WPP Study Group Tutorial

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WPP Study Group Tutorial

• Group created at Vancouver meeting to create test
  definitions and methods for 802.11
• Presentations today will cover WPP from several
  points of view
• Large user
• IC vendor
• Testing community

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Agenda

• Bob Mandeville Iometrix
• Don Berry Microsoft
• Mike Wilhoyte Texas Instruments
• Kevin Karcz UNH-IOL
• Jason A. Trachewsky Broadcom
• Fanny Mlinarsky Azimuth Systems
• Round Table Discussion and QA

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802.11 Requirements forTesting Standards

• Bob Mandeville
• bob_at_iometrix.com

4
WPP

• What is the need for 802.11 metrics
• What problems will they help solve?
• Who will the primary users be?
• How do we go about creating new metrics for
  wireless?

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Two Approaches to CreatingTesting Standards

• IETF (BMWG)
• Based on two-step approach to definitions
• Terminology document (all relevant functional
  characteristics are defined)
• Methodology document
• This method is most appropriate for performance
  testing

• ATM Forum, Metro Ethernet Forum
• Based on ratified standards documents
• Each test definition is referenced to standards
  source text
• This method is most appropriate for conformance
  testing

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IETF BMWG Test Standards Templates

• Terminology Definition Template
• Term to be defined. (e.g., Latency)
• Definition The specific definition for the term.
  Discussion A brief discussion about the term,
  it's application and any restrictions on
  measurement procedures.
• Measurement units The units used to report
  measurements of this term, if applicable.
• Issues List of issues or conditions that effect
  this term.
• See Also List of other terms that are relevant
  to the discussion of this term.
• Methodology Definition Template
• Objectives
• Setup parameters
• Procedures
• Measurements
• Reporting formats

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Conformance OrientedTest Methods Template 1/2
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Conformance OrientedTest Methods Template 2/2
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Sample List of 802.11 Terms to be Defined by
Category

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Requirements forTesting Standards

• Roaming Test
• A practical example which shows poor performance
  related to a lack of test standards definitions
• To roam a 802.11 a/b/g device will
• disassociate from one AP
• search for a stronger RF signal from another AP
• then associate and authenticate with that A
• resume normal data transmission
• Roaming can fail due to
• transient RF conditions
• the time that APs and devices take to complete
  the four step roaming process

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Test Configuration
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Test Procedure

• Roaming Test recorded
• Total Roaming Time Decision Time Switch Over
  Time
• The Decision Time is the time it took the NIC to
  stop attempting to transmit packets to AP 1 after
  the attenuation of the RF signal
• The Switch Over Time is the time it took the NIC
  to complete its association with AP2 after it
  stopped attempting to transmit packets to AP 1
• During the Decision Time cards recognized the
  signal attenuation and invoked proprietary
  algorithms to adapt their rates to slower speeds.
• Switch Over Time ends when the NIC receives the
  APs acknowledgement to its association request.
• This time should only be recorded as valid if
  data traffic from the NIC to the AP successfully
  resumes transmission after association.

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Test Results
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(No Transcript)
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Test Conclusions

• Need to break out Decision Time and Switch over
  Time
• Switch Over Times are as low as 60 milliseconds
  and averages a little over 700 milliseconds over
  all the combinations excepting two outliers which
  took over 8 seconds.
• In the majority Decision Time is the largest
  contributor to the overall Roaming Times.
• Packet traces show that most implementations of
  the rate adaptation algorithms maintain
  transmission at the lowest 1 Mbps rate for
  several seconds after loss of RF signal has been
  detected.
• These algorithms will need to be revisited to
  deliver quality roaming.
• Test standards for measuring roaming times can
  make a significant contribution by aligning all
  vendors and users on a common set of definitions
• This applies to roaming but also to a large
  number of other undefined terms

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Challenges of Operating an Enterprise WLAN

• Don Berry
• Senior Wireless Network Engineer
• Microsoft Information Technology

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Microsofts CurrentWLAN Network

• 4495 Access Points
• 1 AP per 3500 sq/ft
• 15 Million sq/ft covered in 79 countries
• 70,000 users
• 500,000 802.1x authentications per day EAP-TLS
• Supports 5.5 and 11Mbps only

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Wireless Service Challenge

• What is Wireless Service?
• How is it measured?
• What factors impact Wireless Service?
• How do you improve Wireless Service?

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Wireless Service and Support

• Service Goals
• Make Wireless service equivalent to wired
• Offer unique mobile services
• Operational Goals
• Reduce operational costs
• Minimize support personal

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How can WPP Help?

• Produce criteria that reflect the client
  experience
• Offer information that can compare different
  environments Enterprise, SOHO, home

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Desired Outcome of WPPA Perspective From a Si
Provider

• Mike Wilhoyte
• Texas Instruments

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Key Issues We Face Relevant to WPP

• Supporting Customers w/ Custom Test Fixtures
• Are often uncontrolled and therefore
  repeatability is questionable
• May introduce unintentional impairments and
  therefore dont effectively isolate the feature
  under test
• May unintentionally push beyond the boundary of
  the specification
• May stress the system beyond what any user or
  evaluator will do
• May overlook other critical areas of system
  performance
• The complexity of the specification has grown
  since the days of 11b and more than ever,
  performance is setup dependent
• Are tests really apples-to-apples?

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These Issues Result in

• Confusion over unexpected test results
• Resource drain

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A Real Example Customer ACI Test Fixture
RF Shield Room non Anechoic
Over 30 Active APs
TCP/IP
People Observing the Results
Test Plot TCP/IP throughput with increasing
levels of interference from the SMIQ
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Issues With This ACI Test Fixture
Can you imagine trying to repeat any test result
from this fixture in YOUR lab?

• Metal walls in shield room producing multipath
  making the test results depend even more on the
  position of the laptop (in a fade?)
• People (2.4 GHz RF absorbers) in the room
• Over 30 APs active which may couple into the RF
  front-end (even though its cabled) of the test
  AP
• SMIQ produces a non-realistic signal since the
  carrier is always present even though it may be
  triggered externally
• There are ways around this
• The test AP is not isolated from the interference
  and its behavior will affect the test result of
  the DUT
• Rx performance in the same interference
• Deferral behavior in the Tx (CCA) is affected
• Rate adjustment behavior

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A Better ACI Test Fixture Testing the STA
Channel 6
DUT
AP
Atten
clp
Anechoic Chamber 4
Anechoic Chamber 3
30 dB
PA
PA isolates interfering network and is not
affected by traffic in chambers 3,4
Attenuator
clp
in
out
20 dB
pad
Interfering network swept on channels 1-11
pad
30 dB
AP
STA
Anechoic Chamber 1
Anechoic Chamber 2
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A Better ACI Test Fixture Testing a Network
(APSTA)
Channel 6
DUT
30 dB
Anechoic Chamber 3
PA
PA isolates interfering network and is not
affected by traffic in chamber 3
Attenuator
clp
in
out
20 dB
Interfering network swept on channels 1-11
30 dB
AP
STA
Anechoic Chamber 1
Anechoic Chamber 2
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Desirable Outcomes of WPP

• Develop a minimal set of test metrics that are
  relevant to key performance parameters such as
• Robustness
• Throughput/Capacity
• Range
• Develop a Set of Test Best Practices that
• Produce repeatable results
• Achieve the right balance between complexity and
  cost
• The industry will use

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UNH-IOL perspective on WLAN Performance testing

• Kevin Karcz
• March 15, 2004

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A Quick Background

• UNH-IOL Wireless Consortium primarily has focused
  on interoperability and conformance tests for
  802.11, not performance testing
• Have generated traffic loads to observe a DUTs
  behavior under stress, but not specifically to
  measure throughput or related parameters.
• However, QoS is inherently about providing
  performance while sharing limited resources
• Optimization of Throughput , Range, Delay
  Jitter
• Constrained by
• User resources CPU load, DC power
• Community resources Available spectrum,
  aggregate users

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Examples of performance tests

• PHY layer
• Multi-path fading channel emulation using a
  Spirent SR5500 fader.
• What fading models should be used?
• MAC layer
• Throughput varies with Traffic Generator used
• CPU load differs significantly for between
  different vendors. Much greater than CPU load
  for a typical Ethernet device.

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Clear methods of testing are needed

• As we start measuring more performance metrics
• Can each layer of the network be measured
  independently?
• Which metrics need to look at the interaction of
  multiple layers?
• Hassle of real world scenario testing vs. a PHY
  test mode?
• Black box testing requires DUT to authenticate
  and associate with test equipment and interact at
  the MAC layer, not just the PHY layer.

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Some gray areas of testing

• Throughput
• Is throughput measured as a MAC layer payload? At
  IP layer? TCP or UDP layer?
• One DUT may have better PER measurements at the
  PHY layer than a 2nd DUT, but may get worse
  throughput if its rate selection algorithm is
  poor.
• Difficult to maintain consistency in an open
  (uncontrolled) environment
• Can throughput be measured in a cabled
  environment without an antenna?
• What if the DUT has a phased array antenna?
• What if the device is mini-PCI and inherently has
  no antenna?
• Range test
• What if a higher TX level causes higher adjacent
  channel interference and brings the aggregate
  throughput down for a neighboring BSS?
• Power consumption
• Is this just the DC power drain at the cardbus
  card interface?
• Should CPU load be included if the DUT implements
  much of its MAC functionality on a host PC?
• Roaming
• Quickest time 1 STA, 2 APs on same channel
• More realistic AP reboots, Multiple STAs roam to
  new AP on new Channel

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Why WPP role is important to UNH-IOL?

• Open standards are desired for the basis of test
  suite development
• Defining test parameters and standardization of
  Test Scenarios makes comparison of Apples to
  Apples easier
• IEEE focuses on the technical content
• Our interest is the testing, not determining how
  results are utilized

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Why WPP should define the tests

• UNH-IOL follows IEEE PICS for test cases
• More detailed info for test results
• Cases PDA/laptop/AP weight test results
  differently

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Example criteria weighting
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Comments on Wireless LAN Performance Testing And
Prediction

• Jason A. Trachewsky
• Broadcom Corporation
• jat_at_broadcom.com

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Topics

• Test Categories for WPP
• Some Test Configurations

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Test Categories for WPP

• Deciding what parameters are to be considered is
  the challenge.
• How do we transform user perception of
  performance into a set of repeatably-measurable
  quantities?
• Throughput and Range (what environments?)
• Frame latency
• Visibility of APs

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Test Categories for WPP

• How do we transform user perception of
  performance into a set of repeatably-measurable
  quantities?
• Delays in association/authentication
• Host CPU utilization
• Ability to roam without loss of connections
• Etc.

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Test Categories for WPP

• Basic PHY/RF Measurements
• Transmitter Parameter Measurements
• TX EVM or Frame Error Rate (FER) with
  Golden/Calibrated Receiver
• Carrier suppression
• Carrier frequency settling

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Test Categories for WPP

• Receiver Parameter Measurements
• RX FER vs. input power
• Flat channel (controlled through cabled/shielded
  environment)
• Controlled frequency-selective channels (known
  multipath power-delay profile)
• Antenna measurements
• cable/feed losses (S11 and S21)
• gain vs. azimuth and elevation angle
• One can easily take a great receiver design and
  blow away all gains with a bad antenna or lossy
  feed!

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Test Categories for WPP

• MAC Layer Measurements
• rate adjustment behavior
• specific parameters? test conditions?
• association and roaming behavior
• specific parameters? test conditions?
• frame latency
• layer-2 throughput with encryption
• host CPU cycles consumed?

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Test Categories for WPP

• Layer-4 Measurements
• UDP frame loss rate and latency vs. received
  power
• flat or frequency-selective channels?
• TCP throughput vs. received power
• flat or frequency-selective channels?

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Test Categories for WPP

• Open-air Measurements
• Open-air measurements are always subject to
  imperfectly-known time-varying multipath
  power-delay profiles.
• There is substantial variation at 2.4 and 5.5 GHz
  over 10s of msec.

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Test Categories for WPP

• We have established that frequency-selectivity
  due to multipath can result in higher-loss
  channels having higher capacity than lower-loss
  channels.
• The capacity of the channel can vary rapidly.
• This is a more significant problem for systems
  like 802.11a and 802.11g which include a large
  number of possible rates to better approach the
  practical capacity.
• (The problem wont get easier for future WLAN
  standards.)

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Test Categories for WPP

• Open-air Measurements
• What can we do?
• Try to perform as many measurements as possible
  with cable networks.
• Perform open-air measurements in an area in which
  the distance between transmitter and receiver is
  small compared with the distance between either
  transmitter or receiver and any other object.
  I.e., strong LOS.
• Helpful but not sufficient, as even small
  reflections affect channel capacity.

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Test Categories for WPP

• Open-air Measurements
• What can we do?
• Give up and perform a large ensemble of
  measurements and gather statistics.

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Channel Measurement Block Diagram

• Scope provides 10Mhz reference clk for all
  systems
• 3 long interconnect cables connect tx and rx side
• Filt module includes LNA

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Time- and Frequency-Selective Fading
gt 10-dB change in received signal power in some
bins over 60 msec.
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Topics

• Test Categories for WPP
• Some Test Configurations

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System Block Diagram

• Main boardtest system
• Transmit test system

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Boardtest System
controller (attenmach)
DUT
mux-2
atten
REF
coupler
coupler
mux-3
SA
mux-6
power meter
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RF Loop Block Diagram
Does WPP specify RF test fixtures?
Does WPP specify fading channel emulators (no!)
or a set of fading channel profiles (maybe!).
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Multipath Channel 2
Example fixed multipath channel power-delay
profile.
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Comments on Wireless LAN Performance Testing And
Prediction

• Fanny Mlinarsky
• Azimuth Systems
• fanny_mlinarsky_at_azimuth.net

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Ethernet vs. WiFi
Greater protocol complexity more test metrics
Wired test metrics RFC 2285,2544,2889
Wireless test metrics
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Test Metrics
Packet forwarding
Security
Roaming
Behavioral
QoS
Rate adaptation
Encryption
WEP TKIP AES
Authentication
Offered Load
Packet Size
of Clients
of Power Save Clients
Load Of Assoc/ De-assoc/ Re-assoc
RTS/CTS
Fragmentation
EAP-TLS EAP-TTLS PEAP LEAP
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Forwarding Rate Measurement
Open air
Controlled RF
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Controlled Test Environment

• If measurements are not repeatable then the test
  is invalid
• Open air creates unpredictable test conditions
  due to interference and multipath
• Shielded and cabled test environment may be
  necessary for some measurements

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Summary

• The IT managers question How well do mobile
  computing solutions perform in the enterprise?
• The dilemma Standard wired benchmarking
  techniques wont give the right answer
• Verifying function and performance in the
  inherently unstable wireless space calls for a
  new methods metrics
• The answer New methods to test and measure every
  aspect of wireless protocols
• Wireless metrics outnumber traditional Ethernet
  metrics 51