Apple Facetime: Consequences for HSPA networks.

clohamon

http://www.digitalsociety.org/2010/06/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.

And presumably its symmetric.

watty

Actually at average distribution of distance about 6 to 9 users would max out the cell and shrink it to half size.

Same problem with 3G users viewing Youtube.

Speed for ONE user only connected vs likelyhood position in Cell, as signal is 1/4 (and thus speed a bit less) every time you double distance. Area is x4 everytime you double distance


Yes it's symetric.

Why all mobile systems only give rated speed close to mast:

Codes/QAM to for those SNR


From http://www.techtir.ie/comms/fixed-wireless-broadband-better


We have had video calling phones for ages. It was supposed to be one of the "killer apps" for 3G compared to 2G, but it's too expensive and outside of Japan cultural aspects indicate people don't want video calling.
How many use Video, just Audio or even just Text on Skype after novelty of web cam wears off.

It's likely however that the video calling doesn't use full screen resolution.

Largely though the article conclusion is correct.