Question Abnormal access point behavior

Sep 3, 2020
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Hi!

I've recently noticed stability and throughput issues on otherwise reliable wi-fi network and managed to track them down to the 2.4GHz signal from an access point. Problem is, this is likely not the usual "2.4GHz is just bad" issue. On one access point I can get 150~200mbps within line of sight, while on the other I'm barely getting 40mbps on a good day, under the same conditions.

My network consists of the ISP fiber box, a MitraStar GPT-2541GNAC (FCC ID ANATEL 03307-15-08916) and a D-Link DIR-850L (FCC ID KA2IR850LA1). The fiber box is actually a pretty capable device compared to most consumer-level stuff, so I use it as the network's router, with the DIR-850L set up as an access point. Both devices are 802.11b/g/n rated for 300mbps on the 2.4GHz frequency. I know that getting anywhere near 300mbps on a 2.4GHz network is unrealistic, but the same access point that seemingly can't go over 40mbps used to deliver ~150mbps.

My first thought was the usual suspicion that 2.4GHz signals get a lot of interference, so I tested with both access points next to each other and still one would deliver 150~200mbps while the other would barely reach 40mbps. I tried switching channels but it made no difference. Choosing between 20MHz and 40MHz channel bandwidth also didn't change anything.

The next suspicion was that there could be a problem with the (wired) connection between access point and router, but the 5GHz signal from the access point works normally, and I'm getting full gigabit rates between local devices through the lan ports.

Then I tried some older devices I had lying around, and all of them had the same problem. I can get "normal" link speeds on the 2.4GHz band through the router itself, but any other device serving as an access point gets stuck at 30~40mbps. Again, 5GHz and wired through these access points work as expected. One of the devices I tried was a 802.11n repeater and the same problem occurs when repeating the router's 2.4GHz signal: 150~200mbps when connected directly to the router's 2.4GHz, but only 30~40mbps when connected to any other 2.4GHz station on the network.

Some other likely unrelated things that I tested were switching QoS on or off and forcing 802.11n instead of combined g/n. Neither made a difference, so I'm kind of out of options. I'm afraid to buy a new router or access point and end up with the same problem, and I'm also forced to have a working 2.4GHz signal due to the walls of my apartment, which are either reinforced concrete when they serve a structural purpose, or brick and mortar when they don't, so 5GHz can't even get to the next room. Any ideas would be greatly appreciated.

Sorry for the long wall of text and thanks in advance for your help!
 
Sep 3, 2020
2
0
10
0
UPDATE

After some more testing I was able to identify the problem. Even though I had disabled the 5GHz network on it, the ISP fiber box was still transmitting a 5GHz signal alongside the 2.4GHz one with the same SSID, and any 5GHz-capable devices were connecting to it and getting the full 5GHz speeds even though it was supposed to be a 2.4-only signal.
 

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