Absolute Power


iPhone 4 Video Conferencing

This blog originally posted on the Low Power Engineering Community 7/21/11.


I have an opportunity this month to do some tests using my iPhone (AT&T iPhone 4, 32G, iOS 4.3.3, Personal Hotspot) as a true business productivity tool. I’m travelling to Austin, and am hosting a two-hour WebEx video conference call through our WebEx/MeetingPlace infrastructure. I’ve decided to use my iPhone 4 as the wifi connection from my laptop for the WebEx session, as well as for the audio portion of the conference call in MeetingPlace. Supporting hardware is my HP EliteBook 2540p laptop, which is connected via VPN to our company infrastructure through my iPhone 4 personal hotspot.

The first thing to note (if you haven’t seen the TV commercials) is that this setup isn’t even possible on the Verizon iPhone 4 because it requires voice and data transmission simultaneously. So if it works, chalk one up for AT&T.

Setting up the hotspot is very easy. This is a regular tool in my arsenal now, usually used for connecting iPads and iPod Touches to the Internet. Interestingly, I’ve also found around the house that my other family members, who are on more data-limited plans, also like to use my personal hotspot to save their data allocation. The personal hotspot feature from AT&T doubles your data allocation to 4GB per month, so I’ve never had a problem with the data limit on this plan.

With my laptop connected to the Internet via WiFi, the VPN connection into the company is a no problem, and I’m ready to try things out. Starting up the WebEx session goes surprisingly smoothly; I’m worried that latency might be an issue, but no problems. Now the next trick: start up the conference call, as well. Our latest software update in WebEx allows the system to call each participant, including the host, so I enter my phone number, and voila! My phone rings, and I start the conference call. During the next 15 minutes, about 15 to 20 participants join the call and the WebEx, and we’re off.

In order to get as much data as possible during the call, I am recording the time, battery percentage, and signal strength, and have decided to run the first half of the meeting with my iPhone display ON, and the second half with the display OFF (except for occasional interrupts for me to record the data).

Running a full WebEx videoconference with a voice conference in parallel resulted in the following:

– Power consumption during first hour with display ON: 1.87W

– Power consumption during second hour with display OFF: 1.38W

Our first obvious conclusion is that the display seems to draw about 490mW of power at full brightness. This is interesting because in our previous tests (watching Star Trek), the difference in power between the display on full brightness vs. minimum brightness was about 300mW. So now we can approximate the missing piece of information about the iPhone 4 display. It seems to draw around 190mW from powered down to minimum brightness, and an additional 300mW for full brightness.

In addition, the data also continues to fill in our chart of power consumption as a function of signal strength. Previous tests showed the hotspot feature drawing about 920mW when used to stream Star Trek with two to three bars of signal. In this case we can approximate the power for streaming by taking the total power required with display off (1.38W), and subtracting the estimated power for the voice call (750mW, given Apple’s seven-hour talk time estimate), leaving about 630 mW for the personal hotspot with 4 bars of signal strength.

OK, I know this calculation is a stretch. I really have only one data point. The call power is purely a guess from the datasheet, and the volume of data for the WebEx conference vs. the Star Trek movie isn’t controlled (I forgot to reset the data monitor on my phone!). But it’s fun to see that 1) completely mobile video/audio conferencing is a reality, 2) it can all be done on a single smart phone, and 3) the order of magnitude of energy usage is very similar to streaming a movie. Point No. 3 reinforces our previous notion that power dissipation on today’s mobile devices is dominated by communications (radios and antennas), as opposed to computations (gigahertz or gigabytes). And that’s very interesting.


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