Constellations and Modulation

Modulation and Coding Scheme (MCS) Index Values can be used to determine the likely data rate of your WiFi connection. The MCS value essentially summarises the number of spatial streams, the modulation type and the coding rate that is possible when connecting your wireless access point. In reality, the actual MCS will depend on the variables such as hardware design and local interference. If a wireless or WiFi connection cannot be maintained, i.e. there are too many CRC errors being experienced on the link, the MCS value can be lowered which will reduce the error rate (by selecting a more forgiving modulation type/coding rate) but will come at the price of a slower data rate. Whilst the MCS will indicate the data rate of the wireless or WiFi connection, it will not determine the actual usable network throughput.

The table below shows the current MCS index values:

Spatial Streams

Currently, the 802.11n standard allows up to 4 spatial streams using MIMO technology. At a simplified level, MIMO allows the same frequency space to transmit and receive multiple data streams. 802.11ac will increase the number of spatial streams from 4 to 8.

Modulation Type and Coding Rate

This determines how data is sent over the air. Newer and more complex modulation methods can sustain high data rates but generally require less interference and good line of sight (LOS) to the access point. In contrast, older methods such as BPSK can be used to maintain connectivity for high error-rate links. The coding rate is an indication of the percentage of the data stream that is being used to transmit usable network data.

Guard Interval

The Guard Interval (GI) is effectively a very short pause between packet transmission to allow for any false information to be ignored. Longer Guard Intervals make for more reliable wireless. This Guard Interval is necessary to offset the effects of multipath that would otherwise cause Inte r-Symbol Interference (ISI). Guard interval is like pausing between words spoken into a megaphone to overcome echo (sound wave reflection). Legacy 802.11a/g devices use an 800 ns guard interval, but 802.11n devices have the option of pausing just 400 ns. Shorter Guard Intervals would lead to more inte rference and reduced throughput, while a longer Guard Interval would lead to unwanted idle time in the wireless environment. A Short Guard Interval (SGI) boosts data rate by 11 percent while maintaining symbol separation sufficient for most environments.

Unequal Modulation

It refers to using a different modulation type and coding rate on each spatial stream. MCS values 0 through 31 define the same modulation and coding will be used on all streams, while MCS values 32 through 77 describe mixed combinations that can be used to modulate two to four streams. For example, MCS 33 refers to using 16-QAM on spatial stream #1 and QPSK on stream #2, while MCS 77 refers to usin g 64-QAM on streams #1-3 with 16-QAM on stream #4.

Channel Width

This is the width of the channel being used by the wireless link. 802.11n allows channel widths of 20 or 40 MHz. The rule of thumb is the wider the channel, the higher the data rate. Whilst you might be tempted to have 40MHz channels at all times, you can't deploy them when using 2.4GHz WiFi because there are only 3 non-overlapping 20MHz channels available. Additionally, wider frequency channels are more difficult to maintain in challenging environments where RF interference is common.

802.11n APs are required to support at least MCS values 0 through 15, while 802.11n stations must support MCS values 0 through 7. All other MCS values, including those associated with 40 MHz ch annels, SGI, and unequal modulation, are optional. Identifying the MCS values su pported in common by all of your 802.11n devices is a good way to determine the set of data rates that can actually be utilized by your WLAN.