Sticky Clients and Minimum Basic Rate

A client walks between two access points. It only starts looking for a better AP when its signal drops below −72 dBm, so it can cling to a distant AP at a low data rate while a much better one is right next to it. See how AP transmit power and the Minimum Basic Rate (MBR) move the point where it finally roams.

Minimum Basic Rate (MBR)
Client spatial streams
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Dashed circle: where the AP's signal falls to −72 dBm (the client starts probing beyond it). Solid circle: the MBR edge, beyond which the client can no longer decode that AP's beacons. The shaded band is the sticky zone on the walk from AP1 to AP2: the client is still on AP1 although AP2 is stronger. Hatched areas, if any, can't decode either AP's beacons. Zoom out to see the full circles beyond the walls.

Roams from AP1 to AP2 at
Sticky zone (walking AP1 → AP2)
Average data rate on the walk
Extra airtime used

Walking from AP1 to AP2

What's happening

Roaming events

    Why clients stick

    Roaming is the client's decision. Most clients only start scanning for another AP when the current one drops below a threshold, here −72 dBm, and then only move if the new AP is clearly better (the roam margin).

    Above −72 dBm the client doesn't even look, however strong the other AP is. With high AP power, −72 dBm is reached far past the midpoint, so the client drags a slow connection deep into the next AP's cell.

    What MBR does

    The AP sends beacons and probe responses at the Minimum Basic Rate. A higher MBR needs a stronger signal to decode (24 Mbps ≈ −74 dBm, 36 ≈ −70, 48 ≈ −66). Beyond that edge the client stops hearing the AP's beacons and has to move on.

    That only helps if the MBR edge comes before the client's own −72 dBm probing point. With a −72 dBm client, MBR 24 changes nothing; MBR 36 or 48 does.

    Don't compare the MBR with the data rate. Beacons are legacy single-stream frames (48 Mbps is 64-QAM 2/3), while the data rate is 802.11ax with one or two streams. A client can lose 48 Mbps beacons while it still sends data at 103 Mbps: that is MCS 4 over two streams, 16-QAM 3/4, a more robust modulation than the beacon's. What matters is the modulation each frame needs, not the Mbps.

    What actually fixes it

    Lower AP power pulls each AP's −72 dBm edge back towards the midpoint, so clients start looking at the right place. A higher MBR then puts a hard edge on each cell, and removes the slowest data rates that sticky clients would otherwise use.

    Go too far and you create gaps where no AP can be decoded. Watch the MBR circles: they must still overlap.

    Notes and assumptions

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