I decided to write this blog because there appears to be a very common misunderstanding about how Wi-Fi works among end-users and even many network administrators as well. Instead of repeating myself, I can share this link with folks that need a little lesson in 802.11 operation.
Wi-Fi is does not work like AM/FM broadcast radio.
Well, in some ways it does, Wi-Fi radios transmit and receive radio frequency energy (RF) just like AM/FM stations do, but it’s operation is much more complex. If you are stuck in the AM/FM radio analogy, you’ll make several mistakes with Wi-Fi, such as:
- Coverage is considered, not capacity. Again, if Wi-Fi were a one-way radio broadcast like AM/FM radio, you’d only need to provide a strong “Wi-Fi signal” for everything to work well. This leads you down this next path.
- The “Wi-Fi signal” (using this term might be a tell that the person speaking is stuck in the AM/FM radio analogy) is too low, so crank up the AP’s transmit power to make it louder.
- Every problem is thought of as an infrastructure problem, client radios are not considered when troubleshooting.
- Getting hung up on the vendor’s name that is on the access point, without considering what is much more crucial, the overall design that went into the network.
How Wi-Fi Actually Works
Wi-Fi is not a one-way broadcast from AP to clients like AM/FM radio. This is not how Wi-Fi works:
It’s a network. The AP and clients connected to it must all be able to transmit and receive to and from each other, more like this:
Because they are all operating on the same channel, each client or AP must wait for the others to stop transmitting before it can transmit. It works just like Walkie Talkie radios. Only one radio can transmit at a time, everyone else must listen and wait. Additionally, they all need to be close enough to hear each other so that they do not transmit overtop of each other, causing interference that corrupts the communications. The channel they are using is what’s called a shared medium.
If they can’t all hear each other, they will transmit overtop of each other which results in corrupted frames (not packets, Wi-Fi operates at layer 2) that must be retransmitted. The bigger the cell, the worse this problem becomes (the hidden node problem). So when you crank up the transmit power of an AP to increase its coverage, you exacerbate this problem, because the AP is now serving clients that are further apart from one and other.
In many networks, the majority of Wi-Fi clients are smartphones with low-power radios and meager antennas. They already have difficulty hearing other clients further away in the cell. For networks like this, performance can be greatly improved by lowering the transmit power of the AP rather than increasing it.
Further, because the channel is a shared-medium, it has limited capacity. There is only so much available capacity to transmit in a single channel. Faster clients can transmit, well faster, and therefore use less of that capacity, known as airtime. Older or cheaper clients that are slower use more airtime to transmit the same amount of data. It doesn’t matter what vendor’s name is on the access point, airtime is airtime. Once a channel is saturated, that’s it. You can’t add more clients to it without leading to degraded performance. You can’t alter the laws of physics. At this point you need to add another AP to utilize the capacity of a different channel, or replace slow clients with faster ones.
Regardless, it’s worthwhile to intuitively understand the nature of Wi-Fi networks, so that these common pitfalls can be avoided. Many other Wi-Fi best practices that I haven’t outlined here stem from this foundational knowledge. Based on this, can you think of other things that might affect Wi-Fi performance?
Footnote
This is a simplification of 802.11 operation meant to give those new to the subject a casual understanding of how it works. Sometimes 802.11 frames are broadcast, one-way-only, from the AP to all clients in the network. Some management frames and broadcast frames from the wired network are broadcast this way. The important point to remember is that this is the exception, not the rule, and if all clients cannot hear each other, there is still the possibility that this broadcast traffic could be corrupted by another client transmitting over it.
Frame by Frame 
I tend to disagree with your assessment that it is better to turn DOWN the power on an AP. That lowers SNR at the client, lowering MCS, using more airtime and lowering capacity of the cell.
You MAY want to turn down power IF the downstream MCS is significantly higher than the upstream MCS. But if the two devices can maintain communication – the higher the Tx power the better to use less airtime.
But that is just my own humble opinion from doing client testing.
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Thanks for the comment Keith! I’m referring to turning them down from full power like many (all?) AP’s are set to by default. Not always by a lot. It always depends. Certainly there are tradeoffs. Are you designing WLAN’s with AP’s set to full Tx power?
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It depends on the brand. Ruckus AP’s tend to work best with max power… but then again they have adaptive antennas to cut down on causing neighbor CCI.
Other brands cause too much CCI when turned up to max Tx power. It depends on facility, building materials, channel width, etc.
Power is just one variable in the design equation. But I’m more a static power person and think tuning Tx power is part of the design cycle, rather than opting for a dynamic power solution. I’ve seen the auto-power algorithms fail too much, and they usually can’t turn OFF an AP – instead they merely turn down and AP – that tends to keep CCI too high and data rates depressed.
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Yeah, I’ve never had any luck with auto-power algorithms, or RRM in general, either. Static designs have always produced better results for me. What measures do you take to address hidden nodes?
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