We’ve all seen the marketing for APs you can get 2.1 Gbps throughput on this model, this model has two 5 GHz radios and now can achieve speeds of up to 3 Gbps; then when you configure it the actual throughput is nothing even close to those numbers. Let’s dig into this and discuss the expectation vs reality for Wi-Fi.
First, we need to look at how the marketing numbers are achieved. Let’s take a Cisco 2802 AP that is advertised to be able to support 2.6 Gbps. To Cisco’s credit, they do list the MCS that is supported on the AP, in this case, they used VHT (Wi-Fi 5) using 160 MHz wide channels, MCS 8 with 3 spatial streams. While it doesn’t say what power setting is used for testing, they did call out the numbers in the datasheet as theoretical only. I took the information provided and cross-referenced one of my favorite sites, MCSindex.com, see the screen capture below.
Assuming they used the short guard interval, that brings us to about 2340 Mbps. Where are the other 300 Mbps they are claiming? They are counting the 2.4 GHz radio of being able to support an additional 300 Mbps, give or take. This is how they get to the advertised numbers. While I used Cisco as an example here, this is a common practice across all AP manufacturers. To Cisco’s credit, their datasheets show the MCS rates that are supported on that model, and they do state it “Provides a theoretical connection rate of up to 2.6 Gbps per radio…”.
Now, it’s time to explore what we can expect from an AP. In most commercial applications we will not see 80 MHz or 160 MHz channels deployed. We will usually see 40 MHz channels on 5GHz and 20 MHz on 2.4 GHz. Again, looking at MCStables.com to be fair I kept the same 3 spatial streams even though the AP can support 4. With a short guard interval and keeping the same MCS index level, we could get 540 Mbps. Again, see below the screen capture from MCSindex.com
Before we start celebrating that throughput, we must take into account noise on that channel and the usable signal. We take the signal from the AP and subtract the noise that the AP hears. This will give us a useable signal for our Wi-Fi. As the clients move further away from the AP the signal strength drops, but the noise floor usually remains about the same in most environments, not all, but most. This means the usable signal or SNR decreases. How QAM modulation works are there are points in each quadrant and how far the wave is out of phase allows each point to provide many bits. The higher the QAM, the more bits can be provided at each point. With a decreased SNR the points become so close together that it is hard to distinguish between 1 or more of these points. Therefore, what happened is dynamic rate shifting can or will kick in and lower the data rate to the signal will remain usable, because of fewer errors, resulting in fewer retries.
Now we come to the overhead of the Wi-Fi network. Every 102.4ms the AP has to broadcast a beacon frame for each SSID. Clients are moving around the network requiring frames to establish transmit beamforming, along with clients sending a probe, association, and authentication requests (or reassociation, etc) as they roam between APs. Not to mention any other requests like 802.11k asking for the neighbor report to be included in the beacon frame. It is my estimation for a normal network these can use up to about 30% of available airtime. So, these also have to be subtracted from theoretical throughput.
Up to this point, we talked about the APs, let’s now talk about the clients. We have been using the 3 spatial streams in each of the screencaps up to now, but that is what the AP will support. Clients can range across the board form be 1×1:1, 2×2:1, 2×2:2, 3×3:3, or any other combination based upon which standard of Wi-Fi they support. So, while we may be talking about 540 Mbps per AP, the client may only be able to support 180 Mbps or less. That is assuming that the client is on the current standard. If it is on a legacy standard in 5GHz, there may be more header frames added to each frame transmitted. Up to now we primarily looked at the 5 GHz band. If the client is in the 2.4 GHz band, you may see protection mechanisms triggered or RTS/CTS after transmissions if it is an old client, such as 802.11b. Hopefully, no one is still running those clients outside of a warehouse environment. The last point to make on this is the Wi-Fi medium is half-duplex. That means only 1 device including the AP can transmit at any time. Every other device must wait its turn to transmit.
Hopefully, this helped to guide you when deciding on a new AP. While this didn’t cover everything associated with choosing an AP, it should have given you more of an insight into what goes on with Wi-Fi. Proper planning for a network and understanding your client’s capabilities is just as important as choosing the proper AP. If we can better understand the marketing of capabilities, we can make better decisions.
[i] https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-2800-series-access-points/datasheet-
Bibliography
“Cisco Aironet 2800 Series Access Points Data Sheet.” Cisco, August 3, 2021. https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-2800-series-access-points/datasheet-c78-736497.html#Featuresandbenefits.
“MCS Index, Modulation and Coding Index 11n and 11AC.” MCS Index, Modulation and Coding Index 11n and 11ac. Accessed September 13, 2021. http://mcsindex.com/.
