Having a set of walkie talkie radios as a kid was the ultimate in cool toys. Well, except for my TI-99/4A. Anyway, just like every new plaything, we found the limit of our new form of communication very quickly. Once we reached a three or four hundred yards away from each other we weren’t able to understand each other any longer. Bummer.
Those walkie-talkies are a great example of a nicely balanced link. They were identical; both had the same transmit power, antenna and receive sensitivity.
Transmit power is the amount of power that the radio chip actually produces. Power output from a Wi-Fi device is typically measured in milliwatts (mW) or dBm. A 200 mW (23dBm) transmitter is pretty decent in a Wi-Fi access point.
The antenna is responsible for taking the power from the chip and sending it out on to the air. Antennas typically provide gain is signal strength although they don’t have to. In the world of Wi-Fi, antennas are typically measured in dBi. Those nice rubber duck antennas that you see give about 3dBi of signal gain.
This is lesser known of the three factors in our link. Receive sensitivity is how well a device can hear. My wife repeatedly tells me that I’m a good listener. (“Sorry babe, what did you say?”) That means I must have good receive sensitivity. Receiver sensitivity is typically shown in an RSSI (Received Signal Strength Indicator) chart. This chart states at what signal level a certain data rate can be achieved. Here is a made up truncated chart:
-87dBm = 6Mbps
-84dBm = 18Mbps
-73dBm = 300Mbps
-85dBm = 6Mbps
-83dBm = 18Mbps
-70dBm – 300Mbps
Device #1 has better receive sensitivity because it requires less signal to achieve the same data rates as device #2. (keep in mind that these numbers are negative so the closer to zero is higher.)
An imbalanced link is when one device can hear better or transmit higher signal than the other. For example, let’s say that my set of walkie talkies had a “signal gain” dial on it. My brother sets his to 5 and I turn mine up to 11. Now I’m transmitting more power than him and we have an imbalanced link.
Is this link imbalance a problem? Before we answer that, let’s look at the link itself. What defines a good link with our walkie talkies? That’s pretty simple. If each party can understand the other, we have a good link. Oddly enough, this analog link is quite binary in nature. Either you can understand each other or you can’t.
When do you consider that the link failed? The link has failed when there is no longer bi-directional communication. The purpose of walkie talkies is to communicate with each other (two way) not like a radio station (one way). So, for the link to be considered broken, only ONE side of the link has to fail.
Back to the question. Is the link imbalance between our walkie talkies a problem? No, it actually isn’t. All it means is that at a certain distance, bi-directional communication will fail. The fact that one transmitter is higher than the other doesn’t actually matter. This is exactly what happens in a Wi-Fi network.
Have you ever stopped to think about what actually transpires in a Wi-Fi network when a client device can no longer communicate with an AP? I guarantee you, 99.999% of the time it’s ONE side of the link that fails first. If a device transmits and never receives an ACK, the link has failed.
Many a tech document has stated that if you want Wi-Fi to work right, you should lower the transmit power of the AP to the match that of the client. That is a terrible idea. Stream of consciousness as to why:
- The AP has much better receive sensitivity than your client. If you set the Tx power on the AP to 30mW equaling that of the client BUT the AP can hear 6dB better, you still have a seriously imbalanced link.
- Ah… now you are thinking this: If I set my Tx power of my AP to compensate for the client hearing better, now I have a balanced link. What that means is, you set the AP Tx power to 6dB higher than the Tx power of the client. Now, your AP would be transmitting at 120mW (6dB higher) and your client at 30mW. Now you have a balanced link! Perfect! Well, not really.
Bring on the meat!
Some vendors like to use dynamic (non static) antenna gain and / or transmit beam forming (TxBF) to get more signal to the client. But, if the client can’t talk any louder, does that actually improve anything? Read on!
Walkie talkie communication was quite simple. Again, we could either understand each other or we couldn’t. But, Wi-Fi brings in another factor that doesn’t exist in analog communication. Data rates.
Wi-Fi automatically adjusts data rate to accommodate the communication channel. If signals are high and everything is good, it transmits at high data rate. If there is noise and signals are weak, it will transmit at a low data rate. (There are a bunch or rates in between too) With Wi-Fi, signal equals speed and reliability. Unless you are at your maximum MCS (data rate) extra signal will improve your link speed and that equates to improved capacity and throughput.
So what happens if you have that evil unbalanced link and your AP sends higher signals than your client? Great! I’m digging that because it gives me increased data rates on ALL downlink data. Given that many networks have 80%+ of their data going to the clients from the AP, who wouldn’t want more signal, speed, capacity and throughput?