Why are AP transmit powers higher than device transmit powers? What is the design intent?

[snovvman]

When I look at AP FCC filings and their advertised specs, their transmit power typically range between 22 and 30 dBm. At the same time, the intel BE201's max is between 20 and 23 dBm, depending on frequency. I can imagine that phones and IoTs transmit at even lower power.

WiFi is a two-way communication. What's the use if the client can hear the AP but the AP can't hear the client at fringe distances? I understand that AP transmit powers can be turned down, but what is the technical intent for the APs, especially some "gaming" APs to wield so much transmit power? Is this where download speed gets higher than upload speed and the assumption is that many of the traffic is download rather than upload?

Thanks.

 

[tgl]

APs generally have better antennas than clients, simply because there's more space for an antenna. So that allows them to "hear" the clients better, and that allows for a difference in Tx power without changing available throughput.

Beyond that, I think there may be regulatory requirements that clients can't transmit at more than some-number-of-dB less than their AP. Without that, clients would have to independently implement every single local restriction on Tx power, which is not very practical.

But the real bottom line is that people who know what they're doing usually don't run their APs at max Tx power. That just encourages client devices to hang onto connections to APs that can't hear them very well, leading to poor performance.

 

[snovvman]

APs generally have better antennas than clients, simply because there's more space for an antenna. So that allows them to "hear" the clients better, and that allows for a difference in Tx power without changing available throughput.

Beyond that, I think there may be regulatory requirements that clients can't transmit at more than some-number-of-dB less than their AP. Without that, clients would have to independently implement every single local restriction on Tx power, which is not very practical.

But the real bottom line is that people who know what they're doing usually don't run their APs at max Tx power. That just encourages client devices to hang onto connections to APs that can't hear them very well, leading to poor performance.

I agree and understand your points. Yes, APs are purpose designed and Tx power needs to be turned to avoid sticky clients. What I am asking, though, is why are there APs with specs far beyond a typical client. Just because the client can hear the AP because it has powerful transmitters and well-tuned antennas, it does not mean that the client can transmit enough for the AP to hear.

 

[Tech9]

What I am asking, though, is why are there APs with specs far beyond a typical client.

Because most All-In-One home routers are designed to work as single AP with maximum coverage possible as first goal and quality of wireless service second. The create highly imbalanced links to clients in many cases, but are acceptable solution for cost, energy and space.

This is my Ubiquiti U6-Mesh AP options for Tx power:
Auto - system managed on start up, usually defaults on High
High - 26dBm (400mW), 31dBm EIRP
Medium - 18dBm (63mW), 23dBm EIRP
Low - 6dBm (4mW), 11dBm EIRP
Custom - 6-26dBm with 1dBm step (EIRP +5dBi antenna gain)

If the user has single AP only - it has to be on High for maximum coverage. No different than home AIO router. If more than one APs are used - the power has to be lower and closer to the client radios power. This creates more even coverage with balanced Tx/Rx links and better user experience. It comes with increased cost though, higher energy use and more space taken from greater number of devices.

 

[Tech9]

it does not mean that the client can transmit enough for the AP to hear

You are correct. I run perhaps unconventional for most SNB Forum users setup with 4x APs on low power and 4x 40MHz wide channel 5GHz radios in non-DFS. This creates a form of hardware MU-MIMO capable of serving 4x clients at the same time with aggregate wireless throughput up to 1.7Gbps and signal levels in most places -60dBm or better (average -55dBm). The range is excellent without using too much power, interference levels are lower translating into lower latency - user experience first network. Each client gets about 400-440Mbps throughput (AX 2-stream, 574Mbps link rate). I don't participate in Speed Test competitions.

 

[RMerlin]

Your clients are quite often battery-powered or small pocketable devices, so reduced power usage is more important for them.

 

[tgl]

If the user has single AP only - it has to be on High for maximum coverage.

Right. I made the same point in different words today over in the Ubiquiti forums: if you've only got one AP, you might as well set it on max Tx power. The clients might be able to reach it or not, but they have no alternative, and reducing the Tx power won't improve anything.

But as soon as you have more than one AP, you care about whether your clients will make intelligent decisions about which one to connect to. (And, for better or worse, this is the clients' decision not the APs'.) At that point you need to set the APs' Tx power so that further-away APs don't look like they have enough signal to meet the client's threshold of what is enough signal.

 

[Tech9]

One of my UniFi setups is actually UCG-Ultra gateway with single U6-Mesh access point. It works in ~900sqft apartment on High setting. I had an AIO router there, excellent Synology RT2600ac. The only replacement reason was software - integration in Site Manager.

 

[tgl]

What I am asking, though, is why are there APs with specs far beyond a typical client.

I did a little more research on my recollection that there might be regulatory limits, and confirmed that for the 6GHz band: clients are restricted to Tx power at most 6dB less than the AP they are connected to. You can find that in official FCC documents here or here. I did not find anything similar for the lower frequency bands, but maybe I just didn't look hard enough.

EDIT: reading closer, it seems that what these documents actually say is that the legal maximum Tx power for a client device is 6dB less than the legal maximum Tx power for its access point (thus, either 24dBm or 30dBm EIRP according to whether the AP is operating under "low-power" or "standard-power" rules). If the AP owner dials down the AP's Tx power to less than the legal limit, it doesn't seem that the clients have to follow suit.

 

[user114514]

Because of the Shannon-Hartley theorem.

As long as BSSs don't overlap, feel free to increase transmit power to maximize DL speed within regulatory limits. In the US, 5GHz is large enough to accommodate three non-overlapping 160MHz BSSs or seven non-overlapping 80MHz BSSs. Modern STAs are intelligent enough to quickly switch, while legacy devices can simply use 2.4GHz.