Wasted Wi-Fi Q(-Tip)&(802.11)A: Transmit Power, Conducted Power and EIRP

Last week's blog about Wasted Wi-Fi prompted some questions about AP transmit power in the enterprise.  

Let's answer some of those questions by doing a little Q(-Tip)&(802.11)A, an exercise in which we watch a Q-Tip video before Answering a few questions about Wi-Fi.




You've seen the Q(-Tip), now on to the A(nswer)s:

Wasted Wi-Fi is all about APs and/or stations making inefficient use of a Wi-Fi channel.  When Wasted Wi-Fi happens, either data rates are lower than they should be, or Retry percentages are higher than they should be.

The Q becomes, where do equivalent isotropically radiated power (EIRP), transmit power and conducted power fit in?

The A is, higher data rates and lower Retry percentages tend to happen when signal-to-noise ratio (SNR) is higher, and higher EIRP/conducted power/transmit power leads to higher SNR.

In other words, if you talk louder, then people hear louder.  When people hear louder, they have a better chance of being able to make out what you say.

The next natural Q is, what's the difference between EIRP, conducted power and transmit power?

The A's:

-EIRP is the total output power of a Wi-Fi system (AP or client device), including its antenna & spatial multiplexing gains.  (For simplicity's sake, I am ignoring the possibility of using external AP antennas.)

-Conducted power is the output power of a Wi-Fi radio, only.  Antenna & spatial multiplexing gains are not included. 

-Transmit power is the same as conducted power.

...unless, you use Aruba/HPE APs.

Which, of course, leads to the follow-up Q, how does Aruba/HPE define EIRP, conducted power and transmit power?

From an Aruba/HPE perspective, the A's are:

-EIRP is defined the same way everybody else defines it: the total output power of a Wi-Fi system, including its antenna and spatial multiplexing gains.

-Conducted power is also defined the same way everybody else defines it: the output power of a Wi-Fi radio, only.

-Transmit power is the same as EIRP.  (Starting with ArubaOS 8.x)

It's an odd situation, to say the least.  There's one company -- a very important company in the world of Wi-Fi -- that uses "transmit power" as a synonym for EIRP, while everybody else -- Cisco, CWNP, the FCC, Aerohive/Extreme, Mist/Juniper, integrators, instructors, conference speakers -- uses "transmit power" as a synonym for conducted power.

While your humble blogger tends to believe in a person's or company's right to do what they want to do, Aruba/HPE's decision to flip-flop the meaning of "transmit power" does complicate things.

Which leads us to the next Q, what's the bottom line on using the terms EIRP, conducted power and transmit power?

To which there are two A's:

If you are an Aruba/HPE user, then use "conducted power" when referring to the output power of the AP's or client's Wi-Fi radio, and use "EIRP" when referring to the total output power of an AP or client, including the AP's or client's antenna & spatial multiplexing gain.

If you are not an Aruba/HPE user, then use "transmit power" when referring to the radio output power, and "EIRP" when referring to the total system power, including antenna & spatial multiplexing gain.

Now, back to the topic of Wasted Wi-Fi, where our next Q is, why does transmit power (or "conducted power", in Aruba/HPE parlance) need to be equal between AP and client to avoid Wasted Wi-Fi, rather than EIRP?

The A is, because antennas affect both transmission and reception, while transmit power/conducted power only affects transmission.


An AP's antenna affects the AP's ability to transmit and receive.  Therefore, the AP's antenna also affects the CLIENT's ability to transmit and receive.  In other words, the AP's antenna has an equal effect on Wi-Fi data moving both downlink from and uplink to the AP.

An AP radio's transmit power/conducted power does NOT have the ability to affect the AP's ability to receive.  Higher AP transmit power/conducted power may mean that the AP's downlink data gets received with a higher SNR, but it means absolutely nothing on the uplink.  Only the client radio's transmit power/conducted power can be increased to create a higher SNR for uplink data.

If avoiding Wasted Wi-Fi is the goal, then symmetrical uplink and downlink communication needs to happen.  Symmetrical EIRP doesn't matter.  Symmetrical transmit power/conducted power does.  To create a high-performing Wi-Fi network that stays resilient when put under stress, AP transmit power/conducted power must be configured to the same (or at least a similar) level as client transmit power/conducted power.

***

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Twitter: @benmiller

ben_miller at icloud dot com

 

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