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Transmit Power Control

Ed Armstrong
Level 1
Level 1

I have inherited a high density network comprising of older Cisco access points and the wireless clients are failing to roam.   After a site survey I have found that the network is too "hot" with a large number of access points visible in almost all locations.   The TPC control threshold is set to the default -70 dBm and this is what I believe is part of the issue.

According to the "Radio Resource Management under Unified Wireless Networks" document TPC evaluates the transmit power using the following equation:

Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest neighbor above the threshold)

If the transmit power needs to be turned down a TPC hysteresis of at least 6dBm must be met.

If the transmit power needs to be turned up a TPC hysteresis of at least 3 dBm must be met.

It also has the following example:

Tx_Max = 20

Current transmit power = 20 dBm

TPC threshold = -65 dBm

RSSI of the third neighbour = -55 dBm

The TPC equation equates to 20 + (-65 - (-55)) = 10

The transmit power is reduced to 17 dBm as the TPC equation equated to 6 dBm or higher.  So far so good.

According to the WLC Configuration Guides:

In applications with a dense population of access points, it may be useful to decrease the threshold to –80 or –75 dBm to reduce the number of BSSIDs (access points) and beacons seen by the wireless clients. Some wireless clients might have difficulty processing a large number of BSSIDs or a high beacon rate and might exhibit problematic behavior with the default threshold.

So if we take the same example and use a TPC threshold of -80 dBm we get:

Tx_Max = 20

Current transmit power = 20 dBm

TPC threshold = -80 dBm

RSSI of the third neighbour = -55 dBm

The TPC equation equates to 20 + (-80 - (-55)) = -5 so does this mean that the current transmit power is left at 20 dBm?   If so, how does this reduce the number of BSSIDs and beacons seen by the wireless clients?

6 Replies 6

Leo Laohoo
Hall of Fame
Hall of Fame
I have inherited a high density network comprising of older Cisco access points and the wireless clients are failing to roam.

At the end of the day, it's the wireless clients decided which WAP they want to associate to.   So trying to pull your hair out to compute TPC is a waste of your time.  Even worst if you disable TPC and go manual.  Yuck!

I am looking at reducing the access point count in this area (I can always these access points in other areas of the site) and using the TPC to "smooth out" the rough edges.

I am looking at reducing the access point count in this area (I can always these access points in other areas of the site)

Turn ON TPC and disable the WAPs that have very low transmit power.  Then re-check if the other WAPs have "compensated" for the loss of a nearby WAP.  If they don't, then you cannot move the WAP away to another area.

Also check both TPC and DCA. 

leolaohoo,
Thanks for the advice.
My site is a 24x7 facility where the wireless network carries voice traffic so to make any changes I have to provide full documentation before change approval will be granted.    This is is reason I am looking into the TPC mathematics.

iores
Level 1
Level 1

@Ed Armstrong In the first example, you said that the power will get reduced based on the calculated TPC value.

Shouldn't this value be compared to the current Tx value to decide if the power goes up or down?

jagan.chowdam
Spotlight
Spotlight

Have you done any packet captures during the roam failure to see why the client fail to roam?

If not done yet, I would do some packet captures and run debug client in parallel. Then use Cisco Wireless Debug Analyzer to understand the reason. 

Also, use Cisco Wireless Config Analyzer Express (WCAE) to analyze and validate your Wireless Network.

It can perform

  • Configuration Checks
  • RF Health Analysis
  • RF Stats Summarization
  • Upgrade Advisor
  • Channel Stats (per site/flex, covering client, power levels, rogues, etc)
  • Tag/Policy usage
  • RRM analysis
  • Log Message Summarization
  • Ap inventory
  • RF Graph Analysis
  • Client Audits: 8821, iPhone, Drager and others

This will help you in understanding the status of current network. Based on the results you see in WCAE, you can decide the plan of action.

 

Jagan Chowdam

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