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Issues with Common Mesh TSF and Proposed Synchronization Framework

This presentation discusses the issues with maintaining a common mesh TSF in a multi-hop mesh network and proposes a new synchronization framework and peer link offset synchronization protocol.

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Issues with Common Mesh TSF and Proposed Synchronization Framework

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  1. Common Mesh TSF Issues Date: 2007-11-09 Authors: Zhen Xie, Atheros, et al.

  2. Abstract This presentation discusses the issues regarding common mesh TSF and proposes a new synchronization framework and peer link offset synchronization protocol. Zhen Xie, Atheros, et al.

  3. Problem Statement Current draft states that every MP should attempt to maintain a common mesh TSF. In order to “maintain” it, how do we achieve this common mesh TSF in the first place? Distributed network synchronization mechanism. Define a synchronization master and propagate the time base through the mesh network. Neither mechanism scales well in a multi-hop mesh network. Zhen Xie, Atheros, et al.

  4. Issues With Distributed Synchronization No master clock or external reference time. All MPs cooperate to achieve the same time base. IBSS synchronization is not designed for multi-hop environment. Time to convergence could be long Ripple effect of re-synchronization Some part of the network could be out-of-sync. Is the network still functional? Zhen Xie, Atheros, et al.

  5. Issues With Time Base Propagation How to choose a synchronization master? It could be elected. A distributed problem again. MPP could be the master. But what if there’re more than one MPP’s or no MPP at all? Issues with NTP-like protocols A hierarchical tree structure has to be formed first. Frequent re-forming the tree will cause network out of sync. How to reliably transmit the correct time information. Propagation delay, mobility, etc. It takes time to propagate the time information. Some part of the network could be out-of-sync. Is the network still functional? Zhen Xie, Atheros, et al.

  6. Resolutions Define a framework to accommodate different synchronization protocols, as well as vendor-specific synchronization protocols. Vendor could use “out-of-band” mechanisms to achieve common mesh TSF. The “out-of-band” mechanism is out of scope. Define a “Peer link offset synchronization” protocol. Instead of maintaining an offset to the common mesh TSF, an MP should maintain an offset for each peer. An offset should be a per-peer or per-link property. No absolute need to have common mesh TSF. Zhen Xie, Atheros, et al.

  7. Synchronization Protocol Element A Null protocol indicates the MP does not support synchronization. Zhen Xie, Atheros, et al.

  8. Synchronization Framework Synchronization is optional. An MP may support multiple synchronization protocols. An MP shall have only one synchronization protocol active at a time. MPs use “Synchronization Protocol” element in Beacon and Probe Response frames to announce active synchronization protocol. November 2007 Slide 8 Zhen Xie, Atheros, et al.

  9. Peer Link Offset Protocol MP1 MP2 MP3 An MP should maintains an offset to each of neighbors. Zhen Xie, Atheros, et al.

  10. Peer Link Offset Protocol, cont. November 2007 MP1 Mesh Beacons of MP1 MP2 Mesh Beacons of MP2 Offset1 Mesh Beacons of MP3 Offset3 MP3 Slide 10 Zhen Xie, Atheros, et al.

  11. Peer Link Offset Protocol, cont. It does NOT impact power save. An MP should wake up at TBTT of its peer MPs, regardless of which time base to use. An MP can achieve it by keeping track of offset of every neighbor. MDA works too. An MP needs to advertise MDAOP times and neighbor MDAOP interfering times in terms of its own time base. An MP receiving such times should translate them into its own time base by applying the offset. November 2007 Slide 11 Zhen Xie, Atheros, et al.

  12. Summary Define a synchronization framework to accommodate different synchronization mode. Define peer link offset synchronization protocol. Vendor-specific synchronization mode is allowed. Power save and MDA still work without common mesh TSF. Zhen Xie, Atheros, et al.

  13. Link to normative text 11-07-2757-00-000s-mesh-synchronization-normative-text.doc Zhen Xie, Atheros, et al.

  14. Motion To adopt the submission contained in document 11-07/2757r0 and instruct the editor to incorporate the changes into the draft. November 2007 Slide 14 Zhen Xie, Atheros, et al.

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