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An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process

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. Outline. Introduction

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An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process

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  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

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