most roaming No. of unique user who authenticated at each access point Connected to the location of the access point on the map No. of unique users who authenticated at both the access points connected by the line.

15 Number of Access Points Visited Per User

16 The College of Law Generated 86% of authentications and 33% of packets during trace Law students roamed more frequently Reasons –Center of study and work for law students –Early shift to wireless connectivity –Legal community utilizes digital technology

17 Conclusion Avg wireless user connects a small number of times and only to a few APs Well-known APs were used most heavily Authentications have no relation to usage Roaming is limited Design Principles –Wireless networks should focus on location instead of mobility –APs should be focused on departments with online material –Wireless technology is designed for users of modern mobile computers.

18 中正校園無線網路環境 802.1x VPN RADIUS 漫遊（ Proxy RADIUS ）