Troubleshooting Wi-Fi Slowness (Part 2.3- Understanding the MCS Table)

In part 2.2, I briefly explained the difference between wired and wireless networks. In this post, I will focus on understanding the MCS table. If you want to check the MCS table, you can check it online here or offline using my MCS excel table. If you want to understand the math behind it, make sure to check Francois’ post here.

In this post, I will explain how to read the MCS table and how to remove the marketing content from it to get a more realistic view of the expected real world throughput.

So what is the MCS table?

Modulation and Coding Scheme (MCS) Index Values are used to determine the possible data rate of your WiFi connection. Different supported modulation types (BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM) are able to deliver different data rates at varying RF conditions.

As shown in the below modified image taken from WLAN professionals, to achieve the higher data rates that are possible with the more advanced modulation types, the received signal should be stronger and the difference between the signal and noise should be bigger. Having a high signal level alone is not enough. Think about it like if there is some noise around you while you are watching your favorite TV show, if you can’t stop that noise, you most likely will raise the volume of the TV so that you are able to hear it better.

The MCS value mainly summarizes the combination of the number of spatial streams, the modulation type, the coding rate that is possible when the device connects to the wireless access point.

How to read the MCS table?

So let’s take an example to understand how to read the MCS table. What is the maximum data rate that can be achieved by a 1 spatial stream 802.11ac client on 40 MHz channel?

The MCS excel table that I have shared is divided into 3 blocks depending on the technology (2 are only shown in the below diagram)

  1. So we know that it is 802.11ac so we focus on block one (Block 1 – Red Border)
  2. We know it is 40 MHz wide channel so it one of the two columns marked (Dark Navy Border)
  3. Since we need the maximum data rate, we should consider a short guard interval. We now know the exact column (Blue Border)
  4. To get the row block, we need to check the number of spatial streams (Green border)
  5. To get the exact cell, we also need to check the modulation and coding scheme (Yellow table). The intersection of all these tables is the maximum theoretical data rate that can be achieved.

As such, we know now that the maximum data rate that can be achieved by 802.11ac with 1 spatial client on a 40 MHz wide channel is 200 Mbps (MCS 9).

To achieve this data rate

  • The client must support it
  • The AP it is connecting to must support it
  • The RF environment (Signal Strength, Signal to Noise Ratio) must be appropriate as explained in the above section

If any of the above conditions are not met, the client will change its decision and try to connect at lower data rate.

Removing the Marketing from the MCS table

Based on the above it is essential to remove the marketing data that is contained in all vendors datasheets. I will show this based on Aruba 315 AP but the same applies with any vendor AP. You as a customer must learn how to read the MCS table to understand in what conditions are these data rates achieved.

As stated in the datasheet, these numbers are achieved

  • When the 802.11ac client supports 4 spatial streams and we are running in 80 MHz channels. Which clients in your network are 4 spatial stream? Are you using 80 MHz?
  • When the 802.11ac client supports 2 spatial streams and we are running in 160 MHz wide channels? 160 MHz channels are not possible in any production enterprise environment!

A more realistic number is 300 Mbps. It is achieved by a 2 spatial stream 802.11ac client using 64-QAM on a 40 MHz channel.

Two final mind boggling info! The above theoretical 300 Mbps number is

  • Not per client: Always remember Wi-Fi is a shared medium and devices need to contend to send the data. If you connect multiple clients at the same time and try to do an iPerf test to the wired network at the same time, you will not get 300 Mbps from each one! This is very normal.
  • Theoretical Value: The above data rates are the theoretical data rates. If you do a real application throughput test (like file transfer), you will get around 30-50% of that number as real throughput due to the overhead of how Wi-Fi works..

So next time, before you start complaining that Wi-Fi is slow, check your current MCS rate to understand your maximum data rates. Also, you need to check the channel utilization and retry rates. These 3 components MCS, Channel Utilization and Retry Rates are the vital signs of a properly performing wireless network. The next post covers checking channel utilization and retry rates.


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