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An Empirical Analysis of the IEEE An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process802.11 MAC Layer Handoff Process
Arunesh MishraMinho ShinWilliam ArbaughUniversity of Maryland College Park,MD,USA
ACM SIGCOMM Computer Communications Review Volume 33 , Issue 2 (April 2003)
Speaker: Yu Yung-Lin
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Outline
• Introduction
• Handoff process
• Design of the experiments
• Experiment Result
• Analysis of the probe phase
• Conclusion
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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
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Introduction
• 802.11 Wireless LAN architecture
•Wireless LAN Station (STA)•Access Points (AP)•Basic Service Set (BSS)•Distribution System (DS)•Extended Service Set (ESS).
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Introduction
• Two different ways to configure a network
Ad-hocAd-hoc•No structure•Every node can talk to each other
InfrastructureInfrastructure•Fixed APs with which mobile nodes can communicate•APs are connected to DS
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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
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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
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Handoff process
• Reauthentication– The STA attempts to reauthenticate to an AP ac
cording to the priority list– Authentication and a reassociation to the poster
ior AP (new-AP)– Transfer of credentials and other state informati
on from the old-AP to new-AP• This can be achieved through IAPP
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Handoff process
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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
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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
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Design of the experiments
• The clients– IBM Thinkpad T30 with P-IV and 512MB RA
M with three different NICs• Lucent Orinoco
• Cisco 340
• ZoomAir prism 2.5
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Experiments Result
• Probe delay accounts for more than 90% of the overall handoff delay
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Experiments Result
• The wireless hardware used affects the handoff latency
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Experiments Result
• Different wireless
cards follow different
sequence– ZoomAir and Lucent
Different procedure
From the Cisco NIC
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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
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Analysis of the probe phase
• Probe-wait Optimizations
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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
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Analysis of the probe phase
399.8
121
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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 accept
able bounds for VoIP on WLAN
