Tx Power adjustment in 2.4Ghz

[gnijs]

Hello,

We have a deployment with 2702i dual-band radios. We see a lot of Android devices that prefer the 2.4ghz band over the 5ghz band, just because the RSSI is slightly better (3dbm). We want to force more of these devices to 5ghz (some phones are ac wave 1 compatible, i don't understand why you would prefer 2.4 in this case above 5)

Anyway, i was thinking of making the 2.4 Ghz network "worse" by limiting the max Tx power from 30dbm to 20dbm for example, while keeping the 5ghz radio on 30dbm ? This would lower the RSSI of the 2.4ghz network no ? is this done regularly ?

NOTE: most of the APs have today (auto-)power setting of 6 for 2.4ghz, some 1,2 and 4, but never higher as 6.

regards,

Geert

[pfusconi]

Hi,

In order to limit the Tx power of a band you can use RRM:

Go under Wireless>802.11b/g/n>RRM>TPC and limit the "Maximum Power Level Assignment" field.

Tweak that value until you find a good compromise... different clients will need a different signal strenght.

You can find more information here:

http://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-3/b_RRM_White_Paper/b_RRM_White_Paper_chapter_0101.html

Thanks,

Paolo

[Emerson Rodrigues]

Also do you have client bandselect enabled?

[gnijs]

This is indeed what we are going to test first (it was still disabled).

Then, if i don't see a shift of clients, maybe i will mess around a little with the max TX power of 2.4

I must say that i noticed that i only have 20 Mhz "a" channels enabled, no 40 or 80. BUT i believe this will not change the band selection on Android, since it only looks at RSSI strenght of the received 2.4 and 5 ghz signal (not the width of the channel, am i correct ?)

[patoberli]

No.

With only 20 MHz channels, you don't have any AC speeds, nor most of the N speeds. So the achievable speed from the client is actually the same (72 Mbps vs. 72 Mbps in the worst case at perfect signal strength with 1SS (one spacial stream)), which is why the client will choose the one with the stronger signal.

Unless you have a very special setup (with hundreds of latency sensitive clients in the same room), I recommend to enable 40, or even better, 80 MHz channels, with RRM, DCA and TPC running. Also make sure DFS is enabled with the additional channels (if allowed in your country).

[Mitch D · Accepted solution]

This is an interesting question. Are you sure your APs support TX power of 30 dBm? That seems very high especially on the 5 Ghz spectrum. The max TX power of Cisco 3802i's is only 23 dBm on 2.4 and 5 Ghz.

To answer your question think about this: If your AP is transmitting at 23 dBm (200 mW) and your client's max transmit power is only 17 dBm (iPhone 5). The result will be one way communication at the edge of the cell. The iPhone will see the AP at a higher RSSI if it's set to max TX power but when it tries to respond the AP will not acknowledge its frames.

You should design your APs to provide the coverage you need when only running at 14 dBm.

To get more clients to join 5 Ghz versus 2.4 Ghz you could try changing the data rates on the 2.4 Ghz RF profile. Perhaps set 24 or 36 Mbps as Mandatory and disable all lower rates. The higher the mandatory data rate, the less chance a client will be able to receive the beacon frames because it uses a higher modulation that requires more SNR.