October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Measuring IEEE 802.11 Performance in Airplanes

Scott MarstonCabin Systems Tech Center

Boeing Commercial Airplanesscott.e.marston@boeing.com

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Outline

• What’s unique about WLAN in airplanes?• Types of Airplane WLAN Applications

– Open - passenger connectivity– Closed - crew, entertainment, system monitoring, etc.

• Problems with today’s measurements– Signal Strength, SNR, EVM, don’t correlate to

throughput or stability– PER does correlate, but only after link has degraded

• Measurement Requests to 802.11k TG• Measurement Access

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

What’s Unique About WLANs in Airplanes?

• The network moves among different regulatory domains while operating

• The airplane has its own regulatory requirements (EMI, potential failure modes, etc.)

• Very high potential client density– Multiple NICs in every passenger seat– Adjacent Channel Interference– Dynamic RF changes have correspondingly high impact

• Highly reflective physical environment• Potential mix of “open” and “closed” systems• No IT personnel on board to manage or troubleshoot

network problems

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Closed vs. Open Onboard WLAN

• Closed – Airline controls all radios– In Flight Entertainment data distribution to

seat-mounted equipment– Crew Information Services to mobile crew

terminals on board and on ramp– System Status / System Health Monitoring

• Open – Passengers carry on their own radios– Internet / Telephone Connectivity

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Today’s Standard Measurements• Dynamic Frequency Selection and Transient

Power Control are good, in theory– Haven’t had the opportunity to test them yet

• Signal Strength (RSSI), Noise Level– No correlation with link performance– Not repeatable or linear in controlled conditions

• Error Vector Magnitude (EVM)– Only marginally better quality indicator than

RSSI/noise• Packet Error Rate

– Does sometimes seem to correlate to link bit rate, but only after the link has degraded

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

0.0

10.0

20.0

30.0

40.0

50.0

0 500 1000 1500 2000 2500

Data Size, Bytes

Eff

ectiv

e R

ate,

Mb/

s

Multicast, No Backoff

Basic, No Backoff

Multicast w/ Avg Backoff

Basic w/ Avg Backoff

RTS, No Backoff

RTS w/ Avg Backoff •Transmitter Power,•Antenna Pattern/Orientation,•Clear/Blocked Line-of-Sight,•Absorbing/Reflecting Materials (e.g., Bodies in Seats),•Co-Channel Interference,

…all impact Signal to Noise (Interference) Ratio and Media

Access Coordination

Measured Airplane

Performance

802.11a Performance in an AirplaneTheoretical vs. Measured

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

RSSI Variation with NIC and Orientation

5

3

4

7

81

2

6

-55 dBm-62 dBm

-52 dBm

-64 dBm

-60 dBm

-48 dBm

-62 dBm-68 dBm

• Anechoic Chamber• 3 meters distance• Three Identical Client Cards

Orientation NIC #30 NIC #2 NIC #16

1 -60 -50 -55

2 -68 -60 -58

3 -54 -47 -55

4 -65 -60 -68

5 -62 -53 -65

6 -48 -45 -50

7 -62 -60 -65

8 -64 -63 -70

12 dB variation under identical conditions

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Single NIC – RSSI vs. Distance

OrientationNIC #002

@ 3 mNIC #002

@ 6 mDelta,

3 m – 6 m

1 -48 -56 8

2 -58 -60 2

3 -53 -57 4

4 -53 -66 13

5 -53 -52 -1

6 -47 -56 9

7 -52 -64 12

8 -55 -74 19

5

3

4

7

81

2

6

Theoretically expect 6 dB power delta with twice the distance

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

802.11b RSSI in a 747

-120

-100

-80

-60

-40

-20

0

1 2 3 4 5 9 10 11 12 26 27 28 29 30 31 36 37 38 39 40 41 53 54 55 56 57 58 59 60 61 62 63 64 65 66

Seat Row

dB

m

Signal(dBm)

Noise(dBm)

Even Client Signal

Even Client Noise

Odd Client Signal

Odd Client Noise

• RF Signal & Noise strength measured two ways:

-Airmagnet site survey tool

-Client software on individual laptops

AirmagnetSignal

ClientSignal

ClientNoise

AirmagnetNoise

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

High Client Density Implies Adjacent Channels in Close Proximity

802.11a Adjacent Channel Performance can be marginal…

Transmit Spectrum of Two Different 802.11a Chipsets(Actual captured signals shifted in frequency and amplitude

to match single mask)

-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

5190 5200 5210 5220 5230 5240 5250

Frequency, MHz

Pe

ak

Ho

ld, d

Bm

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Coverage Area vs. Interference Area

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 4020 41 42

10 11 12

10 11 12 13

10 11 12 13

2 3 4 5 6

Co-Channel Interference Boundary

Co-Channel Interference Area

DesiredCoverage

Area

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Airplane Channel FadingTest Point 8

-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

5.15 5.17 5.19 5.21 5.23 5.25 5.27 5.29 5.31 5.33 5.35

Frequency, GHz

Am

plitu

de,

dBm

TP8CHR_C_Hb.txt TP8CHR_C_Vb.txt

-31 dB Calibration Offset Applied

36 40 44 48 52 56 60 64

Channel Fade of 10 dB or more is common in airplanes

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

EVM vs. Link Performance

According to Vector Signal Analyzer, EVM for this burst was -14.056 dB, which corresponds to 18 Mb/sec link. But burst was transmitted and received successfully at 54 Mb/sec.

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Measurement Wish List• Network Regulatory Domain Awareness

– Local Spectrum Usage Limits– Local Human Exposure Limits

• What would it take to add “Commercial Airplane” to the list of country codes?

• Adjacent Channel Interference Indication• Meaningful Signal Strength (instantaneous and over

time)• Meaningful Signal Quality (instantaneous and over time)• Why was current link rate selected? (e.g., 36 Mb vs. 54)• Which diversity antenna was used to Transmit or

Receive a given frame?

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Measurement Wish List (cont’d)

• Ideally, there would be a way to tell clients that are not Associated in the Service Set to stay out of certain channels, and/or turn power down

• How about specifying some channel models, including a narrowbody and widebody airplane!

October 2003

Scott Marston, Boeing

doc.: IEEE 802.11-03/810r0

Submission

Measurement Access

• Most onboard network management is either completely local or completely remote:– Local: Maintenance personnel solve problems on

ground between flights– Remote: NOC accesses airplane via satellite or

gatelink

• Aviation Industry already working on network MIB access protocols (SNMP/RMON, etc.) through Airlines Electronic Engineering Committee (AEEC) ARINC standards– http://www.arinc.com/aeec/