Transmit Power WLAN Phone and AP in 5 GHz band

Answered Question
May 3rd, 2010

Hello,

I'm a little bit confused about the different statements in case of the transmit power for wlan phones (7925G, 7921G)

and LWAP (1252agn) working with an 4402 WLC (6.0.196) in the 5 Ghz band.

In the actual version of the 792x Deployment Guide they speak about a maximum transmit power of the phone of 16 dbm (40 mw).

At the same time the AP is able to transmit at a maximum of 17 dbm (50 mw) in European domain.

Now when i connect to the web gui of a wlan phone and look at the wlan info, there is shown a tx power of 17 dbm.

Furthermore when i watch the network profiles => advanced config, there is also shown a max tx power of 17 dbm.

In the end, since we've upgraded our WLC to 6.0.196, there are only 5 power levels left which can be set manually via the TPC on the WLC,

but when configuring a single AP via the WLC's web gui there are 7 power levels available.

1) What is the max. tx power of the 7921G / 7925G? Is it 16dbm or 17 dbm

2) If it is 16 dbm. Why is it shown as 17 dbm in the phone's web gui? And wouldn't it be better to reduce the AP to 16 dbm as well,

    so that the tx power of the phone and AP are matching? Or increasing the phone's tx power to 17 dbm?

3) What are the 5 power level, which can be set manually via the TPC of the WLC, are standing for? 1 = 17 dbm, 2 = 14dbm, .......

Thanks for anwering all these questions,

Regards,

Frank

I have this problem too.
0 votes
Correct Answer by dancampb about 6 years 8 months ago

OK, this may get a bit tricky.  If look at table 4-8 in the following link.  You can see that the maximum tx power is based on the channel and also on how many of the tx radios are enabled.  So both 16 and 17dBm are valid.

http://www.cisco.com/en/US/docs/wireless/access_point/channels/lwapp/reference/guide/1140_chp.html#wp1146965

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dancampb Mon, 05/03/2010 - 07:42

The maximum transmit power of the phone or AP really depends on the regulatory domain that it is configured to use.  Which part numbers did you purchase for the phone and AP?

Frank Wagner Mon, 05/03/2010 - 08:39

Thanks for your quick respronse.

The part numbers are as follows:

Phone: CP-7925G-EC-CH1-K9

AP:      AIR-LAP1252AG-E-K9

Regards,

Frank

Frank Wagner Mon, 05/03/2010 - 23:50

Ok. That's for the 1140 Series.

For the 1250 Series (which we have) it looks a little bit easier.

One Tx antenna => 17 dbm ; Two Tx antennas => both 14 dbm => total 17 dbm

But what, if the AP is only powered via PoE 15,4 W, so that the AP runs at "degraded power"?

Are both Tx antennas active and only the higher 802.11n data rates are affected or is only one Tx antenna actice?

And what about the phone now.......16 dbm as described in deployment guide or 17 dbm as shown in phone's web gui?

In conclusion, if the phone really only transmits at a maximum of 16 dbm and the 1250 Series AP can transmit 17 dbm, that would mean, that the maximum allowed power level to be set on the 1250 would be 2 (14 dbm).

If we would set power level 1 (17 dbm), this could cause problems, because the AP would be able to "speak louder" than the phone, right?

George Stefanick Tue, 05/04/2010 - 21:10

Hi Frank,

1 db you are splitting hairs really. With a dense voice deployment your phone will not have issues. In fact i have a deployment where the APs were at 100mW  2.4 GHz(customers request only god knows why!), and we didnt see any issues ...

Where you may see issus is where you have a client attached to an AP and the client walks away from coverage like leaving the building ... but even at 1 or 3 db you wouldnt likely see an issue...

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