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Estimate of network bandwidth for iPhone 4 FaceTime

By 9 June 2010 13 Comments

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.

13 Comments »

  • Some noteworthy posts – June 9 2010 | Technology for Mortals said:

    [...] Estimate of network bandwidth for iPhone 4 FaceTime From my estimates, FaceTime video conferencing on iPhone 4 will take 667 Kbps to 2 Mbps. If my assumptions about the resolution and frame rate of FaceTime is correct, it explains why FaceTime is limited to Wi-Fi operation. [...]

  • aeleth said:

    You don’t need 30fps for a video call. 5-15 frames may be perfectly acceptable. Apple’s wifi-only policy is because of AT&T’s network limitations.

  • George Ou (author) said:

    @aeleth

    “You don’t need 30fps for a video call. 5-15 frames may be perfectly acceptable. Apple’s wifi-only policy is because of AT&T’s network limitations.”

    You don’t “need” 960×640 resolution and we could just do 160×120 video conferencing at 5 fps. That would certainly solve the problem. The only problem is that it is no longer at a quality level that people want, so the Wi-Fi restriction is a very good solution because most people have access to Wi-Fi most of the time.

    Now your own comment contradicts yourself. First you say we don’t have to do 30 fps and that it can be dropped to 5 fps (which would cut bandwidth down to 100 Kbps), then you blame it on AT&T’s network limitation. That would certainly improve the bandwidth situation, but that would mean that your “solution” is suggesting that Apple can change the situation.

    The reality is that the network limitation is something all wireless operators face. LTE makes the situation better, but it still won’t allow for widespread video unicasting.

  • Rudy said:

    Steve Jobs is a perfectionist. He wants things to look good. That means max resolution and FPS. Everything else has to catch up to his “vision”.

  • Me said:

    By ‘perfectionist’ you mean douche.

    And what sort of idiot actually thinks the statement “most people have access to Wi-Fi most of the time” is true. What sort of iPhone apologist excuse is that?

  • Digital Society » Blog Archive » Mobile networks aren’t for Video on Demand said:

    [...] estimated that the new FaceTime video conferencing application for the new iPhone 4 will probably consume [...]

  • bigRoN said:

    So… I have an AT&T 3G card that I currently plug directly into my PC. If I get a CradlePoint Personal WiFi hotspot, I can use the same network that the iPhone would otherwise be using to allow the iPhone 4 to use FaceTime when no other WiFi was available.

  • Andrew said:

    How fast is your broadband connection though? The ‘A’ in ADSL means ‘asymmetric’: I have a 10mbps download speed but only 0.5mbps upload speed. Those are the official numbers; in every-day use the upload speed maxes out around 400kbps. That’s a fairly typical upload speed for domestic broadband.

    So whatever protocols and compression FaceTime uses, it can’t exceed those limits.

  • George Ou (author) said:

    @Andrew

    Most video conferencing software have variable rates, so 400 Kbps would likely be the low end of the spectrum if you want reasonable image quality.

    Most 10 Mbps broadband connections have 1 Mbps up.

  • Anonymous said:

    [...] [...]

  • Josh Skidmore said:

    Facetime averages about 300Kbps/sec or roughly 38KB/sec over wifi.

    This is an interesting conversation. Regardless of Steve Jobs’ “we’re working on the carriers” comment, I believe that the 3G/EDGE issue is more of a latency/packet-loss issue more than speed or network load. If you’ve pinged over 3G, you’ll notice the delays and dropped packets.

    I would assume that wifi is also a first attempt effort at this. There’s a lot more that needs to be incorporated into the UI. For instance, a quality/buffer indicator to allow the user to have some insight on the call status. If the call begins to get delayed due to lost packets, there’s currently no way for the user to know other than loss of responsiveness.

  • Jim said:

    I’m doing video and audio right now on Iphone cross platform and using only 30Kb/s per side at high (15-30) frames per second and 320×288. (-: Should even work on for Iphone to PC with PC on dial up modem. HAHA

    Its very clear. I’m trying to work it out to offer to everyone on my servers.

    Imagine if you wanted to make a phone call but you could only call people with same service on same phone as you… How useful is that? My solution is platform independant – Iphone to Mac, Iphone to PC, Iphone to Iphone, Iphone to Android all work with very low bandwidth reqs and nice high quality. Destinations matter – I want to enable video calling that is prolific as regular phone calling and I have the simple tech to do it. Its working, so there is no reason not to offer it.

  • James said:

    Does facetime work with 512 kbps speed? please respond

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