I'm trying to figure this one out and I'm a bit confused. We've got a WiFi access point in a specific room in the office where we have about 3-4 people working and we'll see a large fluctuation in the overall Transmit Rate as perceived by the client. The trick to this whole thing is that the Signal or Noise don't fluctuate at the same time -- the SNR is almost always around 45 dBm.

We have up to 4 15" Retina MacBook Pro (Mid-2014) at any given time on this one AP. The AP is a Meraki MR18. We have disabled 2.4 GHz due to excessive channel usage from other APs in and out of the local office network. It's fixed on channel 120 on the 5 GHz band, which is showing a very low channel utilization (less than 10%).

The transmit rate will fluctuate drastically between an average of 120 Mbps, down to around 8 Mbps. During the fluctuation, there is no noticeable drop in signal, or gain in noise; as far as we can see, there is very little (if any) change in the SNR. The most we'll see is a shift in SNR from 46 dBm to 40 dBm.

In most cases, the dropout only lasts for up to 15-seconds before the Transmit rate jumps back to the 120 Mbps average.

We believe this loss of transmit rate is causing dropouts in our VoIP system (VERY import) and we're getting disconnected from certain network resources. The system dropouts only occur when we see the transmit rate drop to the <10 Mbps range -- there are no system disconnects if we attach to the network via ethernet.

The average RSSI value is around -48 dBm and average Noise value is around -95 dBm.

Does anyone have any insight as to what would cause a drop in Transmit rate, but no change in either Signal or Noise?

EDIT: I've changed the channel from 120 to 140. It's now overlapping 136-144 (due to wide-band 5GHZ). I'm testing it today for the dropouts.

EDIT2: After sitting on channel 140 for a day or two and only seeing maybe 80-140 Mbps in the transmit rate, I moved it to channel 161 and our transmit rated jumped to about 300 Mbps! So far it's not dropped any clients for a couple of days so that's great news!

Thanks for the tips on the DFS channels, the particular AP allows you to choose if you want to use them or not.

  • I'm curious to know if changing the channel made a difference. – Spiff Mar 19 '15 at 17:47

"Fixed on channel 120" sounds pretty weird for North America.

First, that's a DFS channel (Dynamic Frequency Selection; a method to avoid interfering with radar installations), so you're not supposed to be able to "fix" and AP to that channel. You can suggest it try channel 120 to start, but if it detects a radar energy pulse, it has to change to another channel where no radar energy pulses are detected.

Second, channels 120, 124, and 128 are also TDWR (Terminal Doppler Weather Radar; used by airports) channels, and aren't supposed to be available for use on consumer gear. Those channels are only for professionally-installed gear, and the professional installers are supposed to be trained to see if the installation is within X miles of an airport using TDWR, and if so, look up what frequencies that particular airport's TDWR system uses, and make sure to select a channel that doesn't overlap with the nearby airport's TDWR.

Your profile says you're in Austin TX, and Austin doesn't have a TDWR-using airport as far as I can tell. But if you've actually moved to the Dallas/Fort Worth metroplex or the Houston area, you might be running afoul of a TDWR airport.

Anyway, this may all be a red herring, it just seemed weird. I guess you could try changing your channel to a non-TDWR channel and see if it makes any difference.

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