1. Greenfield VoIP Transmissions Cause False RADAR Triggers
September 2006
doc.: IEEE /1458r0
January 2008
Greenfield VoIP Transmissions Cause False RADAR Triggers
Date:
Authors:
Chan et al. (Cisco Systems)
Joonsuk Kim, Broadcom Corp.

2. Recap of previous investigations on this issue
January 2008
Recap of previous investigations on this issue
In LB 97 (Draft 1.0), there were CIDs which pointed out that GF transmissions can be falsely detected by legacy devices in the DFS band as radar
We performed experiments and presented a submission, 07/0329r2, in March 2007 (Orlando) to discuss the results
We showed that a widely used receiver hardware gave false positive radar detects for VoIP traffic using Greenfield mode
Strawpolls showed a significant fraction of the TGn Coex Ad Hoc members are concerned with this problem, but more investigations should be done to be certain
Subsequently, we performed a set of measurements with another legacy 11a receiver and presented them in submission 07/2849r0 in Nov 2007 (Atlanta)
Again, false positive radar detects are observed for VoIP traffic using Greenfield mode
Strawpoll showed an even more significant fraction of the TGn Coex Ad Hoc members – a majority – agreeing for a text change to address this
Chan et al. (Cisco Systems)

3. The GF-DFS problem seems a generic 802.11a problem
January 2008
The GF-DFS problem seems a generic a problem
Since then we’ve continued to perform testing with various 11a chipsets and vendor implementations
In our analysis, we also found that detection of the bin-5 radar profile which is part of the minimum present-day FCC DFS certification strongly resembles a “GF voice detector” – thus this is a widespread problem
In our measurements, our latest tests have shown at least two 11a chipsets and two vendors that would have falsing issues due to GF voice transmissions
With the same GF-DFS problem occurring on independently-implemented legacy 11a hardware, this is compelling evidence that GF-DFS is a serious 11n issue that we need to address
Chan et al. (Cisco Systems)

4. Recommendations: Preferred
January 2008
Recommendations:
802.11n should be changed to prevent GF’s potentially disruptive effects to legacy 11a devices in the DFS bands
There’re two options to solving this problem:
1. Prohibit Greenfield operations in DFS bands or
2. Define a suitable mechanism to prevent Greenfield operation in DFS bands in the presence of legacy 11a devices
Simple to implement since it reuses existing 11n schemes to signal when GF can be used.
Involves only a software upgrade/change.
More importantly, this mechanism doesn’t affect 11n GF evolution path, as 11a devices get phased out in the next few years, GF wouldn’t be prevented from use due to this prohibition.
True to the definition of having a “greenfield”.
Preferred
Chan et al. (Cisco Systems)

5. January 2008
Illustration of Option 2’s proposed mechanism: AP detects non-HT OBSS (1/4)
Operation on a DFS Band
Covered by proposed text changes in Slide
HT Greenfield Transmissions
HT Greenfield Transmissions
Beacon
HT Greenfield AP
Non-HT AP
HT Greenfield Clients
During operations or when establishing a BSS,
an HT Greenfield AP receives beacon from a
non-HT AP, thus detecting a non-HT OBSS.
Chan et al. (Cisco Systems)

6. January 2008
Illustration of Option 2’s proposed mechanism: AP detects non-HT OBSS (2/4)
Operation on a DFS Band
Covered by proposed text changes in Slide
HT Capabilities Info Field
Beacon
Greenfield bit:
HT Greenfield AP
Non-HT AP
HT Greenfield AP sets its Greenfield support bit from 1 to 0.
Chan et al. (Cisco Systems)

7. January 2008
Illustration of Option 2’s proposed mechanism: AP detects non-HT OBSS (3/4)
Operation on a DFS Band
Covered by proposed text changes in Slide
HT Mixed Mode Transmissions
HT Mixed Mode Transmissions
Beacon
HT Greenfield AP
Non-HT AP
HT Greenfield Clients
Greenfield transmissions are then suppressed across this BSS.
Non-HT OBSS is not disrupted by 11n BSS.
Chan et al. (Cisco Systems)

8. Covered by proposed text changes in Slide 13-14.
January 2008
Illustration of Option 2’s proposed mechanism: AP detects non-HT OBSS (4/4)
Operation on a DFS Band
Covered by proposed text changes in Slide
After waiting 30 min…
HT Capabilities Info Field
Greenfield bit:
HT Greenfield AP
Non-HT AP
If non-HT AP does not exist anymore, HT Greenfield AP can set its Greenfield support bit from 0 to 1 after thirty minutes
Chan et al. (Cisco Systems)

9. Covered by proposed text changes in Slide 13-14.
January 2008
Illustration of Option 2’s proposed mechanism: A client detects non-HT OBSS
Operation on a DFS Band
Covered by proposed text changes in Slide
HT Mixed Mode Transmissions
HT Greenfield Transmissions
HT Greenfield Clients
Beacon
Non-HT AP
HT Greenfield AP
Greenfield transmissions are also suppressed by individual STAs if they detect any non-HT OBSS. (Note scenario here, the HT GF AP is out of range from the non-HT AP.)
Chan et al. (Cisco Systems)

10. January 2008
Illustration of mechanism under consideration: Client notifies GF AP of non-HT OBSS (1/2)
Operation on a DFS Band
Not covered by proposed text changes in Slide
Should we consider this too??
HT Mixed Mode Transmissions
HT Greenfield Transmissions
HT Greenfield Clients
Beacon
Some action frame to notify detection of non-HT AP.
Non-HT AP
HT Greenfield AP
HT Greenfield Transmissions
HT GF clients also send some action frame to notify the HT GF AP of the non-HT OBSS.
HT Greenfield Clients
Chan et al. (Cisco Systems)

11. January 2008
Illustration of mechanism under consideration: Client notifies GF AP of non-HT OBSS (2/2)
Operation on a DFS Band
Not covered by proposed text changes in Slide
Should we consider this too??
HT Mixed Mode Transmissions
This protocol is more involved, as an action frame is created, etc.
HT Mixed Mode Transmissions
HT Greenfield Clients
Beacon
Some action frame to notify detection of non-HT AP.
Non-HT AP
HT Greenfield AP
HT Capabilities Info Field
HT Mixed Mode Transmissions
HT GF AP subsequently sets its Greenfield bit to 0 and suppress its BSS from using GF, for at least 30 min since the last non-HT OBSS notification was received or detected.
Greenfield bit:
HT Greenfield Clients
Chan et al. (Cisco Systems)

12. January 2008
The potential for false radar detects with GF is due to DFS requirements for wide pulse radar signals
Results from previous investigations lead us to postulate something fundamental must have been violated by GF transmissions…
It turns out, according to FCC Part 15 Subpart E (Appendix on DFS compliance), one of the waveforms required to be successfully detected looks exactly like typical VoIP traffic
This is the long-pulse radar or “bin-5” waveform:
[Excerpted from Appendix to FCC Part 15 Subpart E]
Chan et al. (Cisco Systems)

13. Option 2 only requires a minor change to the 11n draft
January 2008
Option 2 only requires a minor change to the 11n draft
TGn Editor: Editing instructions preceded by “TGn Editor” are instructions to the TGn editor to modify existing material in the TGn draft. As a result of adopting the changes, the TGn editor will execute the instructions rather than copy them to the TGn Draft.
TGn Editor: Please insert the following new subclause following subclause Channel management at the AP and in an IBSS, on page 213 of D3.02.
HT Greenfield transmissions in DFS bands
The requirements described in this subclause apply only when the HT STA is operating in a regulatory class subject to DFS procedures.
A Non-HT OBSS Scan operation is a passive or active scan of the channel on which a STA currently uses or intends to use. During a Non-HT OBSS Scan, the channel scan duration is a minimum of 200 TU when scanning passively and a minimum of 20 TU when scanning actively.
Before an HT STA starts a BSS supporting the HT Greenfield mode, it shall perform a Non-HT OBSS Scan to search for any existing non-HT BSSs.
Before an HT STA changes the Greenfield bit in the HT Capabilities Info field in a beacon or probe response from 0 to 1, the HT STA shall perform a Non-HT OBSS Scan to search for any existing non-HT BSSs.
Chan et al. (Cisco Systems)

14. January 2008
(Continued)
An HT STA shall set the Greenfield bit of its HT Capabilities Info field in a beacon or probe response to 0 to indicate no support of HT Greenfield mode when there is one or more non-HT BSS overlapping (a non-HT BSS may be detected by the reception of a Beacon where the supported rates only contain Clause 17 rates). The value of this bit can be changed after a minimum of 30 minutes have elapsed since the last detection of an overlapping non-HT BSS.
An HT STA shall not transmit PPDUs in the HT Greenfield format when there is one or more non-HT BSS overlapping (a non-HT BSS here may be detected by the reception of a Beacon where the supported rates only contain Clause 17 rates) for a minimum of 30 minutes since the last detection of an overlapping non-HT BSS.
<End of editing instructions.>
Chan et al. (Cisco Systems)

15. Option 2 solution resolves three comments:
January 2008
Option 2 solution resolves three comments:
LB 115 (Draft 3.0) comments:
CID
Commenter
Type of Comment
Part of No Vote
Clause
Comment
Proposed Change
Resolution
5123
Chan, Douglas T Y
9.13.3
Transmission of GF preambles in DFS bands can cause DFS false alarms on legacy STAs. Thre is nothing in the DFS regulations that indicate radar may be ignored if preceded by MAC protection. Therefore protection is ineffective for GF preambles in DFS bands.
Prohibit GF in DFS bands.
Accept; as in editor instructions in submission 08/0111r0.
5795
Stephenson, Dave
5363
Hart, Brian
Chan et al. (Cisco Systems)

16. January 2008
Strawpoll
Task Group n should design the specification to appropriately account for and prevent GF’s potential disruptive effects to legacy 11a devices in the DFS bands. Accept the resolutions proposed for CIDs 5123, 5363 and 5795 in 08/0111r0, i.e. the draft text changes with editor instructions on Slide of 08/0111r0.
Yes No
Abstain
Chan et al. (Cisco Systems)

17. January 2008
References
“Compliance Measurement Procedures for Unlicensed-national Information Infrastructure Devices Operating In The Mhz and Mhz Bands Incorporating Dynamic Frequency Selection”, Appendix to Revision of Parts 2 and 15 of the Commission’s Rules to Permit Unlicensed National Information Infrastructure (U-NII) devices in the 5 GHz band, FCC 06-96, June 30, 2006.
Submission 07/0329r2
Submission 07/2849r0
Chan et al. (Cisco Systems)

18. January 2008
Backup slides
Chan et al. (Cisco Systems)

19. The bin-5 algorithm explained
January 2008
The bin-5 algorithm explained
In order to detect this waveform, a typical bin-5 algorithm will look for:
Pairs (or triplets) of pulses between 50 to 100 usec wide with spacing of 1 to 2 ms between pulses
[Excerpted from Appendix to FCC Part 15 Subpart E]
Chan et al. (Cisco Systems)

20. The bin-5 algorithm and typical VoIP traffic
January 2008
The bin-5 algorithm and typical VoIP traffic
But this corresponds exactly to a typical VoIP transmission !
For example, a 230-byte client transmission (76 usec) followed ms later by the AP's 230-byte transmission (76 usec)
If the AP detects three or more such pairs in a sliding 12-second window, a radar detect will be triggered
Note: The ACKs do not affect this detection at all, since the "bin-5" algorithm only processes the usec pulses
So legacy receivers implementing the minimum requirements for DFS compliance would produce false positive radar detects whenever in the presence of GF VoIP transmissions
VoIP traffic pattern used in experiments
Chan et al. (Cisco Systems)

21. Detrimental consequences expected from GF on DFS Bands
January 2008
Detrimental consequences expected from GF on DFS Bands
Operations of legacy a networks which have no concept of Greenfield mode would be disrupted by their false detects from GF transmissions by moving to another channel each time
Many mesh network architectures use the 5 GHz band for backhaul
A single voice call using GF transmissions could bring down a mesh tree while it changes channel.
A small number of GF APs using efficient channel selection can totally occupy the 5 GHz band and cause a mesh network outage.
This type of behavior also facilitates possibilities of simple denial of service attacks
There is nothing in the DFS regulations that indicate radar may be ignored if preceded by MAC protection. Therefore protection is ineffective for GF preambles in DFS bands.
Chan et al. (Cisco Systems)

22. Excerpted from Nov 2007 (Atlanta) Coex Ad Hoc Minutes:
January 2008
Excerpted from Nov 2007 (Atlanta) Coex Ad Hoc Minutes:
Chan et al. (Cisco Systems)

23. Recap of previous investigations on this issue
January 2008
Recap of previous investigations on this issue
Chan et al. (Cisco Systems)

24. Recap of previous investigations on this issue
January 2008
Recap of previous investigations on this issue
GF transmission sent according to a typical VoIP packet stream
50 packet sequences at each power level …
Chan et al. (Cisco Systems)

25. Recap of previous investigations on this issue
January 2008
Recap of previous investigations on this issue
Chan et al. (Cisco Systems)