Estimate of network bandwidth for iPhone 4 FaceTime
There’s quite a bit of excitement about Apple’s new “FaceTime” mobile video conferencing application on the new iPhone 4, but many people are lamenting the fact that it only runs over Wi-Fi and not the 3G network and wondering why. While I haven’t been able to test or measure FaceTime, I can make some fairly reasonable estimates of how much bandwidth it would require.
First I have to make some assumptions about the resolution (which will have to be confirmed at a later date when the hardware is available to me). I’ll assume that FaceTime runs at the native screen resolution of 960×640 and that it operates at 30 frames a second (fps). That would mean that FaceTime videos have a theoretical bandwidth requirement that is 2/3 of a 1280×720 (720P) video.
720-30P (720P at 30 fps) video conferencing generally requires 1 to 3 Mbps of network bandwidth which means the video is compressed 663:1 or 221:1. That means the compression codec is discarding 99.5% to 99.8% of the uncompressed video data. If we use the same compression level on 960×640 resolution FaceTime, it would require 667 Kbps to 2 Mbps of bandwidth which is 50 to 150 times more bandwidth intensive than a typical GSM phone call.
These compression levels are already extremely high and broadcast video or Blu-ray video tend to limit compression at 75:1 ratio, so it’s unlikely we can increase the ratios further without an even bigger drop in quality. For a mobile device like the iPhone 4, compression will be an even bigger challenge because of the amount of movement in the video.
From a network capacity standpoint, it can be problematic if a sufficient number of users in the same cell tried to use FaceTime over a 3G network simultaneously. At 667 Kbps, just 20 FaceTime users would saturate a 14 Mbps HSPA based 3G cell even if we assumed no scarcity at the base station backhaul, and that every user was in perfect range operating at the peak transmission/receive rates, and that there was no other traffic on the network. Wi-Fi isn’t nearly as congested because much fewer users can attach to a short-range Wi-Fi base station. If my assumptions about the resolution and frame rate of FaceTime is correct, it explains why FaceTime is limited to Wi-Fi operation.