Let’s Talk Transmit Power. Let’s Talk EIRP

By Samuel Clements, Blog Contributor
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I distinctly recall the first time I logged into an Aruba controller. It was several years back and I believed that I 'simply' wanted to find out what the transmit power of an existing AP was. Knowing my way around an industry full of controllers, I was looking for the transmit power of the AP (usually shown in dBm) and was more than a little surprised when the information I found was not what I was expecting. You see, in Aruba-land, the default expression of power in the GUI of the WLC is in EIRP.


Now, before we go any further, it's worth painting a bit of a picture about what EIRP is, and where it's commonly used in the industry. When you talk about regulatory domain validation (here in the states, that's the FCC), they're concerned about the total output power of a complete solution. In the Wi-Fi world, this is output power from the APs radio plus the gain of the antenna. This two-part equation is very similar to the magnifying glass example. If you step outdoors, the power of the sun is likely not enough to hurt you too badly (at least short term), but if you use a magnifying glass to focus the suns output power, the resultant focus of energy is more than enough to start a fire! In this example, the APs radio is the sun, and the antenna is the magnifying glass - a completely passive component, but since it's an 'energy focuser' it's a critical piece to the total solution of regulatory domain validation. The FCC, in this case, is concerned about SAR (Specific Absorption Rate), or the focus of the magnifying glass, not the raw output power of the APs radio (in Aruba-speak, Conducted Power Set).

The reason this is important is that EIRP on an Aruba WLC is an expression of the total output power leaving the antenna of the AP. The reason I was confused was this - you can arrive at the same EIRP number in a variety of ways - by simply changing the antenna and the Conducted Power (transmit power) of the AP:

9dBm transmit power + 11dBi antenna = 20 dBm EIRP

18dBm transmit power + 2dBi antenna = 20 dBm EIRP

Now, don't get me wrong, EIRP is a very important number, especially from a regulatory domain compliance perspective (just ask the FCC!), but I was curious about its intended use for an average Wi-Fi administrator, especially since most vendors express output power as the energy leaving the radio (Conducted Power), not in EIRP. If you're not exceeding EIRP (and really, your WLC should make sure that's not occurring behind the scenes), what really is the use?

At Aruba Atmosphere 2018, I had a chance to sit down with Eric Johnson of Aruba Outdoor AP fame and chat about why this choice of EIRP in the Aruba WLC GUI. He was gracious enough to go 'on the record' with me and explained it as one primary benefit with two notable exceptions:

The Benefit

Since you're expressing the total output power of the solution, this is a representation of what the client will hear. If you're expressing just raw transmit power, this is not what is received at the client, much like the suns energy isn't what's reaching you (in the example above). Since the energy leaving the AP is the same in the two examples above (low antenna gain + high output power compared to high antenna gain + low output power), expressing the total energy output in EIRP makes sense if you're looking to understand what the energy is that will ultimately hit the client (and define your cell edge). In short, you're concerned about reaching the client (giving them bars on their Wi-Fi signal indicator) and this is the primary method of doing that.

Notable Exception #1

Since EIRP is Conducted Power + antenna gain, the real variance comes in when you change antennas in your deployment. It's easy enough to back out antenna gain from EIRP and get the transmit power (Conducted Power) of your radio, so why all the fuss? In the overwhelming majority of deployments, a typical Aruba customer standardizes on a single model of AP. These are most commonly integrated antenna APs and if you have 2,000 of the same model, once you figure out the gain of one, you have the gain of all of them. Some customers introduce some variance to this equation by having a model of outdoor APs, in which case the total volume of concern for those looking to know Conducted Power is two models (two different antenna gains).

Notable Exception #2

Since EIRP does not take into account the uplink (from the client to the AP), there is no way for EIRP to express the 'health potential' of the client uplink. Since antenna gain is reciprocal (the same properties that allow it to transmit well in one direction, also allow it to hear well in the same direction), many WLAN designers want to understand the gain of the antenna in order to best understand not only the AP to client link, but the client to AP link in return.

The upshot of these two exceptions is, if you're looking to know the Conducted Power of your Access Points radio, you should remember these AOS 8.0 commands to identify the gain of your APs antenna. By removing the gain from the EIRP shown in the GUI, you're left with Conducted Power - or what the rest of the world calls, transmit power.