1. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 1 Resolutions to 20/40 MHz Coexistence in 2.4 GHz Issues Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair stuart@ok-brit.com as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at.http:// ieee802.org/guides/bylaws/sb-bylaws.pdf stuart@ok-brit.compatcom@ieee.org Date: 2007-05-10 Authors:

2. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 2 Abstract High level overview of 20/40 MHz coexistence in 2.4 GHz and resolutions.

3. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 3 Issues Scan and detection on power up Legacy BSS with periods of only beacon traffic Behavior in overlapping 20/40 HT BSS Scanning responsibility Clarification of requirements in 5 GHz

4. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 4 Scan and Detection on Power Up Scenario: Existing Legacy BSS with data traffic, and new HT 20/40 OBSS added with partial overlap on the secondary channel. 2412 241724222427243224372442244724522457246224672472 CH1CH6CH11 2400 2483.5 CH3CH2CH4CH5CH7CH8CH9CH10 Secondary Ch 5Primary Ch 1 Legacy OBSS Ch 6 Possible Behavior: If the AP does a scan on power-up (not clearly specified in LB 2.0), then it would detect the Legacy OBSS, and stay in 20 MHz mode, until the data traffic on the Legacy OBSS stops for at least 30-minutes, as detected by one or more 40 MHz STA scans. If the AP does not scan on power-up, then the Legacy OBSS performance would be impacted for at least 30 minutes until the 40 MHz STA’s in the HT 20/40 BSS detect the Legacy OBSS.

5. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 5 Resolution in 07/YYYY Modified text clearly mandates in 11.9.8.3 overlapping BSS scanning at immediately before 20/40 BSS startup –“An FC-HT-AP-19 may set the Secondary Channel Offset field to a non- zero value in transmitted HT Information elements beginning at BSS startup provided that two sequential overlapping BSS scan operations are performed immediately before BSS startup, and that no BSS width trigger events are detected during those scans.” –“After BSS startup, the AP shall perform at least one overlapping BSS scan of the channels either itself or through its associated STAs, before making a transition from a 20 MHz BSS to a 20/40 MHz BSS.” The detection of legacy data traffic has been changed to beacon detection The scan interval have been made into a MIB variable with a default value of 60 seconds

6. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 6 Legacy BSS with periods of only beacon traffic Scenario: Existing legacy OBSS with beacons only, and a new HT 20/40 network is added that has partial overlap on the secondary channel. 2412 241724222427243224372442244724522457246224672472 CH1CH6CH11 2400 2483.5 CH3CH2CH4CH5CH7CH8CH9CH10 Secondary Ch 5Primary Ch 1 Legacy OBSS Ch 6 Possible Behavior: The HT 20/40 network will operate in 40 MHz mode, and when the Legacy OBSS begins transmitting data on the Legacy BSS, the performance of the Legacy OBSS will be significantly impacted for roughly the first 30 minutes, or until the 40MHz STA’s in the HT 20/40 BSS detect the traffic on the Legacy OBSS, and the HT 20/40 BSS reverts back to 20 MHz mode For example: Every night, your neighbor’s HT network will go to 40MHz mode (as your network is only beaconing), and every morning, your legacy network will have a severe performance penalty for the first 30 minutes of operation.

7. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 7 Resolution in 07/YYYY The detection of legacy data traffic has been changed to beacon –9.20.4: “The following events are defined to be BSS width trigger events: –a) on any 40 MHz affected channel, reception of a Beacon frame that does not contain an HT Capabilities element” The scan interval have been made into a MIB variable with a default value of 60 seconds

8. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 8 Behavior in overlapping 20/40 HT BSS 2412 241724222427243224372442244724522457246224672472 CH1CH6CH11 2400 2483.5 CH3CH2CH4CH5CH7CH8CH9CH10 Secondary Ch 6Primary Ch 2 Secondary Ch 7Primary Ch 11 Possible Behavior: The behavior is non-deterministic. Depending on which of the two OBSS’s detect the overlapping secondary channel first, one or both of the HT OBSS’s will drop to 20 MHz. When one of the networks switches to 20Mhz, the overlap would no longer exist and it may stay in 40Mhz mode. Depending on the data traffic on the secondary channels, the two BSS’s could oscillate between 20 and 20/40 roughly every hour

9. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 9 Resolution in 07/YYYY New scanning text in 11.9.8.3 remedies this scenario The second HT BSS that starts up would scan and not be able to establish a 20/40 BSS –Has the option to start one, but only if completely aligned with existing one

10. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 10 Scanning Responsibility It wasn’t clear who had responsibility for scanning: –40 MHz capable STAs? –All associated STAs? –40 MHz capable STAs that indicate use of 40 MHz? Resolution: –New definition for forty megahertz capable high throughput stations (FC- HT-STA & FC-HT-STA-19 & FC-HT-STA-17) FC-HT-STA: an HT-STA that included a non-zero value in the Supported Channel Width Set field (indicating its capability to operate on a 40 MHz channel) of its most recent transmission of a frame containing an HT Capabilities Information element –In 11.15.1a “An overlapping BSS scan operation is defined as a passive or active scan of all 40 MHz affected channels when the scanning STA is an FC-HT-STA-19.” –& “An FC-HT-STA-19 shall perform at least one OBSS scan every dot11BSSWidthTriggerScanInterval seconds…”

11. doc.: IEEE 802.11-07/0623r0 Submission May 2007 Eldad Perahia, Intel CorporationSlide 11 Clarification of requirements in 5 GHz It wasn’t clear whether scanning and detection and reporting of intolerance was required in 5 GHz Resolution: –The 20/40 scanning and reporting mechanisms may be used in 5GHz, but 20/40 MHz operation has fewer restrictions in 5GHz –9.20.3a: “An FC-HT-STA-17 shall set the Forty MHz Intolerant bit to 0 in transmitted HT Capabilities elements and HT Information fields in frames that use either a unicast or broadcast/multicast Address1 value “ –11.15.1a: “FC-HT-STA-17 are not required to perform overlapping BSS scan operations.” –11.9.8.3: “An FC-HT-AP-17 may start a 20/40 MHz BSS on a channel if a Beacon has been detected on the intended secondary channel during the scanning described in this subclause.”