AP transmit power vs wireless phone transmit power for site survey

[piotrPaszk]

Hi All,

It is recommended to match Transmit Power of en AP and Wireless Phone to avoid one-way communication.

I have a 3702i and 7925G which I have planned to use for AP-on-the-stick survey.

With DTPC enabled, the AP adjusts transmit power of the phone accordingly but I am trying to understand how does it provide match of EIRP.

To be more precise:

If I use 14 dBm on the ap I see on the phone that transmit power is used 14dBm but there is still mismatch of EIRP as the 3702i have antenna gain of 4dBi and the phone I assume is much lower which we do not know much ( I have not found this in any documentation). Cisco documentation provides som transmitter output power of for example IEEE 802.11b is 50 mW (17 dBm) but I do not know if this is max TX or max EIRP. 

If I use 17dBm on the AP I see 17dBm on the phone but there are again different antenna gains. 

I came across some article which states "changing the AP antenna gain will change the RSL in both directions, but changing the AP transmit power affects the RSL at the client only. If the transmit power is matched, changing the AP antenna gain will alter the RSL by the same amount in both directions. "

How should I configure this in the real life scenario?

Best regards

Piotr

[Scott Fella]

It's best to go lower ham what he max on a device would be.  For example, using 11dbm or 14dbm would be recommended.  You would try to match the AP's EIRP with that of a client device.  If you plan on doing high density for example, your max TX when surveying might be 11dbm and can be lower at times.  You need to plan for density along with coverage. Typically if you plan for density, you will always have more than enough coverage and TX power would be lower than you expect.

Hope this sort of explains things. 

-Scott 

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[piotrPaszk]

Hi Scott,

You would try to match the AP's EIRP with that of a client device.

- this is what I am struggling with.

If you use tx 11dBm on the AP with DTPC, that forces the phone to use tx 11dBm but they still have different antenna gains. Unless that works like this: AP 11dbm + 4dBi is equal to what I see on the phone. But the phone never shows 15dBm which would match EIRP on the AP  but decrement of 3dBm like 17,14,11 which is the same as TX on the ap not EIRP. So I understand that It does not help to reduce the TX on the AP because TX on the client will be proportionally reduced to match that TX anyway. That brings us to the same discrepancy of EIRP regardless of TX value on the AP.

Typically if you plan for density, you will always have more than enough coverage and TX power would be lower than you expect.

I agree with you but I want to carry out Ap-on-the-stick survey in some places to see roughly cell sizes for future planning in Ekahau.

BR

Piotr

[George Stefanick · Accepted solution]

This is more art then science. Generally I always lead with 25mW on the AP TX and ignore the antenna gain. Other wise you will be powering down the APs even more with some smaller antennas. 

[Scott Fella · Accepted solution]

Don't over think this.  Bottom line, you don't want to over power what the client can do. With multiple APs covering an area, does it really matter what the antenna gain is for a client?  Makes more sense when you are using an external antenna.  It's good to keep the power to a db that the clients still can communicate with the AP.  Even if the power on the AP is higher like outdoor wireless for example, devices still work, you just need to set the proper expectations to the user. 

Like George mentioned, start at 25mW and do your survey.

-Scott 

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