1. 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. 2 Outline Introduction Handoff process Design of the experiments Experiment Result Analysis of the probe phase Conclusion

3. 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. 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. 5 Introduction Two different ways to configure a network Ad-hoc No structure Every node can talk to each otherInfrastructure Fixed APs with which mobile nodes can communicate APs are connected to DS

6. 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. 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. 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. 9 Handoff process

10. 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. 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. 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. 13 Experiments Result Probe delay accounts for more than 90% of the overall handoff delay

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

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

16. 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. 17 Analysis of the probe phase Probe-wait Optimizations

18. 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. 19 Analysis of the probe phase 399.8 121

20. 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