hostapd: Add cell_density data rates option

Add a cell_density option to configure data rates for normal, high and
very high cell density wireless deployments.

The purpose of using a minimum basic/mandatory data rate that is higher
than 6 Mb/s, or 5.5 Mb/s (802.11b compatible), in high cell density
environments is to transmit broadcast/multicast data frames using less
airtime or to reduce management overheads where significant co-channel
interference (CCI) exists and cannot be avoided.

Caution: Without careful design and validation, configuration of a too
high minimum basic/mandatory data rate can sacrifice connection stability
or disrupt the ability to reliably connect and authenticate for little to
no capacity benefit. This is because this configuration affects the
ability of clients to hear and demodulate management, control and
broadcast/multicast data frames.

Deployments that have not been specifically designed and validated are
usually best suited to use 6, 12 and 24 Mb/s as basic/mandatory data
rates.

Only usually seek to configure a 12 Mb/s, or 11 Mb/s (802.11b
compatible), minimum basic/mandatory rate in high cell density
deployments that have been designed and validated for this.

For many deployments, the minimum basic/mandatory data rate should not be
configured above 12 Mb/s to 18 Mb/s, 24 Mb/s or higher. Such a
configuration is only appropriate for use in very high cell density
deployment scenarios.

A cell_density of Very High (3) should only be used where a deployment
has a valid use case and has been designed and validated specifically for
this use, nearly always with highly directional antennas - an example
would be stadium deployments. For example, with a 24 Mb/s OFDM minimum
basic/mandatory data rate, approximately a -73 dBm RSSI is required to
decode frames. Many clients will not have roamed elsewhere by the time
that they experience -73 dBm and, where they do, they frequently may not
hear and be able to demodulate beacon, control or broadcast/multicast
data frames causing connectivity issues.

There is a myth that disabling lower basic/mandatory data rates will
improve roaming and avoid sticky clients. For 802.11n, 802.11ac and
802.11ax clients this is not correct as clients will shift to and use
lower MCS rates and not to the 802.11b or 802.11g/802.11a rates that are
able to be used as basic/mandatory data rates.

There is a myth that disabling lower basic/mandatory data rates will
ensure that clients only use higher data rates and that better
performance is assured. For 802.11n, 802.11ac and 802.11ax clients this
is not correct as clients will shift around and use MCS rates and not the
802.11b or 802.11g/802.11a rates that able to be used as basic/mandatory
data rates.

Cell Density

0 - Disabled (Default)
Setting cell_density to 0 does not configure data rates. This is the
default.

1 - Normal Cell Density
Setting cell_density to 1 configures the basic/mandatory rates to 6, 12
and 24 Mb/s OFDM rates where legacy_rates is 0. Supported rates lower
than the minimum basic/mandatory rate are not offered.
Setting cell_density to 1 configures the basic/mandatory rates to the 5.5
and 11 Mb/s DSSS rates where legacy_rates is 1. Supported rates lower
than the minimum basic/mandatory rate are not offered.

2 - High Cell Density
Setting the cell_density to 2 configures the basic/mandatory rates to the
12 and 24 Mb/s OFDM rates where legacy_rates is 0. Supported rates lower
than the minimum basic/mandatory rate are not offered.
Setting the cell_density to 2 configures the basic/mandatory rates to the
11 Mb/s DSSS rate where legacy_rates is 1. Supported rates lower than the
minimum basic/mandatory rate are not offered.

3 - Very High Cell Density
Setting the cell_density to 3 configures the basic/mandatory rates to the
24 Mb/s OFDM rate where legacy_rates is 0. Supported rates lower than the
minimum basic/mandatory rate are not offered.
Setting the cell_density to 3 only has effect where legacy_rates is 0,
else this has the same effect as being configured with a cell_density of 2.

Where specified, the basic_rate and supported_rates options continue to
override both the cell_density and legacy_rates options.

Signed-off-by: Nick Lowe <nick.lowe@gmail.com>

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