1. Synchronization of HCCA APs in OBSS
January 2011
doc.: IEEE /0xxx
January 2011
Synchronization of HCCA APs in OBSS
Date: 2011, January 18
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. January 2011
doc.: IEEE /0xxx
January 2011
Abstract
As noted by several commenter's on 164, the HCCA TX OP Advertisement scheme suffers from inefficiency due to "gaps" caused when TSs go away or new TSs cannot be scheduled because of fragmented allocations. In addition, because the APs time clocks are not synchronized, there is a potential clock drift problem that causes the allocated time slots to also drift. The scheme was based upon the 11s scheme but in that case, there was a synchronization scheme.
This presentation looks at the ‘drift problem’ and proposes to use the “adjust TSF to the fastest” scheme.
This presentation addresses CIDs 1159 and 1254 in LB 170
Graham Smith, DSP Group
Graham Smith, DSP Group

3. Drift TSF timer can have 0.01% error.
January 2011
Drift
TSF timer can have 0.01% error.
Therefore worse case is 20us per Beacon (100TUs)
OR 200us /second, 12ms per minute
Therefore relative drift is significant
Graham Smith, DSP Group

4. January 2011
The Drift Problem
Graham Smith, DSP Group

5. Synchronize the Beacons
January 2011
Synchronize the Beacons
APs can synchronize their Beacon timing to the slowest clock. This is straight out of 11s.
The basic mechanism is:
Establish the relative offset of the TSFs by noting the advertised TSF in the other AP Beacon and comparing to own TSF.
For each subsequent Beacon, measure the difference in the local TSF and the advertised TSF in the other AP (accounting for the offset)
If difference is positive, say N, then subtract the difference, N, to the TSF (or stay still for N)
If listening to two Beacons, then take the higher result and add that
Same as correcting on each one individually if they are seperated
Graham Smith, DSP Group

6. January 2011
Simulations
In these simulations the APs synchronize to the fastest TSF. Final proposal is to synchronize to the slowest
Note the permanent error which is the timing error per Beacon
Graham Smith, DSP Group

7. Proposal Add text along the lines as per this presentation
January 2011
Proposal
Add text along the lines as per this presentation
Use 11s text with changes from ‘mesh STA’ to “HCCA AP”
See 11/0020r1
Graham Smith, DSP Group