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Analyzing the Beacon Collision Probability in Mesh Networks

This study examines the beacon collision problem in mesh networks, where beacons from hidden nodes can collide, causing peer link timeout and discovery difficulties. The study proposes possible solutions and asks for feedback on tolerable collision probabilities and the use of existing or new beacon frames.

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Analyzing the Beacon Collision Probability in Mesh Networks

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  1. Analyzing the Beacon Collision Probability in Mesh Networks Date: 2007-11-12 Authors: Michelle Gong (Intel)

  2. What is the beacon collision problem? • In a mesh network, each MP sends out a beacon periodically • When two MPs are hidden from each other, their beacons may collide • Depending on their clock drifts, their beacons may be colliding for a prolonged period of time, causing peer link timeout, discovery difficulties, and wasting medium time MP-A MP-B MP-C Michelle Gong (Intel)

  3. In a mesh network, what is the beacon collision probability? • Before we try to solve the beacon collision problem, let’s first examine whether it is indeed a problem Michelle Gong (Intel)

  4. The beacon collision problem is approximately modeled as the birthday paradox • This is a very rough model that intends to give us some intuition regarding the beacon collision problem • Two levels approximation are utilized • A beacon interval is divided into discrete time slots, each of which is twice the beacon duration • Taylor series approximation collision Beacon interval Michelle Gong (Intel)

  5. In a group of 23 or more randomly chosen people, there is more than 50% probability that some pair of them will have the same birthday. • Because the beacon interval is divided into n time slots, the probability of a beacon falling into any of the time slot is 1/n • Assuming there are k hidden MPs in a two-hop network, the probability of no more than one beacon falling into a time slot is: • The probability of beacon collision is therefore: Michelle Gong (Intel)

  6. The beacon collision probability can be high when the beacon interval is short and there are many hidden nodes • Beacon transmission time: 250us Michelle Gong (Intel)

  7. Increasing the beacon interval can significantly reduce the beacon collision probability Michelle Gong (Intel)

  8. The beacon collision probability is small when the beacon interval is 10 seconds Michelle Gong (Intel)

  9. The root causes for the beacon collision problem are hidden nodes, fixed delivery time, and broadcast • Hidden MPs: Hidden nodes are the result of the network topology. The hidden node problem cannot be solved without changing the network topology • Beacons are sent periodically at TBTTs: Replace the beacon with an action frame that does not contain timing information and is not sent at a fixed time • Only feasible for MPs that do not need 11s synchronization support and do not go into power save mode • Beacons are broadcast: Replace the beacon with an unicast frame • Only feasible for maintaining peer link connections • Only feasible for MPs that do not need 11s synchronization support and do not go into power save mode Michelle Gong (Intel)

  10. Beacon frames are essential for the operation of synchronization and power management • Beacon frames carry TSF timing information, which is the base for the synchronization operation • Beacon frames are sent at fixed time so that MPs in PS know when to wake up and exchange packets with each other • Many existing 802.11 devices including mesh APs already deployed today are not SW upgradeable to be 802.11s compliant if an action frame must contain the TSF timing information Michelle Gong (Intel)

  11. In summary, the beacon collision problem cannot be completely avoided but it can be mitigated • When the beacon interval is small, i.e. 100ms, the beacon collision probability can be high • Increasing the length of the beacon interval and reducing the size of the mesh network reduces the collision probability • The questions are: • Do you think the beacon collision probability is tolerable for your mesh networks? Is < 1%, < 0.1%, or < 0.01% tolerable? • Should we define a feature to mitigate this beacon collision problem or should we define a notification method when beacons start to collide (implementers can decide whether to act upon it or not)? • Do you think 11s should reuse the existing beacon frame type or define a new action frame to replace the beacon? Michelle Gong (Intel)

  12. Straw Poll 1 • Do you think the beacon collision probability is tolerable for your mesh networks? • > 1% is tolerable: • < 1% is tolerable: • < 0.1% is tolerable: • < 0.01% is tolerable: Michelle Gong (Intel)

  13. Straw Poll 2 • What should we do about this beacon collision problem? • Define a feature to mitigate this beacon collision problem • Define a notification method when beacons start to collide (implementers can decide whether to act upon it or not) • Do nothing Michelle Gong (Intel)

  14. Straw Poll 3 • Do you think 11s should reuse the existing beacon frame type or define a new action frame to replace the beacon? • Reuse the existing beacon frame type • Define a new action frame type Michelle Gong (Intel)

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