doc.: ieee 802.11-04/0748r0 submission july 2004 spilman, azimuth systems test methodology for...
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July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Test Methodology for Measuring BSS Transition Time
Jeremy SpilmanAzimuth Systems
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Outline• Methodology• Topology• Procedure• Results
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Why define test methodology?
• Can be a selection criteria for proposals in TGr
• Highlights issues with current standard• A test methodology can be used for real
world testing as well as a theoretical analysis– Allows apples-to-apples comparisons– But must be based on observable events
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Different ways to cause a roam• Rolling cart• Power off AP• “Cone of Silence”• RF Attenuators
– Open Air– Controlled Environment
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Use cases require controlled environment
• Both APs always on• Must control amount of BSS overlap• Figure 1 implies client moves between BSSs smoothly• Methodology must be flexible for additional use cases
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Controlled Environment Topology
Combiner
WirelessSniffer
Combiner
WirelessSniffer
EthernetHub
Ethernet SnifferTraffic Source / Sink
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Device Topology• 2 APs, 1 Client, 1 Server• 2 Wireless Protocol Analyzers
– Positioned between each AP and Client
• 1 Ethernet Sniffer
• Packets Captured on:– Both AP Channels– Wired Ethernet on Server
• Traffic generated between Client and Server
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Controlling Path Loss
• Each station is cabled into a shielded chamber– Greater than 120 dB isolation required between
station and AP to guarantee loss of association
• Path loss between stations is controlled programmatically with step attenuators– Dynamic range of 80dB is sufficient
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Packet Capture• On Wired Ethernet:
– Last Data Packet before Roam– First Data Packet after Roam
• On Wireless Protocol Analyzers– Above packets, plus:– 802.11 Management frames sent during roam
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Key packets during a roam• Some intervals may not exist
Last Unack’d Datat RETRY
Last Ack’d Data
t SCAN
t ASSOCIATE
t DATA
First Probe Request
First Beacon
Auth Request
Auth Response
Assoc Request
Assoc Response
First Ack’d Data
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Client Association:
First data packetafter roam
Stimulating a Transition
t RETRY t SCAN
Last data packetbefore roam
t ASSOCIATE
t ROAM
t DATA
AP1 AP2
max
min
Atten
AP1
AP2
Attenuator Sweep Time
Inter-Roam Delay
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Test Results, Total Roam Time
0
2
4
6
8
10
12
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39Test Iteration
To
tal
Ro
am
Tim
e (
s)
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Test Results, Transition Intervals
0
1
2
3
4
5
6
7
8
9
A B C D E F G H I
Client Card, Average over 40 Transitions
Ro
am
Tim
e (
Se
co
nd
s)
T-Data
T-Associate
T-Scan
July 2004
Spilman, Azimuth Systems
doc.: IEEE 802.11-04/0748r0
Submission
Conclusion
• Repeatability and flexibility of controlled environment is very beneficial
• Capturing 802.11 management packets gives visibility into underlying performance variations