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:
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.
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.
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.
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
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.