1. Common Mesh TSF Issues Date: 2007-09-12 Authors: September 2007
Month Year
doc.: IEEE yy/xxxxr0
September 2007
Common Mesh TSF Issues
Date:
Authors:
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
September 2007
Abstract
This presentation discusses the issues and resolutions regarding common mesh TSF.
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company
John Doe, Some Company

3. September 2007
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.
Zhen Xie, Atheros, et al.John Doe, Some Company

4. Issues With Distributed Synchronization
September 2007
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.
Zhen Xie, Atheros, et al.John Doe, Some Company

5. Issues With Time Base Propagation
September 2007
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.
Zhen Xie, Atheros, et al.John Doe, Some Company

6. September 2007
Resolutions
Define a framework to accommodate different synchronization modes, as well as vendor-specific synchronization modes.
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 synchronization mode” mode.
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.
Zhen Xie, Atheros, et al.John Doe, Some Company

7. Synchronization Mode Identifier
September 2007
Synchronization Mode Identifier
Octets:1 1 4 2 ID
Length
Version
Active Path Selection
Protocol Identifier
Metric Identifier
Congestion Control Mode Identifier
Channel
Precedence
Synchronization Mode Identifier
Mesh Capability
Mesh Configuration element
Synchronization mode identifier values
OUI
Value
Meaning
00-0F-AC
Peer-link synchronization mode
1-255
Reserved for future use
Vendor OUI
0-255
Vendor specific
An MP should request synchronization from a peer MP only if the peer MP supports synchronization and has a matching synchronization mode identifier.
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company

8. Peer Link Synchronization Mode
September 2007
Peer Link Synchronization Mode
MP1
MP2
MP3
An MP should maintains an offset to each of neighbors.
Neighbor Table of MP2
Neighbor MPs
Offset
MP1
Offset1
MP3
Offset3
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company

9. Peer Link Synchronization Mode, cont.
September 2007
Peer Link Synchronization Mode, cont.
MP1
Mesh Beacons of
MP1
MP2
Mesh Beacons of
MP2
Offset1
Mesh Beacons of
MP3
Offset3
MP3
Slide 9
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company

10. Peer Link Synchronization Mode, cont.
September 2007
Peer Link Synchronization Mode, cont.
It does NOT impact power save.
In current draft, an MP wakes up at TBTT of every neighbor, 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.
Slide 10
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company

11. September 2007
Summary
Define a synchronization mode identifier to accommodate different synchronization mode.
Define peer link synchronization mode.
Vendor-specific synchronization mode is allowed.
Power save and MDA still work without common mesh TSF.
Zhen Xie, Atheros, et al.
Zhen Xie, Atheros, et al.John Doe, Some Company

12. September 2007
Straw Poll #1
Should “Synchronization Mode Identifier” be included in the draft to allow different synchronization modes?
Zhen Xie, Atheros, et al.

13. September 2007
Straw Poll #2
Is there any interest to investigate in the direction of “Peer Link Synchronization Mode”?
Slide 13
Zhen Xie, Atheros, et al.

14. September 2007
Straw Poll #3
For simplicity, should references to “common mesh TSF” be removed from the draft, unless there’s a proven “in-band” algorithm to achieve such a common mesh TSF?
Slide 14
Zhen Xie, Atheros, et al.