General BusinessTechnology

How Fast Will It Go?

By April 1, 2019 No Comments
man on cell phone

“How fast will it go?”

This is the second-most-popular question I am asked when talking with individuals who are new to Wi-Fi technology. (If you haven’t read my blog on THE most frequently-asked question, go check it out!). I am always hopeful to have the ability to answer this question before the sale to set proper expectations. However, I’ll sometimes get that frantic post-sales call, text, or email with something along the lines of, “I’m not getting anywhere close to 1.3 Gbps or 1.7 Gbps. What did I do wrong?!”

My answer: “It depends.”

I get it – it’s the most frustrating answer in the industry. However, this prompts a conversation based around mcsindex.com and explaining that the PHY rate is a starting point and many things draw down on that number.

In most cases, I tend to deploy in 20 MHz or 40 MHz wide channels.  This is because I have a limited number of channels to work with, and I want to only use each channel once.  Now the channels can be reused, but I have to make sure the AP using that channel can’t hear another AP using that same channel.  If it does, they will each pause giving the other a chance to use the channel and significantly cut your throughput.

By statically assigning a smaller channel width, I am putting in a self-imposed throughput limit. That 1.3 Gbps for a three-stream AP running 80 MHz-wide channels won’t be capped at 600 Mbps @ 40 MHz wide channels or 300 Mbps @ 20 MHz wide channels.

As a point of reference, how many streams does your device support? Streams is the number of links that your device can communicate on simultaneously. Most phones support one or two streams, and laptops will support two or three. In the case of a three-stream AP that can support 1.3 Gbps, it will also require a device that supports all three streams to reach the highest speeds possible. Using 80 MHz wide channels, a three-stream device will have a PHY rate of 1.3 Gbps. A two-stream device will have a PHY rate of 867 Mbps, and a single-stream device will have a PHY rate of 433 Mbps.

So now let’s apply both lessons into one example.  Let’s say the installing engineer decides to deploy at 20 MHz wide channels so they can have the maximum number of channels to reuse, and the device they are testing with only supports two streams.  Even though the AP has a PHY rate of 1.3 Gbps, the new PHY rate based on this configuration is 173.3.

Notice how I keep using the term PHY rate?  That is because this is the highest theoretical number that could be achievable.  In the real world, we have to contend with things like the fact that we will want to transmit and receive data.  The PHY rate is based on traffic only going one direction.

We also have to contend with overhead – all the extra packets that are sent in addition to your data to coordinate the communication over the air.

Distance from the AP and noise level are also two factors to consider. The further you get from the AP, the less-clear the signal is.  Additionally, competing signals from either other Wi-Fi devices or devices like baby monitors that aren’t Wi-Fi enabled but use the same frequency to communicate will create noise that the client and AP must contend with. This will result in the client and access point using simpler codes to communicate resulting in slower overall throughput.

If you are looking at mcsindex.com, there is an index number of 0-9 on the right side of the chart.  In order to hit the highest data rates, you need to have an MCS index of 9.  This means that the AP and client have a really good connection. As obstacles like distance and noise begin to interfere with that connection, the MCS index lowers therefore lowering your PHY rate.

Now, enterprise-level solutions will have features set to optimize these conditions as much as possible.  For example, the APs will have the ability to self-organize and automatically adjust power levels and channel numbers to avoid two APs being able to hear each other on the same channel.

Another feature that helps optimize these (and similar) scenarios is the ability to select which antenna on an access point communicates with which clients.  By using different antenna patterns, we can optimize the client experience resulting in a higher speeds.

Keep in mind, the items we’ve talked about here are NOT unique to any one manufacturer of Wi-Fi equipment.  Everybody must work with these variables.  However, if you want a good way to determine how well a Wi-Fi solution performs, set up testing that creates the same environment and test multiple manufacturers. One of the key differences between manufacturers will be how well they perform in a difficult RF environments.

These are only a few of the most critical things that impact the speed of a client device on a Wi-Fi network.  It can be difficult to predict exactly what throughput levels your clients device will see; but the next time an engineer responds with, “It depends,” when you ask how fast the client will go, you will definitely have a much better understanding as to why.

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