MCS Table and How To Use it

Today in our training video, we’re going to talk about one of my favorite things, the MCS table, and how you can use it.

Here is the link to François’ MCS table mentioned in the video.

The MCS table evaluates the quality of the RF environment- the RF media that devices are working in, which is reflected in every single transmission.

Every transmitter device, whether it be an AP or a client, will make an internal decision of which MCS it is going to use.

MCS summarizes and categorizes Wi-Fi parameters such as modulation, coding scheme, guard interval, and channel width.

MCS Table for 802.11n and 802.11ac

MCS Parameters

High Throughput Modulation and Coding Scheme (HT-MCS)
Used by 802.11n. Represented by an integer in the range of 0-76.

Very High Throughput Modulation and Coding Scheme (VHT-MCS)
Used by 802.11ac. Represented by an integer in the range of 0-9.

Modulation Scheme
Defines the phase and amplitude required for bit computing, from BPSK to QPSK to 16-QAM, 64-QAM, and 256-QAM.

Rate of bits transferred and Forward Error Correction. A 1/2 Coding means two bits are transferred, and one is received. Minimizing the coding scheme would entail sending the data faster while losing robustness.

Data Width
Specifies the channel used: 20MHz, 40MHz, 80MHz, and 160MHz.

Guard Interval
Waiting time or pause between each packet transmission. 802.11n has 400ns, and 802.11ac has 800 ns. The smaller the guard interval, the faster the throughput.

Minimum SNR and RSSI
Determines the minimum SNR and RSSI required for a specific MSC index.

Using the MCS Table to determine the MCS index

 Define the capacity of the client device, such as the data width and Wi-Fi designation.
 Determine and associate the range of such parameters in the table.

For example, an 80MHz wide-capable client and supports 802.11ac. This means the AP and the client can support all data transfer defined by the range of the client device, from 6.5 MCS to 1300 MCS.

MCS Index supported by 802.11ac, 80MHz

 After determining the MCS index, the client device will send a DHCP request to the radio chipset.
 The AP’s radio will decode and demodulate the request. If no error occurs, it will send back an ACK with an MCS index supported by the client device. Every transmitter chooses which MCS to use.