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

我清楚地记得我第一次登录Aruba控制器。几年前，我相信我只是想找出现有AP的发射功率是什么。知道自己在一个充满控制器的行业中，我一直在寻找AP的发射功率（通常在DBM中显示），当我发现的信息不是我所期望的信息时，我感到有些惊讶。您会看到，在Aruba-land，WLC的GUI中的权力默认表达在EIRP中。

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发射功率 + 2DBI天线= 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?

在2018年Aruba Altersere，我有机会与Aruba Outdoor AP的Eric Johnson坐下来，并谈论为什么在Aruba WLC GUI中选择EIRP。他足够亲切，可以与我一起“记录”，并将其解释为两个显着例外的主要好处：

The Benefit

由于您要表达解决方案的总输出功率，因此这是客户所听到的内容的表示。如果您只是表达原始传输电源，这不是客户收到的，就像太阳的能量不是接触到您的东西（在上面的示例中）。由于离开AP的能量在上面的两个示例中是相同的（与高天线增益 +低输出功率相比，低天线增益 +高输出功率），如果您想了解什么，则表示EIRP中的总能量输出是有道理的能量最终将击中客户（并定义您的单元边缘）。简而言之，您担心与客户联系（在其Wi-Fi信号指示器上给他们酒吧），这是这样做的主要方法。

Notable Exception #1

自从附近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

自从附近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.