Transcript September 2004 - Mentor

September 2004
doc.: IEEE 802.11-04/1162r0
Systems Supporting Devices Under
Test (DUTs)
Mike Goettemoeller
Texas Instruments – Santa Rosa, CA
Submission
Slide 1
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
Topics
• Overview
• Test platform
– Issues
– Mitigating Issues
• Conclusion
Submission
Slide 2
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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
Submission
Slide 3
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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
Submission
Slide 4
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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 redesigning the entire system.
Submission
Slide 5
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
Will all tests run on all platforms?
• Forwarding rate tests may not be possible
given the lack of sufficient memory in
embedded devices.
Submission
Slide 6
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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
Submission
Slide 7
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
Example of Good/Bad Regulation
PCMCIA Supply Voltage vs. Current Consumption
3.400
3.300
CB Power supply voltage [V]
3.200
a
b
c
d
3.100
e
f
g
h
3.000
i
j
k
2.900
l
m
n
o
2.800
p
q
r
2.700
0
100
200
300
400
500
600
700
800
900
Current Consumption [mA]
Submission
Slide 8
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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.
Submission
Laptop/PC/?
Voltmeter
Slide 9
+
V
-
DC
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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.
Submission
Slide 10
+
Laptop
Voltmeter
-
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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
Submission
Slide 11
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
Laptop Interference Example
Submission
Slide 12
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
Laptop Interference Example (II)
Submission
Slide 13
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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.
Submission
Slide 14
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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.
Submission
Slide 15
Mike Goettemoeller, Texas Instruments
September 2004
doc.: IEEE 802.11-04/1162r0
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.
Submission
Slide 16
Mike Goettemoeller, Texas Instruments