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Mighty iPhone Power Ranges II (With iPads)

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About a year and a half ago, yours truly wrote about WiFi transmit power levels in iPhones.  Things have changed since then.  And possibly the biggest change (to iPhones, at least) is how aggressive iPhones are in modifying transmit power levels.  

In the “Mighty iPhone Power Ranges” blog post, I wrote about the value of setting AP transmit power levels to approximately the same level as client/station device power levels.  Over the past year or so, more and more client/station devices have started using adaptive power levels.  A typical implementation would force a device to lower its transmit power when receiving a strong signal from the AP and raise its transmit power when the AP’s signal is weak.

The unanswered question is, “just how vast are these ranges of transmit power levels?”  Can a smartphone or tablet go as low as half power?  10% power?  0.0001% power?  Those differences could have a major effect on a WLAN infrastructure’s ability to handle a variety of devices.

I decided to do a quick, unscientific test of device transmit power levels while on a non WiFi-equipped plane.  I sat the device directly adjacent to my 2012 MacBook Air (which has a 2-stream 802.11a/b/g/n WiFi radio) running in monitor mode.  Then I let the device send Probe Request frames looking for nearby WiFi networks.  I used the received signal strength in those Probe Request messages to see if my WiFi devices were changing power levels.

The first device I checked was a 3rd generation iPad, which has 1 stream 802.11a/b/g/n radio.  I let my iPad send Probe Requests for while.  Initially I saw this:

The screenshot above shows that my MacBook Air’s WiFi adapter was receiving Probe Request frames at a signal strength just below -20 dBm.  
After a while, my capture started to look like this:
Notice how the received signal strength is now about 10 dBm higher, hovering around -10 dBm.  That means that my iPad sort of got frustrated with having no AP to associate with, and decided to start using a higher transmit power in the hopes of reaching an AP that is further away.  This behavior is bad for battery life, but would seem to indicate that a 3rd generation iPad can handle connecting to an AP that uses a very high transmit power.
After looking at the iPad, I moved on to my iPhone 5.  My iPhone 5 has a 1 stream 802.11a/b/g/n radio, just like the 3rd gen iPad does.
My initial capture started out looking very similar to the initial iPad capture:
The received signal strength from the iPhone 5 is maybe a few dB lower than the received signal from the iPad, but it’s close.
But then look what happened:

The iPhone started using a LOWER transmit power when sending Probe Request frames.
And then it dropped lower:

And lower:

All the way down until the capture laptop sitting right next to the iPhone was receiving a signal strength below -70 dBm:

Could this be?  Is it possible that iPhones actually lower their transmit power when probing?  In some ways that would be good.  It would mean less interference in large venues like hockey arenas when users fail to connect to guest WiFi.  But in the enterprise it could be bad.  It could result in frustrated users.  A user might be told by support personnel that the WiFi is up and running, but the extremely low transmit power used when doing active scanning (that’s the 802.11 term to sending Probe Request frames in hopes of receiving a Probe Response) could cause the iPhone to think that no APs are nearby.
This was an unscientific test, so there could be other reason for these odd results.  Maybe the iPhone 5 was probing on channel 4 while my laptop was capturing on channel 1.  Maybe the signal strength just appeared to be lower because the channels were off.  Maybe iPhones have different transmit power levels for data and management (which includes Probe Requests/Responses) frames.  Maybe it’s a temporary bug in Apple’s iPhone code that will eventually be changed.
Whatever the reason for the odd changing of transmit power, it is something to take note of if you support iPhones.  And it’s yet another reason that sniffing WiFi often reveals information that is unavailable from vendor documentation.
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Written by sniffwifi

January 10, 2014 at 12:52 am

Mighty iPhone Power Ranges

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Oh, those darned iPhones. Can’t live with ’em, can’t keep your job without ’em.

The vagaries of iPhones and other station devices are the most difficult part of managing a WiFi network, but there are some things that can be done on the infrastructure to try to make your stations work better. One of those things is lowering your AP transmit power to a level that more closely matches your client station’s transmit power. 

My main man G.T. Hill (of Ruckus Wireless) recently wrote a blog post discussing why this post is bullshit. Now I’m going to tell you why his blog post is bullshit. (sorry, G.T.)

G.T.’s primary point is that is is borderline mentally handicapped (politically correct term) to turn your AP’s power down. His theory is that even if your client stations transmit at low power levels, having a high AP power level at least allows the from-AP data rates to stay as high as possible. (G.T. goes on to add that most traffic is downstream, thus making it all the more important to maintain high from-AP data rates. I have found this to be incorrect, so double-sorry, G.T.)

I wanted to check out an example using my iPhone (which, from what I gather, transmits at 10 dBm) connecting to an AP in a small office (which probably transmits at 17 dBm or 18 dBm). I ran a Skype test call so that I would make sure that both the AP and the iPhone would transmit plenty of frames.

Just as G.T. hypothesized, the higher transmit power of the AP allows the AP to transmit at a high rate.  The small office AP was acting as an 802.11g AP (even though it’s probably an 802.11n AP configured to use TKIP encryption) and, sure enough, the small office AP used the highest possible rate (54 Mbps) for just about every transmitted frame:

Also as G.T. hypothesized, the rate for frames transmitted by the small office AP would likely have been lower if the AP’s power level were reduced to match the iPhone’s power level. We can deduce that from the fact that the iPhone was routinely transmitting frames at a lower rate (48 Mbps) than the AP:

As expected, G.T. is correct about the rates. If your primary goal is to have the highest possible rate for all frame transmissions on your WiFi network, chances are that setting your AP and station transmit power levels to the highest possible value will help you achieve that goal. 
The problem with this whole discussion about rates is that having high rates really should not be your primary goal. High rates are attractive because it’s fun to see a big number when you mouse over your system tray (or, for us Mac OS X users, when he hold the Alt key and click on the WiFi icon in the top menu bar), but for most enterprise WLANs high rates fall somewhere around fifth on the list of things to look for when evaluating whether the WLAN sucks or not. 
My criteria for not sucking:
  1. Drops/re-authentications are rare (meaning that every captive portal sucks)
  2. Roaming works for mobile devices
  3. Discovery traffic is minimal
  4. Retrys are low (indicating that whenever the wireless gets busy, it’ll still probably work)
  5. Rates are high
(Please notice that throughput/goodput, packet sizes and single station usage are absent from this list. One could write an entire blog post about how much time is wasted analyzing those numbers.)

High percentages of Retrys are more damaging than low rates (in most cases) because Retrys waste channel time. There is a finite amount of time available for data to get across each channel. When a Retry happens, the time for the failed frame was wasted because all stations and APs must stay quiet during each frame transmission. Low rates for data frames also waste time, but for most enterprises the budget is there to install plenty of APs so that a decent signal can be had anywhere.

Now take a look at the Retry numbers for the communication between my 10 dBm iPhone and the 17/18 dBm office AP (while also taking a look at how much extra time it takes to do menial tasks when you go cheap and use Wireshark instead of WildPackets OmniPeek or Fluke AirMagnet WiFi Analyzer):

Frames sent by the AP to the iPhone: 1650

Retry frames sent by the AP to the iPhone: 155 (9.4%)

Frames sent by the iPhone to the AP: 1973

Retry frames sent by the iPhone to the AP: 329 (16.7%)

Both 9.4% and 16.7% Retrys are bad numbers (though this was for a small office, where bad numbers are expected because of neighboring WiFi networks), but 16.7% is a helluva lot worse. About 1.77 times worse.

And why is it worse? Most likely because the AP transmit power is set too high. The iPhone has no idea what the AP transmit power level is, so when the iPhone receives a frame at a good signal strength, the iPhone assumes that a high rate can be used when the iPhone transmits. The iPhone doesn’t realize that its transmitted frames go out about 7 or 8 dB lower than the AP’s frames. The end result is the iPhone using rates that are too high (and if I were able to show you the entire capture of the iPhone’s transmitted frames, you would see many failed attempts at 54 Mbps), causing lots of Retrys on the channel and thus, SLOWING THE ENTIRE CHANNEL DOWN.

(I cannot make the point strongly enough that RETRYS SLOW THE ENTIRE CHANNEL DOWN. A massive Retry percentage like 16.7% makes my iPhone slower, every other device on my iPhone’s network slower, every other device on any other network the AP is offering slower and every other device on any network that any AP on my channel slower. Retrys are a bad, people. And the only way to get an accurate gauge of Retrys is by sniffing WiFi. It’s why I cared enough about sniffing to start a free, advertising-bereft blog.)

The bottom line here is that even though my friend G.T. has phenomenal taste in fast food, a lovely wife/kids/family and a solid history of only spending money on things that will help his career or increase in value (sorry, G.T., I couldn’t resist), I believe he is wrong here. I have found that designing a WiFi network with AP power levels lower to match station power levels works best, and my experience gathering statistics from real-world WiFi sniffing supports that.

Written by sniffwifi

June 27, 2012 at 5:07 pm