In a Dormitory or
Multi-dwelling facility the ratio of AP to Client count is not as impacting as
say an open enterprise office environment with real time voice and video. The
key concern about selecting good minimum data rates is to optimize the
efficiency of the network by encouraging and enforcing client connections to
AP's that are closest to them. Selecting a relatively high minimum data rate in
a dense environment enforces client selections of higher data rates which
improves airtime utilization. This efficiency equals higher capacity and lower
latency throughout the surrounding cells. A minimum data-rate of 18 Mbps with
no supporting data-rates below effectively tells a client that they must
complete association and use a minimum data-rate of 18 Mbps or it will not be
If the selected data
rate is too low, then you will have clients holding onto AP's that are far away
from them rather than roaming to a closer AP. This creates interference, and
the lower data rates waste airtime since the radios must be on the air longer
for the same size transaction.
If the data rate
selected is too high, then you will create coverage holes between cells within
the coverage area - a Higher data rate effectively reduces the cells coverage
area by requiring higher levels of signal to function. As you get closer to the
AP - higher data rates are possible. Using 18 Mbps as an example only - with no
data rates supported below effectively reduces the overall coverage of the
cells by regulating access. The minimum mandatory data rate (18 Mbps) will not
be heard as wi-fi beyond a certain distance from the AP as the signal level
will have dropped to low for demodulation. This ensures that a client can not
hear the beacon of an available AP until it gets close enough to demodulate it.
Increase the data rate to 24 Mbps - and the effective cell grows smaller,
decrease it to 9 Mbps and the cell edge moves farther away from the AP and the
coverage area becomes larger.
The data rate used by
a client or an AP in either direction is a function of signal to noise ratio
(SNR). Clients and AP's alike are built to lower or raise the data rate in
response to changing conditions on the over the air link. Each data rate has a
minimum SNR that it must have to support the conversation. All of this is
automatic and beyond our control - particularly with the clients side. However,
we can enforce some control from the network.
Data rates can be
configured in 1 of 3 states:
- Mandatory–the client MUST
support the data rate and operate at the minimum or above in order to associate
- Supported–the AP supports it and the
client may use it to communicate with the AP
- Disabled–the AP does not support it
and if disabled will not answer a client using that data rate
The "First" Mandatory data rate that you select determines what speed
all of the beacons and other management traffic use. The second mandatory data
rate that you set (always higher) serves as the default multicast data rate if
you are not using the default which is auto. Any rates that are supported below
the Mandatory data rate - should be set to disabled. Enabling lower data rates
encourages sluggish roaming by allowing the client to hang on to the AP when it
really should be roaming.
In all cases for 2.4 GHz - data rates 1,2,5.5, and 11 should always
be disabled - you will only have true legacy 802.11b devices if you're a
hospital, warehouse, or retail chain most likely.
The above three settings apply only to legacy data rates - and not to
the HT or VHT rates. This is because all HT and VHT clients and AP's only use
the 802.11a protocol for management traffic and beacons - this is by design to
accommodate backward compatibility. It is not necessary, or advised, to adjust
the HT /VHT permitted data rates. Let the clients figure those out for
The data rates that you select are dependent upon your deployment
style. With strong 5 GHz coverage (-60 dBm cell edge) - the first mandatory
rate can easily be 18-24 Mbps with 12 and below disabled. In higher density
environments 24, 36, and even 48 have been used to help the clients spread out
on the available AP's.
So, how do you select the correct data rate? Start conservatively.
In a nearfield deployment the goal is to keep the AP and the clients
near one another - so a fairly aggressive data rate should be supported. If
your deployment is a combined enterprise (hallways and common areas) and
nearfield (individual room coverage) then you only need enough coverage for a
room ad a slight overlap with the hallway with the door open- and a very high
data rate will work fine. However, if you are also covering hallways from the
rooms - then you will need to take that into account and a lower data rate will
be called for. What you want to achieve is a good and well enforced overlap
with the next cell to facilitate good roaming. Here's an example of how to tune
- Start with 12 Mbps as the
first mandatory data rate.
- Monitor coverage hole
alerts (first likely configure coverage hole threshold,the default is -80 you
likely want -75). This alerts on clients that are below the coverage threshold
as seen by the AP. If your getting coverage holes on interior spaces (it is
common to get alerts on AP's near entrances - people do leave) then reduce it
to 9 Mbps.
- You can also monitor which
AP's clients are connected to using Prime and CMX - and if you see clients
skipping through 2 available AP's and attached to the 3rd - then the
data rate is too low for your deployment. You would want to increase the data
rate one step to 18 Mbps.
- Even better, do your post
deployment survey and check your roaming and coverage and adjust up or down
based on those results (Strong coverage = -60 dBm at 5 GHz as a lot of clients
are hard of hearing). If you do an active survey and maintain a ping to the
network on the internal adapter you would be looking for increased packet drops
in cell overlap areas.
In a dormitory near field deployment the goal would be to ensure that
clients are connecting to available AP's at a sufficiently high data rate.
Given the above example - it's a process of tuning. Tuning is not necessarily
required - however once you fine tune the configuration -you will enjoy a lot
of trouble free existence. Obviously we are discussing one building, area, or
floor. And the configuration your configuration results may well apply to
multiples, but it likely doesn't apply to every conceivable installation under
your control - hence the reasons for using AP groups and RF Profiles.