Wifi transmission power?

[Ola Malmstrom]

I recently purchased an RT-AX92U to use as another Access Point. My main router is an RT-AX88U.

To my surprise, the transmit power on the AX88U is 14 dBm. On the AX92U it is 23 dBm. Both set to max power.

Why is the max power of those two routers so different?

I use the Analiti tool to gather information on my network. It seems to be a very useful tool. However I am suspicious on the transmit power figures. What do they really mean? Why are they not negative?

 

[L&LD]

Are they both on the same firmware level?

 

[ColinTaylor]

I presume you mean -14 dBm and -23 dBm? For a phone app to report such high power output you must be putting it right up to the router?

 

[Ola Malmstrom]

Are they both on the same firmware level?

No. The AX88U is on the latest Merlin FW. The AX92U is on the latest Asus version

 

[L&LD]

No, I mean, are they both on 386.xx or 388.xx (or something else...).

 

[Ola Malmstrom]

To ColinTaylor: No. It seems like the app on the phone somehow get information from the router and the APs. It doesn't matter how far away I am. The figures are not negative which is what I expected

No, I mean, are they both on 386.xx or 388.xx (or something else...).

The RT-AX88U is on Merlin 386.8. The RT-AX92U is on Asus 3.0.0.4.386_46061.

Note that the figures displayed in the app are not negative. When I set the AX-92U on max transmit power the app shows 23 dBm. When I put it on min it shows 13 dBm. The AX-88U shows up as 14 dBm on max. Both routers are on the 5 GHz network.

As far as I understand 23 dBm = 200 mW. 13 dBm = 20 mW. 14 dBm is 25 mW.

Could it really be that the max transmit power on the AX92U is 200 mW and on the AX88U only 25mW? I just wonder why the available transmit power on the AX88U seems to be only 25mW.

 

[L&LD]

Are the same Control Channels used on both routers?

(That app seems flakey).

 

[ColinTaylor]

To ColinTaylor: No. It seems like the app on the phone somehow get information from the router and the APs. It doesn't matter how far away I am. The figures are not negative which is what I expected

Ah, OK. It must be reading the TPC Report from the WiFi management frame.

Could it really be that the max transmit power on the AX92U is 200 mW and on the AX88U only 25mW? I just wonder why the available transmit power on the AX88U seems to be only 25mW.

Those numbers can be very misleading in my experience as you have to adjust for the number of streams. That said, there will also be differences between different bands and different channels.

 

[sfx2000]

TPC is client power control advised by the Access Point...

-TPC, Transmit Power Control, is actually a feature of 802.11h along with DFS
by which the access point can define local rules for maximum transmit power.
There are many reasons why this would be used.

One could be that the administrator wants to set another set of rules than
the regulatory domain maximum because of more specific local rules or
environment.

Another could be that the administrator knows it is a very dense Wi-Fi
deployment with a intense coverage : therfore APs set themselves to a
lower transmit power (thanks to the RRM algorithm) and TPC is a static
way to force clients to also lower their power and therefore lower
their coverage so that they do not disturb neighbor clients/APs that
are on the same channel.

 

[tgl]

Could it really be that the max transmit power on the AX92U is 200 mW and on the AX88U only 25mW?

Are they on the same channel? Do they both report the same country code? I'm fairly sure that 200mW is the legal power limit for some channels in some jurisdictions, and 25mW is the limit in others.

There's some fairly dense tables at Wikipedia that will tell you what the regs are for different channels in different places.

 

[Ola Malmstrom]

Thank you for some very interesting information!

I'm in Europe, Sweden. I have attached a similar table particularly for Sweden, Europe.

There are no restrictions for 200 mW / 23 dBm max power output on my main router.

The channel settings for my routers are
AX88U (router): 80 MHz, control channel 36, extension auto
AX92U (AP): 80 MHz, control channel 128, extension auto
AX58U (AP): 80 MHz, control channel 64, extension auto

I really have no complaints. The roaming works fine with the transmit power set to minimum on the APs.

But..... I would still like to understand why the current max power output of my main router is 25 mW (the AX88U), not 200 mW (the AX92U). Not that I really need it,

 

[ColinTaylor]

But..... I would still like to understand why the current max power output of my main router is 25 mW (the AX88U), not 200 mW (the AX92U). Not that I really need it,

As I said, those numbers are misleading. I must admit that even after looking at RF testing reports and comparing it to those numbers the precise mathematics is beyond me. The best I can work out is that the TPC number is the conducted power of a single antenna. So you need to add up the combined power of all active antennas and adjust for directional antenna gain. Only then would to have an EIRP number. So despite what that TPC number might suggest you are in fact going to get nearly 200mW EIRP on RLAN 1 and almost 1W EIRP on RLAN 2.

 

[sfx2000]

So despite what that TPC number might suggest you are in fact going to get nearly 200mW EIRP on RLAN 1 and almost 1W EIRP on RLAN 2.

Again - TPC is for the client power limits, not for the Access Point - see above

 

[ColinTaylor]

Again - TPC is for the client power limits, not for the Access Point - see above

Yes I know but:

TPC Report​

For stations to know how to tune transmission power, it helps to know the attenuation on the link. TPC Report information elements are included in several types of management frames, and include two one-byte fields (Figure 4-44). The first, the transmit power, is the transmit power of the frame containing the information element, in units of dBm. The second, the link margin, represents the number of decibels of safety that the station requires. Both are used by the station to adapt its transmission power, as described in Chapter 8.

The point being that the software that the OP is using appears (I'm guessing here) to be reporting this value. And this value exactly matches the output power value returned by access point's wireless driver.

 

[Ola Malmstrom]

Thank you Colin, sfx and tgl for an interesting discussion! I have certainly learnt something new here.

 

[tgl]

I'm in Europe, Sweden. I have attached a similar table particularly for Sweden, Europe.

Interesting, thanks! You'll notice that this table does show 25mW as the maximum power permitted in the RLAN 3 (UNII-3) band. RLAN 1 is 200mW, and RLAN 2 is 1000mW --- or at least that's the current regulations. I wonder if the router firmware knows that, though. Many jurisdictions have been opening up more spectrum and raising the power limits for WiFi recently, so it's possible that the AX88U is adhering to some older rules with lower power limits. (Are you running current firmware? Sadly, I remember seeing people whine on these forums about how slow ASUS is to absorb the results of regulation changes.)

And, as @ColinTaylor points out, there are multiple factors involved in deciding what's the "power output". I believe the channel bandwidth enters into it, as well as the number of antennas in the router and whether they have any directional gain. There may be less difference than you think between these routers when they are measured according to the way the regulations define power output.

 

[sfx2000]

wireless-regdb is what most devices use - it's updated regularly, but OEM's do need to update their versions from time to time...

Not sure how Asus does this for their builds - there can be regulatory issues where they may not want to recertify devices if channels/bands/limits change over time

kernel/git/sforshee/wireless-regdb.git - Wireless regulatory database for CRDA

git.kernel.org

 

[Ola Malmstrom]

Got an idea - changed the channel for the AX88U from 64 to 100.

The output power as reported jumped from 14 dBm (25mW) to 21 dBm (125 mW). So the AX88U obviously has an algoritm to adjust the power output based on the legal requirements. The performance also jumped about 200 Mbps - so I adjusted both APs to minimum to reduce interference.

The reason why I started this exercise was that I discovered a neighbor using the same channel as we. To reduce interference I changed channels without being aware of the implications on output power. Which made my test person (my wife) complain......