More VOIP over WLAN: With low bitrates disabled, decrease the beacon interval for more responsive AP switching

2016 November 4
by Daniel Lakeland

It seems that beacons from your APs are how your wireless client (say a smartphone) determines which access point to connect to. Beacons obviously take up time that would otherwise be used for transmitting data, so that suggests reducing them in order to make more time available for transmission of useful stuff, but, with your low bitrate modes turned off, beacons will be transmitted at 6000 kbps or 12000 kbps and hence be pretty short compared to their duration if you were allowing 1000 kbps modes. So this suggests actually increasing their frequency so that it is quicker to switch between stations.

Typical settings are something like (in hostapd on OpenWRT)

    option beacon_int '100'

These counts are in 1.024 ms intervals, which which suggests that every 102.4 ms a beacon will be transmitted. Round off, around 100ms between beacons. So, if you're running around your house at 10mph you will travel about 1.5ft in 100ms which suggests your beacons could occur between longer intervals. But, it takes a while to re-associate with a new AP. Typical VOIP packets are sent at 20ms intervals, and this suggests that if you're dissociated from an AP for say 2 beacons, you're going to drop 10 packets, which is a noticeable audio delay.

Instead, let's drop the beacon interval to 50 and then if you're dissociated for 2 beacon intervals that's 100ms total and you drop about 5 packets, an amount that might be covered by typical jitter buffers in hard/soft phones.

I just literally ran around my house while talking to a test number that I have set up, the test number records for 30 seconds and then plays back the recording... with beacon=250 I definitely had little drop-outs in audio, with beacon=50 even running around my house to change position as fast as possible, no noticeable audio dropouts occurred.



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