1. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 1 Issues for Coexistence in 2.4 GHz ISM 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: 2006-12-06 Authors:

2. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 2 Abstract 40 MHz TX in the 2.4 GHz ISM considered harmful. There are no practical mechanisms for protecting Bluetooth and other IEEE 802 non-dot11 networks. Hidden-node and power-save operations pose intractable problems. Current proposals are worse than “nuke and pave over” as a solution.

3. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 3 Scope of the Problem Coexistence with different networks requires different approaches. –802.11b/g stations in overlapping BSS. –Overlapping non-802.11 networks, e.g. Bluetooth. Passive protection mechanisms can overcompensate for interference from other microwave emitters. –Microwave ovens, for example. User experience may provide incentive to abuse. –Moral hazards for equipment vendors and network operators. –May exacerbate rather than mitigate coexistence problems.

4. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 4 Separable Concerns How do HT stations know about other users in the secondary channel? What do HT stations do when they learn of other users of the secondary channel? How do other users of the secondary channel perform in the presence of any proposed coexistence mechanism as opposed to with non-HT stations?

5. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 5 Detecting Users of the Secondary Channel Unsolved problems abound. –A Bluetooth WPAN twenty meters away from the AP is too far away to hear short-range transmissions. Only local stations will switch to 20 MHz, while all the others will stay at 40 MHz. –A configurable threshhold may cause stations to ignore other users, e.g. a typical VoIP call falls below proposed minimum activity threshold. –Cooperation between co-located interfaces is no solution. –A properly power-managed station is dozing most of the time and will not listen for secondary channel activity.

6. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 6 What Happens When Users Collide? The proposed solutions may be worse than the problem. –Constraining to 20 MHz for configurable detection interval causes bizarre user experience. –Long periods of constrained channel width may appear like broken rate adaptation. Power cycling the AP appears to “fix” it (until the next time secondary channel activity is detected.) –Performance sensitive vendors may set activity threshold high enough that 20 MHz fallback never happens. –No requirement that HT stations check the secondary channel before resuming 40 MHz transmissions.

7. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 7 How Other Users Respond… HT stations will not detect most Bluetooth networks. –No 20 MHz fallback, except maybe at co-located interfaces, so they will have no protection. Informal experiments show Bluetooth is unusable in the presence of 40 MHz transmission. Power-managed stations will not detect overlapping 802.11b/g BSS stations. –They will come out of power-save mode to transmit 40 MHz and step all over the overlapping BSS in the secondary channel.

8. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 8 What Can Be Done? Require very low fixed activity thresholds and short detection intervals to 1) minimize false appearance of broken rate adaption, and 2) remove moral hazard in setting activity threshold too high. More aggressive rules about detection, e.g. require stations to detect Bluetooth explicitly. …AND… A mechanism for distributing information about detection of users in the secondary channel to all the stations in a BSS.

9. doc.: IEEE 802.11-06/1899r1 Submission December 2006 james woodyatt / Apple Computer, Inc.Slide 9 Conclusions The existing family of compromise proposals are ineffective, and an ineffective coexistence mechanism is worse than none at all. An effective coexistence mechanism is highly unlikely to achieve sufficient support from the membership. …THEREFORE… Removing 40 MHz transmissions from 2.4 GHz ISM band is the only practical compromise.