September 2004

Mike Goettemoeller, Texas Instruments

Slide 1

doc.: IEEE 802.11-04/1162r0

Submission

Systems Supporting Devices Under Test (DUTs)

Mike GoettemoellerTexas Instruments – Santa Rosa, CA

September 2004

Mike Goettemoeller, Texas Instruments

Slide 2

doc.: IEEE 802.11-04/1162r0

Submission

Topics• Overview• Test platform

– Issues– Mitigating Issues

• Conclusion

September 2004

Mike Goettemoeller, Texas Instruments

Slide 3

doc.: IEEE 802.11-04/1162r0

Submission

Overview• The systems supporting the device under test for STAs –

PCs, laptops, PDAs, VoIP/cell phones, etc can and will drastically affect the performance of the wireless device under test.

• Hardware Issues– Voltage regulation of device – the ability to supply the proper

current necessary under heavy traffic conditions– Interference from the system supporting the DUT– Insufficient memory for buffering packets in high throughput

situations• Software Issues

– For PCs and laptops, operating system must be able to support requirements (minimum RAM, CPU, etc) of the DUT

September 2004

Mike Goettemoeller, Texas Instruments

Slide 4

doc.: IEEE 802.11-04/1162r0

Submission

How does one isolate the DUT from the system supporting it?

• Assuring voltage regulation of system• Assuring interference from system is

negligible• Assuring device has proper memory and

correct OS to support device

September 2004

Mike Goettemoeller, Texas Instruments

Slide 5

doc.: IEEE 802.11-04/1162r0

Submission

Are there cases where it is not possible to isolate the system from

the device it supports?• For embedded devices such as cell

phones/VoIP phones, it may be impossible to isolate the wireless subsystem without re-designing the entire system.

September 2004

Mike Goettemoeller, Texas Instruments

Slide 6

doc.: IEEE 802.11-04/1162r0

Submission

Will all tests run on all platforms?

• Forwarding rate tests may not be possible given the lack of sufficient memory in embedded devices.

September 2004

Mike Goettemoeller, Texas Instruments

Slide 7

doc.: IEEE 802.11-04/1162r0

Submission

Assuring Voltage Regulation

• Device performance can degrade if the regulation of the supporting system dips.

• Two ways to mitigate– Use an extender card (for pcmcia, mPCI) or cut

supply lines (USB cable) and provide power via an external supply known capable of supplying proper regulation

– Test system under heavy load to verify it can supply correct regulation

September 2004

Mike Goettemoeller, Texas Instruments

Slide 8

doc.: IEEE 802.11-04/1162r0

Submission

Example of Good/Bad RegulationPCMCIA Supply Voltage vs. Current Consumption

2.700

2.800

2.900

3.000

3.100

3.200

3.300

3.400

0 100 200 300 400 500 600 700 800 900

Current Consumption [mA]

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September 2004

Mike Goettemoeller, Texas Instruments

Slide 9

doc.: IEEE 802.11-04/1162r0

Submission

Testing System for Voltage Regulation Under Load

• To test the system under load, insert an extender card for pcmcia/mPCI slot or expose cable wires for USB to gain access to the supply lines.

• Step a potentiometer load across the terminals and measure the voltage/current supplied by the system.

• Once the specification current for the interface (mPCI, PCI, USB) has been met, stop the test.

• If the voltage regulation degrades with an increase in current, the system supply may be incapable of giving reliable results and an external supply should be used for wireless testing.

DC

Laptop/PC/?

+V-Voltmeter

September 2004

Mike Goettemoeller, Texas Instruments

Slide 10

doc.: IEEE 802.11-04/1162r0

Submission

Alternative Method• Insert the DUT in an extender

card and put the device in near continuous transmit (streaming UDP). Transmission typically requires more current due to the power amplifier.

• Measure the voltage across the extender card supplying the DUT. If it dips greatly, it may be insufficient for the DUT.

• A similar method can be utilized for testing any other interface.

Laptop

Voltmeter+

-

September 2004

Mike Goettemoeller, Texas Instruments

Slide 11

doc.: IEEE 802.11-04/1162r0

Submission

Can the System Generate RF Interference?

• The systems supporting the devices under test can generate RF interference which can inadvertently affect wireless performance

• This interference can come from embedded 802.11 devices (mPCI in laptops), Bluetooth devices, or the laptop subsystems themselves.

• These RF ‘spurs’ can cause false packet detects, trigger CCA unnecessarily, etc

September 2004

Mike Goettemoeller, Texas Instruments

Slide 12

doc.: IEEE 802.11-04/1162r0

Submission

Laptop Interference Example

September 2004

Mike Goettemoeller, Texas Instruments

Slide 13

doc.: IEEE 802.11-04/1162r0

Submission

Laptop Interference Example (II)

September 2004

Mike Goettemoeller, Texas Instruments

Slide 14

doc.: IEEE 802.11-04/1162r0

Submission

Finding RF Noise Sources

• The obvious first step is to disable any RF devices - 802.11, Bluetooth, etc.

• The next step is to determine if there is any RF interference being generated by the laptop, PC, etc.

• Using a spectrum analyzer, it is easy to scan the air surrounding the system and look for interference.

September 2004

Mike Goettemoeller, Texas Instruments

Slide 15

doc.: IEEE 802.11-04/1162r0

Submission

Mitigating RF Noise Sources

• The easiest method is to separate the DUT from the system – for USB, this is easy. A cable can be used isolating the DUT from the system.

• For pcmcia/mPCI, an extender card does an ok job to mitigate the effects. The level of interference decreases greatly with distance.

September 2004

Mike Goettemoeller, Texas Instruments

Slide 16

doc.: IEEE 802.11-04/1162r0

Submission

Conclusion

• In wireless testing, the systems supporting the DUTS can and do greatly affect the performance of the wireless devices.

• In non-embedded devices, it is necessary to mitigate the effects of the system to truly judge the wireless performance.