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Choosing an operating channel for an AP with the best SNR (Signal To
Noise ratio) is important. But since one of the major sources of interference
in our network is our own clients and AP's - Transmit Power Control is equally
as important. DCA and TPC work hand in hand to manage the RF in our
environment. Power largely determines our cell boundaries, there are other
variables (see
High Client Density Design Guide ), but
power is one of the primary determining factors. The goal is to maximize the RF
coverage in the environment - without causing co-channel interference. It's a
balancing act of sorts. Since we cannot control the clients TX power (not all
clients will support DTPC, an optional portion of the CCX specification) we
only have our AP's to work with. Maximizing the AP's coverage and minimizing
it's interference potential then is the job of TPC.
What does TPC do?
TPC uses the TX
neighbor and RF Neighbor lists generated by the NDP process. RSSI organized
lists built on how we hear other AP's (RX Neighbor) and how other AP's hear us
(TX Neighbor), to form a picture of how every AP is heard by every other AP
within the RF Neighborhood and RF Group. Based on this information TPC sets the
transmit power of each AP to maximize the coverage and minimize co-channel
interference. TPC will adjust the Tx power up or down to meet the required
coverage level indicated by the TPC Threshold.
Like DCA, TPC runs on
the RF Group leader and is a global algorithm that can be sub configured in RF
profiles for groups of AP's in an AP group.
There are two
versions of this algorithm since version 7.2 known as TPCv1 (or just TPC), and
TPCv2. The purpose of these two algorithms is essentially the same - the
calculations and how they are implemented differ greatly. We will discuss each
below and give their strength's and potential caveats.
Figure 1. TPC Maximizes
Coverage, Minimizes CCI
TPC algorithms run on
the RF Group Leader along with DCA. They are configured and run separately from
DCA. Three modes may be configured:
Automatic–Runs
every 10 minutes
On Demand –invoke
a power level change once, then freeze till the next demand request
Fixed–allows a
user selected power level to be applied to all AP’s in the RF group
TPCv1
TPCv1 is known as
coverage optimal mode - and is the default method for power control in RRM. The
algorithm runs on the RF Group Leader, and calculates Tx power on a per AP per
radio basis for every member of the RF group. Control over TPCv1 relies on a
user tunable setting - RRM Power Threshold. This combined with the information
gathered from the third neighbor in an AP's neighbor list, is used to make
decisions on an AP's transmit power.
The RF Power
threshold is used to control the cell boundaries of the AP's and hence the
coverage behavior of the system. The default value of -65 dBm was in place
until rel. 4.1.85.0 (4.1 MR1), which changed the default value to -70 dBm.
Valid entries for this are -50 to -80 dBm. The RF Power Threshold is set on the
controller and should be the same for every controller in an RF Group. Good
results are generally observed with values ranging between -68 dBm and -75 dBm.
However certain scenarios will require higher or lower settings. The RF Power
threshold determines what an AP does in response to how other AP's hear him. In
a situation like a high ceiling - the AP's might hear each other fine - but
down on the floor the clients are having issues.
The Third loudest TX
neighbor is used because this is the number of non-overlapping channels
available for 2.4 GHz. As an aside, tests have been done with the 8th neighbor and more -while this would seem to make more
sense in 5 Ghz, there is no real advantage realized. Also, very few regulatory
domains agree on the number of channels they permit - so 3 is the number.
Calulating
Tx_Ideal–Ideal Power
The TPCv1 algorithm
runs as a two stage process–first determining what the ideal Tx power for a
radio would be (Tx_Ideal).
Tx_ideal = Tx_max +
(TPCv1_Threshold - RSSI_3rd)
Tx_Max—the
maximum supported power for a given radio
TPCv1_Threshold—User selectable RRM power threshold - default
-70 dBm version 4.2 and forward -65 dBm before
RSSI_Third—The
Third loudest AP in the AP TX Neighbor list
Note
This is the TX–not the RX neighbor–see above
If Tx_Ideal is
higher than Tx_current, then a power increase is recommended.
If Tx_Ideal is
lower than Tx_current, then a power decrease is recommended.
Evaluating a TPCv1
Change Recommendation
The second part of the process
involves evaluating the recomended results of the first part and deciding to
implement it or not. Since changing the Tx power of an AP also changes the cell
boundaries - it can be disruptive to clients. To ensure that a change is
necessary a hysteresis is applied.
For a TX power
Increase–Hysteresis = 3 dB
For a Tx Power decrease–Hysteresis = 6 dB
So determining if a Tx change is recommended looks like this:
Tx_Curr - Tx_Ideal = N
If N is =/< Hysteresis - No Change is recommended
If N is > Hysteresis a change is recommended
Implementing a
Recommended Power Change
If a power change is recommended it
will be implemented by the following rules:
Decreasing Power–power is
decreased 1 level (3 dB) at per TPCv1 interval (600 seconds Default) and the
effect would be a gradual reduction and allow for settling in the environment
Increasing Power–the power level is set to increase 1 level(3 dB)
per iteration of TPCv1 until Tx_Ideal or hysteresis is reached.
Before a power change is recommended- it is sanity checked for validity
in the RF neighborhood. As an example, we'll use a use case where the AP being
set does not have a 3rd neighbor (classicaly resulted in a power
level 1 assignment before smoothing was introduced in version 6.1 of the RRM
algorithms). Before applying the new power level to the AP, a check of the AP's
neighbors is made to see what they're operating at. In the case where there is
no 3rd neighbor we would look at the two existing neighbors and
ensure that the recommended power level is in alignment with they're neighbors
and themselves.
Figure 2. TPCv1 Smoothing Algorithm function
The recommendation is matched against the power levels being used for
our neighbors neighbor list of AP's, and an average of averages is developed
for any neighbor who is on the list at or above the TPCv1 threshold (-70 dBm by
default).
Once the recommendation is validated it is passed to the AP for
implementation by TPCv1. Let's say the power needs to be decreased by 12 dB to
match Tx_curent to Tx_ideal. In all cases the power will be decreased by 3 dB
(one step) during each TPCv1 cycle. Then on the next run of TPCv1, the entire
process is repeated and if power still needs to be changed- will apply another
single 3 dB step. The process is the same for a TX increase to be implemented,
1 power level per iteration will be applied and evaluated.
Like DCA, TPCv1 runs on the RF Group leader, changes in DCA - will
affect changes in TPCv1 and vice versa - so it's good that they work together
to balance the infrastructure. TPCv1 runs at the same interval as DCA, by
default every ten minutes. TPCv1 has no knowledge of channel and assumes that a
neighbor entry could be on the same channel at any time (reasonable since DCA
runs independently).
On Demand mode schedules a TPCv1 run on the next regularly scheduled
boundary, that is if running in default mode, TPCv1 will run on the next 10
minute interval - however the calculation will be held then and TPCv1 will not
run or update until you select On Demand again.
Note
When all APs boot up for the first time (new out of the box), they
transmit at their maximum power levels (power level 1). When APs are power
cycled or rebooted, they use their last configured power settings. Transmit
Power Control adjustments will subsequently occur as needed.
TPCv2
Cosmetically TPCv2 is
almost invisible to the user, in fact it shares all the same configuration
parameters on the GUI that are involved in TPCv1. It is however very different
under the hood. First, it does not use the 3rd neighbor method or
anything else really other than the neighbor lists from TPCv1. The primary
difference is that TPCv1 is based on a received energy measurement at the
neighboring AP. TPCv2 calculates a cell boundary between two AP's based on the
measured RF distance between them, and optimizes the coverage based on that
calculation.
Figure 3. TPCv2 Two AP
example
The problem to be
solved is similar to filling a box with balls. Larger balls fill the box
quicker, but leave larger amounts of open space between the balls that still
could contain more balls. So - add some smaller balls, and fill in those spaces
as well. This is essentially the same problem to be solved in getting maximum
cellular coverage - larger balls will fill the space quicker but also leave
larger open areas, so the equation seeks to optimize the size of all the balls
in order to maximize the coverage provided. By increasing and decreasing the
cell size, and minding overlap the solution arranges variable size cells for
optimum coverage.
Figure 4. TPCv2 Multi-ap
Result Example
TPCv2 runs to
completion, and power changes are handled continuously. If a cycle concludes
that a particular AP's power needs to be raised or lowered the algorithm will
run continuously adjusting the power and re-checking the results until Ideal_TX
is reached. TPCv2 runs considering all AP's neighbors as equals regardless of
channel (default mode), and in channel mode which only considers AP's that are
on the "SAME" channel as the AP being solved. Channel mode is enabled at the
command line of the RF group leader only. Channel mode is a good choice for
High Density Deployments - as this will minimize co-channel interference, while
Maximizing coverage and signal between 2 AP's on the same channel. Adjacent
channel interference will be addressed by DCA.
(Cisco Controller) >config advanced 802.11a tpcv2-per-chan enable/disable (disabled by default).
Using Channel mode
increases the power significantly.
TPCv2 also adds a
radio Utility feature. As the algorithm runs, it models different combinations
of power to reach a solution. As it does this it keeps track of the utility of
a given radio. TPCv2 will run 10 iterations every TPC Interval (600 seconds) if
a particular radio is only need for 3 of the 10 solutions – that radio is
marked as 30% utility. You can see the results of this in the show advanced
802.11a/b sum command.
Other data in this
show command includes the last 3 octets of the AP BSSID, and RX neighbor and TX
neighbor counts and values for each AP. For instance - AP_1 has a total of 5
RxNbrs - his closest RX neighbor is AP3 at -21 dBm. This is a very useful
output and can be used to select AP's for shutting off 2.4 Ghz radios in overly
dense situations - such as this example above. From above - we could get good
coverage with 3 of the 6 AP's.
Note
It is highly
advisable to look at the AP's position and coverage on the map with a good
understanding of the desired coverage requirements before shutting off radios.
This data should be used only as a guide.
The other show
command you will want to know is:
show advanced 802.11a/b txpower
(Cisco Controller) >show advanced 802.11b txpower
Leader Automatic Transmit Power Assignment
Transmit Power Assignment Mode................. AUTO
Transmit Power Update Interval................. 600 seconds
Transmit Power Threshold....................... -70 dBm
Transmit Power Neighbor Count.................. 3 APs
Min Transmit Power............................. -10 dBm
Max Transmit Power............................. 30 dBm
Update Contribution
Noise........................................ Enable
Interference................................. Enable
Load......................................... Disable
Device Aware................................. Disable
Transmit Power Assignment Leader............... Cisco_69:9a:64 (192.168.10.8) (::)
Last Run....................................... 539 seconds ago
Last Run Time.................................. 0 seconds
TPC Mode....................................... Version 2 Per-Channel NO
TPCv2 Target RSSI.............................. -67 dBm
TPCv2 VoWLAN Guide RSSI........................ -67.0 dBm
TPCv2 SOP...................................... -85.0 dBm
TPCv2 Default Client Ant Gain.................. 0.0 dBi
TPCv2 Path Loss Decay Factor................... 3.6
TPCv2 Search Intensity......................... 10 Iterations
TPCv2 Plan Quality Index.................. Overall -0.5 Coverage 33.2 CCI 3.0 Ratio 1.0
TPCv2 Target Plan.............................. To be reached in 7 TPC runs
AP Name Channel TxPower Allowed Power Levels
-------------------------------- ---------- ------------- ------------------------
upstairs_3602e *1 *7/7 ( 4 dBm) [22/19/16/13/10/7/4/4]
AP_2702E *6 *8/8 ( 1 dBm) [22/19/16/13/10/7/4/1]
downac 1 2/8 (20 dBm) [23/20/17/14/11/8/5/2]
upac *6 *2/8 (20 dBm) [23/20/17/14/11/8/5/2]
NOS_3600 1 6/7 ( 7 dBm) [22/19/16/13/10/7/4/4]
AP_3502 *6 *8/8 (-1 dBm) [20/17/14/11/8/5/2/-1]
1602I_.560e.1b97 11 6/6 ( 7 dBm) [22/19/16/13/10/7/7/7]
This command shows you
the configurations for both TPCv1, TPCv2, identifies which is in use (you can
only use one) as well as where the TPC threshold and the TPCv2 Target RSSI are
set for the global configuration. TPCv1 Threshold and TPCv2 Target RSSI can
both be set differently within an RF Profile, and you will not see that in this
command. The show command also lists all the AP’s WITH their allowed powers as
well as the current power level. Keep in mind – the allowed powers are for the
current channel assignment, see this example where that makes a difference in 5
Ghz
Regardless of the TPC method you
choose you will have the option to limit either the maximum or minimum power
settings allowed. TPC Min/Max is a setting that unlike TPC, runs on every
controller. It is designed as a safety to prevent going too low or too high in
power. The effect is that no matter what TPC sends to the radio, if it is above
the Max or below the minimum, the TPC Max or Minimum Value overrides the global
assignment.
Why would you want to do this you may ask? Well, we don't always get
the AP where we would like it in an installation. The classic use case for
which this feature was created is all the AP's being mounted in a central
hallway with the intended coverage being on either side of the hall. Because
the AP's can see one another quite well in the hallway - power will be reduced
to meet the criteria between the AP's and may not be loud enough to reach the
edges of the rooms on either side of the hall. Moving the AP's into the rooms -
and staggering them down the hall would be better solution - but also may not
be possible for a number of reasons. In this case you could use TPC min to
ensure that the power levels required to reach the users was indeed honored.
Now, this will increase co-channel interference in the hallway itself - however
very few of the users will be in the hallway.
Another good example of where to use this would be a lecture hall or
classroom that is configured for High Density. People absorb RF, and when the
room is full of people - the amount of RF energy you see at the floor could be
attenuated by 5 or as much as 10 dB in extremely dense cases. When the room is
empty, the power levels of the AP's will drop because the propagation has
improved and when it is full, you will need more power (5-10 dB more). If you
let TPC manage this without any guidelines, it will eventually apply enough
power, but that could be 30 minutes into the class that lasts 1 hour. Setting
TPC minimum at the required full class level will ensure that they have plenty
of signal at the beginning of class. Yes, the AP's will be louder for all other
times, however the unused AP's will only be sending beacons during those times
- so not a concern really as the interference will be minimal.
TPC Min and Max settings are entered in dBm NOT Power level index. For
this you will want to know the allowed powers for the AP model you are
configuring. Power level index is a scale 1-8 from (1)Max to (8) Minimum power
for the AP. Not all AP’s support 8 Power Levels. The max power an AP can
transmit differs by band, and in 5 GHz will be lowest in the UNii1 band
(channels 36-48), higher in Unii2 and Unii2e (52-64, 100-140) and highest in
Unii3 (149-165). The advantage to entering this in dBm is that all AP’s
regardless of channel will exhibit the same power assuming the selection is
supported in all 3 ranges, else the AP will be set to the power level it
supports in the band closest to the dBm value entered. An AP’s allowed powers
list is just that – the power levels that the AP can support. To see the
allowed powers for the AP’s on your network, from the controller CLI - show
advanced 802.11a/b summary. Both the channel and the allowed powers for the AP
are displayed. From the AP CLI show controller d0/d1 (d0=2.4 and d1=5 GHz).
Power level 1 always relates to the max power that can be made at 6
Mbps - Non-BF (Beam Formed).
Entry of TCP Min/Max values can be done at the GUI -
Wireless> 802.11a > RRM > Tx Power Control(TPC)
Figure 5. TPC configuration dialogue WLC GUI
It is also supported within RF profiles under RRM.
Figure 6. RF Profile - RRM config dialogue - WLC
TPC Min/Max Values apply regardless of TPC version in use. Note in the
examples above - the default values are - Max =30 dBm and Min = -10 dBm which
is effectively off - as AP's do not support these power levels.
Again, this is a per controller/RF profile setting and does not apply
to all AP's in the RF Group but only to ones local to the controller where the
setting is made.
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