Before the iPhone 4 with Apple’s FaceTime application was released to the public, I estimated 667 Kbps to 2000 Kbps based on the assumption of full screen quality at 960×640 resolution.  It turned out that my assumption that Apple would opt for full screen resolution would be wrong and the actual resolution looks to be substantially less 1/6th the pixel count.  The actual bandwidth people are measuring is 100 Kbps to 392 Kbps.

Mark Gurman did a recent test indicating that FaceTime uses an average of 392 Kbps when there is a lot of movement in the video.  AnandTech measured between 100 to 150 Kbps but the screen shots they show have a resolution of 400×267 but some of the captured images don’t even fill all the pixels with quality pixels.  Based on what we are seeing here, FaceTime doesn’t begin to use 1/6th of the pixels of the iPhone 4 “Retina” display.

The question now becomes why FaceTime is still banned on AT&T’s 3G network if it “only” consumes 392 Kbps.  If we assumed that a 3G cell has 7 Mbps of total capacity (under ideal conditions), that would only support 17 users on an entire 3G cell meant to service hundreds if not thousands of customers.  Furthermore, the actual FaceTime capacity of a 3G network under real world conditions where some wireless consumers are in bad coverage areas which drags down the speed of the entire shared network.

Furthermore, figuring out the capacity constraints of applications isn’t straightforward.  A typical 802.11g Wi-Fi network has an actual capacity of 22 Mbps yet Apple Store employees have explained to us at Digital Society that a Wi-Fi access point only supports 3 FaceTime users.  One would assume that 22 Mbps of capacity can support 55 400-Kbps applications concurrently but that’s only possible with 100% efficiency.  Wireless networks that allow end points to use the network without scheduling often end up being less than 10% efficient due to random packet collisions which spawns a lot of retransmit requests which bog down the network even more.  This same phenomenon also explains why Wi-Fi wireless networks are horrible at supporting VoIP despite the tiny amount of bandwidth usage.