Many articles have been written on how to properly calculate the quality of a WiFi connection. In this blog we will introduce one concept, the modulation and coding scheme (MCS) index, and show how it can be used to understand the quality of a WiFi connection for Windows and Mac OS computers. Modulation refers to the way data is formatted prior to transmission. The coding scheme refers to how the data is encoded while being transmitted.
Modulation and Coding Scheme (MCS) Table
The Modulation and Coding Scheme (MCS) index is a list of modulation schemes and coding schemes that are currently used to send data from WiFi devices.Modulation schemes are different methods used by each client and access point to encode data onto a radio signal before transmission. For example, binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK) are simple modulation techniques for encoding messages. The simplicity of these techniques, however, can lead to an increase in errors, and in turn lowering performance. Quadrature amplitude modulation (QAM), on the other hand, uses a more complex method of encoding to maximize throughput speeds and reduce errors. However, QAM requires higher signal strength and signal-to-noise ratio (SNR).
Understanding how the MCS index is calculated is not as important as knowing how to use information from the MCS index table and your device to your advantage. In general, the higher the MCS index number, the higher the MCS rate / max data rate of the connection.
Below a short summary of how to read the MCS Index table.
- Start by opening this MCS Index table in a new window.
- Focus on the first three columns. These are the MCS index values for HT=802.11n, VHT=802.11ac, and HE=802.11ax. MCS rates do not exist for technologies prior to 802.11n. 802.11 a/b/g have less variable data rates like 1, 6, 11, 54 Mbps...also know as non MCS rates.
- Now locate the spatial stream column. The max MCS index value and MCS rate is dependent on the number streams supported. The higher the spatial streams supported by the device the higher the index value. Note: Most mobile devices (tablets / smartphones) are single or dual stream only. Some laptops have more than 2 stream capability.
- Locate the channel width columns (20 MHz, 40 MHz, 160 MHz). As you can see the MCS rates increase as the channel bandwidth increases.
- Locate the Modulation column. Again, as the modulation complexity increases the MCS rates increase.
- Knowing the wireless data rate currently used by the device can be used to determine the corresponding MCS value.
With this information, you should be able to read the MCS Index table. In the next blog post, we will dive into improving the MCS Index / MCS Rates / Maximum Data Rates possible over a given WiFi connection.