Aug 22, 2022
Modulation and coding schemes (MSCs) – Understanding Network and Security for Far-Edge Computing
The speeds provided in the preceding table are best-case scenarios. They assume an optimal SNR, which, in turn, allows the use of a modulation and encoding scheme that gets a higher data rate. Each generation of Wi-Fi has a different matrix of MCSs. The following is the MCS index table for 802.11ac (Wi-Fi-5):
| Modulation | FEC Coding Rate | Data Rate | |
| MCS0 | BPSK | 1/2 | |
| MCS1 | QPSK | 1/2 | 2x faster than MCS0 |
| MCS2 | QPSK | 3/4 | 3x faster than MCS0 |
| MCS3 | 16-QAM | 1/2 | 4x faster than MCS0 |
| MCS4 | 16-QAM | 3/4 | 6x faster than MCS0 |
| MCS5 | 64-QAM | 2/3 | 8x faster than MCS0 |
| MCS6 | 64-QAM | 3/4 | 9x faster than MCS0 |
| MCS7 | 64-QAM | 5/6 | 10x faster than MCS0 |
| MCS8 | 256-QAM | 3/4 | 12x faster than MCS0 |
| MCS9 | 256-QAM | 5/6 | 13.3x faster than MCS0 |
Figure 3.30 – 802.11ac modulation and coding schemes
Each of the MCSs shown has two parameters:
Modulation: In this context, modulation refers to the particular 802.11x modulation type in use. Some modulation types are very sensitive to noise while others tolerate it well. However, the robustness of a modulation type is achieved by reducing how sensitive it is – and this means a lower bit rate.
FEC coding rate: This describes how many bits transfer data, and how many are used for forward error correction. A coding rate of 5/6 means for every 5 bits of useful information, the coder sends 6 bits of data. In other words, there’s one error bit for every 5 data bits:
Figure 3.31 – Impact of MCS on data rate for 802.11ac
A Wi-Fi-5 or Wi-Fi-6 access point will negotiate the best MCS that it can, given the interference it is experiencing. Wi-Fi devices tend to express the SNR as a single number in dB, which represents the amount of signal above whatever noise is present.
A laptop 1 meter away from an access point with no obstructions would have an SNR of ~50 dB, and be able to operate at MCS9 (100% max speed). A second laptop far away or in a different room might only see an SNR of ~25 dB and be stuck at MCS3 (30% max speed).
Here are some practical steps that can help your device negotiate a faster MCS to its access point:
Reduce devices per AP: Try to have only 3-4 devices per AP where possible
Change Wi-Fi channels: Utilities such as NetSpot can help with this
Increase AP signal power: Some APs default to a lower power level than they are legally able to use
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