I have a very similar problem, at a small hotel. Two baseboxes (2.4GHz) were covering it just fine since Summer 2014. Two weeks ago, I upgraded them from 6.1X to the latest stable and shortly after, complaints started coming in about very low speeds. Indeed, I noticed high ping jitter and low speeds when many devices connect. One of the APs is performing slightly better than the other. I am still trying to figure out the reason. Problem is, I am not sure whether the upgrade caused the problem and I also do not remember what the previous version was (my bad, I know, but I did not really expect to downgrade with so many versions difference...)
Question: instead of completely rejecting poor signal clients, is there a way to use features/parameters such as frame-lifetime and others, to prevent them from bogging down the network?
Passionate about networks
Enthusiastic about Mikrotik
MTCNA | MTCRE | MTCINE
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