madwifi has a feature called 'staggered beacons' where beacons for each vap are distributed evenly across the beacon interval. When 'staggered beacons' is not enabled the driver sends all beacons (and any associated power-save buffered frames) quickly one after another at each beacon interval.
staggered beacons:
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non-staggered beacons:
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This is currently unimplemented in dadwifi (in dadwifi beacons always are sent rapidly one after another). Support for staggered beacons should be added to dadwifi.