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I have a Cisco WAP4410N AP device.

Since yesterday, it is supposed to serve a network of about 15 (max 20) clients at the same time, but not all clients will be using bandwidth simultaneously, since it's a mixed network of notebooks/tablets/smartphones. It is configured to work in B/G mode (instead of default B/G/N) because it appeares to have a stronger signal that way.

Somewhere in settings (Wireless > Advanced Settings), there is a following screen:

Part of the administrative interface in question

Note the "Load Balancing" section above. Embedded help page says this:

Load Balancing

Load balancing enables the access point to reject any newer wireless client to associate when the utilization reaches the configured threshold. If you do not want it, select Disabled.

Administration manual makes it a little bit clearer what's the intended use for this feature:

Snippet from WAP4410N Administration Manual

First question: In which scenario would it make sense to load balance clients between SSID's emitted from same AP? From my understanding of wireless networks, a single AP would become saturated with max 25 clients, and in my mind, it is pointless to load balance on the same device, all while sharing same channel and BSSID. Please someone correct me if I'm wrong.

(I'll try to sneak in two more related question, if anyone is able to answer them, so I don't repeat similar questions.)

Second (sub)question: Manual says that "utilization" is actually current CPU utilization of the unit. Does it mean that if utilization is near 100% most of the time, unit is becoming saturated and is working sub-optimally? It's weird to me that a single AP unit (and a quite pricey one at that!) would get to 100% utilization with some 10-15 clients...?

Third (sub)question: Is capacity of AP to handle more clients going to improve when N-mode is disabled (the way I did), and AP is set to work in G (or mixed B/G) mode?

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The purpose of load balancing is not for a scenario where there's just one AP. It's intended for a scenario with multiple APs. When you set up multiple APs, you always put them not-too-far from each other. You want the APs to overlap, a little, to be able to hop between APs. You don't want one APs signal to completely die out before getting into the next APs coverage, because then you'd have disconnections when the first APs signal has already died out, but the next APs signal is not there yet, or too weak.

So, you have overlaps. This is where the load balancing comes in. If you're standing in a place where you're picking up signals from 2 APs, the device will naturally prefer the one with a stronger signal. But if that AP is already at it threshold, it will push the device to the next AP.

The purpose is to balance the load between AP, as to not overload them as devices, and also to balance the network throughput between router ports, giving everyone about the same speed

You COULD also use this threshold with just one AP, if you really want. Then the AP will just deny devices over its threshold. Not a smart idea in any case I can think of, but just said this to further bring out the idea of load balancing.

Also, it's not "load balancing between SSIDs" on the same AP. The load balancing is per SSID, between APs.

Hope all is clear.

Sorry, me not sure about the other 2 questions. Leaving that for others :)

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I don't know the device, but you propably don't load balance between ssid, but between accesspoints, so enabling it, having only one (reachable) accesspoint serving the ssid, will most likely bring up errors.

the client will always try to connect to the ap with best signal and strenght, so this ap can reject the connection to give load to another ap that doesn't have too many connections ...

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I haven't heard of this 25 limit before. Researched it a little now, couldn't find a mention about it. I did find (and seen myself) limits on devices, depending on whether they're home grade or business grade. Some home grade routers may have a limit as low as 15. But no inherent limit. Ah, yes, there is an inherent limit. It's 2007 connections ;) Without knowing the device's limit, consider its limit to be 50 connections.

So, after understanding load balancing, and when it is used, I think your question about load balancing based on CPU utilization should be answered as well.

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I think this would have been better as an additional note at the end of your other answer than as an altogether separate answer. – kasperd Dec 20 '15 at 10:37
@kasperd, agree. I've been using the various Stack Exchange sites for years, but I never wrote anything here. I'm new as an active user. Not familiar yet with the etiquette. I apologize. Thanks for pointing out. – Dr.Ping Dec 20 '15 at 10:40
You can always edit your answers and delete one of the two. – kasperd Dec 20 '15 at 10:50
@Dr.Ping there isn't an inherent limit on 802.11 protocols. The 25 limit is more of a practical limit of what an access point can handle in the real world. Even 25 concurrent clients seems too high to me. Wifi does not do well with multiple clients. – Cha0s Dec 20 '15 at 11:37
@Cha0s yes, known. The OP is not so much asking about best practices, but rather to understand the settings – Dr.Ping Dec 20 '15 at 11:39

Every radio that is in range of the AP and connected to the AP will increase the amount of collisions and also reduce the available bandwidth available to each client. Lets assume Wireless N 5Ghz you have 450mbps available, which translates to 93.75 mb/s and that is without interference and with 100% signal strenght with one single device connected to the AP

Divide the 97.75 mb/sec by the number of connected devices and there will be collisions so the effective maximum speed for each client will be reduced. Pretty soon you get to the point of diminishing returns

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In what system of units does 450 mbps translate to 93.75 mb/s? Baudot? – Michael Kjörling Dec 20 '15 at 14:45
@MichaelKjörling Since mbps means millibits per second and mb/s means millibits per second, I think the answer to your question is none. – kasperd Dec 20 '15 at 23:59
@kasperd Indeed, but I was trying to be generous. With a conversion factor of 4.8 between the numbers, the closest I could think of (with any reasonable interpretation of "mbps" and "mb/s", respectively) was 5-bit Baudot. – Michael Kjörling Dec 21 '15 at 8:13

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