1. An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process Arunesh Mishra Minho Shin William Arbaugh University of Maryland College Park,MD,USA ACM SIGCOMM Computer Communications Review Volume 33 , Issue 2 (April 2003) Speaker: Yu Yung-Lin

2. Outline • Introduction • Handoff process • Design of the experiments • Experiment Result • Analysis of the probe phase • Conclusion

3. Introduction • Present an empirical study of this handoff process • The primary contributor to the overall handoff process is probe • Discuss optimizations on the probe phase

4. Introduction • 802.11 Wireless LAN architecture • Wireless LAN Station (STA) • Access Points (AP) • Basic Service Set (BSS) • Distribution System (DS) • Extended Service Set (ESS).

5. Introduction • Two different ways to configure a network • Ad-hoc • No structure • Every node can talk to each other • Infrastructure • Fixed APs with which mobile nodes can communicate • APs are connected to DS

6. Handoff process • Definition • Mobile node moves from coverage area of one AP to that of another AP • Handoff process can be divided into two distinct logical steps • Discovery • Reauthentication

7. Handoff process • Discovery • The client needs to find the potential APs • Accomplished by a MAC layer function: scan • The card listens for beacon message on assigned channels. • Created a candidate set of APs prioritized • Two scanning mode • Passive • Active

8. Handoff process • Reauthentication • The STA attempts to reauthenticate to an AP according to the priority list • Authentication and a reassociation to the posterior AP (new-AP) • Transfer of credentials and other state information from the old-AP to new-AP • This can be achieved through IAPP

9. Handoff process

10. Design of the experiments • The wireless network environment • The umd network • 35 Cisco350 APs distributed on channel 1,6,11 • Open authentication • The nist network • 17 Soekris APs distributed on channel 1,6,11 • Open authentication • The cswireless network • 8 Lucent APs on other 8 different channels • Use WEP

11. Design of the experiments • The sniffer system • For umd and nist network(APs are on 1,6,11) • Two linux machines • One with one wireless NIC • One with two wireless NICs • Sniffing the 3 channels • For cswireless network • Six linux machines • One with one wireless NIC • Five with two wireless NIC • Sniffing all 11 channels

12. Design of the experiments • The clients • IBM Thinkpad T30 with P-IV and 512MB RAM with three different NICs • Lucent Orinoco • Cisco 340 • ZoomAir prism 2.5

13. Experiments Result • Probe delay accounts for more than 90% of the overall handoff delay

14. Experiments Result • The wireless hardware used affects the handoff latency

15. Experiments Result • Different wireless cards follow different sequence • ZoomAir and Lucent Different procedure From the Cisco NIC

16. Analysis of the probe phase • Probe-wait latency • STA waits on one channel after sending the probe request • Total probe delay : t • N*MinChannelTime≤ t ≤ N*MaxChannelTime

17. Analysis of the probe phase • Probe-wait Optimizations

18. Analysis of the probe phase • Improvement • Set MinChannelTime = 6.5ms • Set MaxChannelTime = 11ms • We can get the probe delay on 11 channel • 11ms * 11 channels = 121 ms

19. Analysis of the probe phase 399.8 121

20. Conclusion • Contribution • Detailed analysis of the handoff process • Optimizing two parameters to improve the handoff latency • Future work • Reduce the latency of the handoff within acceptable bounds for VoIP on WLAN