July 2005

C Trecker, Azimuth Systems

Slide 1

doc.: IEEE 802.11-05/0757r0

Submission

Test Methodology for measuring Fast BSS Transition Performance

Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

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Date: 2005-07-20

Name Company Address Phone email Chris Trecker Azimuth

Systems 31 Nagog Park, Acton, MA 01720

(978) 263-6610 ctrecker@azimuthsystems.com

Authors:

July 2005

C Trecker, Azimuth Systems

Slide 2

doc.: IEEE 802.11-05/0757r0

Submission

Abstract

This presentation proposes test metrics and test methodologies to characterize fast BSS transition performance.

July 2005

C Trecker, Azimuth Systems

Slide 3

doc.: IEEE 802.11-05/0757r0

Submission

Outline• Definitions and Metrics• Methodology• Topology• Procedure• Results

July 2005

C Trecker, Azimuth Systems

Slide 4

doc.: IEEE 802.11-05/0757r0

Submission

Definitions and Metrics• Fast BSS Handshake Time: Time between FBT Auth Request and

FBT (Re) Assoc Response (Base Mechanism). When pre-reservation is used, time between FBT (Re) Association Request and FBT (Re) Association Response.

– This time can be precisely measured.

• Fast BSS Transition Time. Time between MSDU delivery to DS from AP1 and AP2 as a STA executes a FBT transition from AP1 to AP2. Should also measure the time downstream DS – STA MSDU time.

– This time is measured both on the DS (FBT Time DS) and on the air (FBT Time Air)

– This time can not be precisely measured unless using WIFI test equipment configured to act as a STA in a controlled environment.

– This time will generally be measured statistically over several transitions at different data rates.

July 2005

C Trecker, Azimuth Systems

Slide 5

doc.: IEEE 802.11-05/0757r0

Submission

Fast BSS Handshake Time

Observed BSS Transition Time

FBT Handshake Time

Auth Req Auth Resp Assoc Req Assoc Resp

packets observed on the air

July 2005

C Trecker, Azimuth Systems

Slide 6

doc.: IEEE 802.11-05/0757r0

Submission

Fast BSS Transition Time

MSDU-AP1 ACK

Last possible instant where frames can pass through old

AP

MSDU-AP2 ACK

First possible instant where frames can pass through new

AP

Theoretical BSS Transition

Time. . . . . .

Observed FBT Time on air

packets observed on the air

MSDU-AP1 MSDU-AP2

. . .packets observed on the DS

Observed FBT Time on DS

July 2005

C Trecker, Azimuth Systems

Slide 7

doc.: IEEE 802.11-05/0757r0

Submission

Methodology• Force a Fast BSS Transition

– Base Mechanism– Pre-reservation

• Measure Fast BSS Handshake• Measure Fast BSS Transition on DS and

over the air• Iterate to allow for statistical aberrations• Use different data rates.• This allows for apples-to-apples

comparisons in FBT performance

July 2005

C Trecker, Azimuth Systems

Slide 8

doc.: IEEE 802.11-05/0757r0

Submission

Different ways to cause a fast-BSS Transition

• STA or AP initiated for reasons other than a degrading RF signal

• Rolling cart• Power off AP• “Cone of Silence”• RF Attenuators

– Open Air– Controlled Environment

July 2005

C Trecker, Azimuth Systems

Slide 9

doc.: IEEE 802.11-05/0757r0

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 2005

C Trecker, Azimuth Systems

Slide 10

doc.: IEEE 802.11-05/0757r0

Submission

Controlled Environment Topology

Combiner

WirelessSniffer

Combiner

WirelessSniffer

EthernetHub

Ethernet SnifferTraffic Source / Sink

July 2005

C Trecker, Azimuth Systems

Slide 11

doc.: IEEE 802.11-05/0757r0

Submission

Device Topology• 2 APs, 1 Client, 1 Server• 2 Wireless Protocol Analyzers

– Positioned between each AP and Client• 1 Ethernet Sniffer (assumes DS is ethernet)

• Packets Captured on:– Both AP Channels– Wired Ethernet on Server

• Traffic generated between Client and Server

July 2005

C Trecker, Azimuth Systems

Slide 12

doc.: IEEE 802.11-05/0757r0

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 2005

C Trecker, Azimuth Systems

Slide 13

doc.: IEEE 802.11-05/0757r0

Submission

Packet Capture• On Wired DS:

– Last Data Packet before Roam from AP1– First Data Packet after Roam from AP2

• On Wireless Protocol Analyzers– Above packets, plus:– 802.11 Management frames to measure FBT handshake.

July 2005

C Trecker, Azimuth Systems

Slide 14

doc.: IEEE 802.11-05/0757r0

Submission

Client Association:

First data packetafter roam

Stimulating a Transition

t PRE-RES t RETRY

Last data packetbefore roam

t FT

t ROAM

t DATA

AP1 AP2

max

min

Atten

AP1

AP2

Attenuator Sweep Time

Inter-Roam Delay

July 2005

C Trecker, Azimuth Systems

Slide 15

doc.: IEEE 802.11-05/0757r0

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

Tota

l Roa

m T

ime

(s)

July 2005

C Trecker, Azimuth Systems

Slide 16

doc.: IEEE 802.11-05/0757r0

Submission

Conclusion• Fast BSS Transition Performance metrics will

allow for valid comparisons between different implementation algorithms and vendor solutions

• Repeatability and flexibility of controlled environment is very beneficial

• Valid system FBT performance measurement will depend on statistical analysis