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How Much Data Does VoIP Use?

Monday, April 07, 2008

I thought about this after reading about the onerous terms and conditions that AT&T and Verizon Wireless have on their mobile data service. Basically, if you go over 5 gigabytes of data in a month, you may end up paying $0.50 per megabyte overage--in other words, $500 a gigabyte.

With that in mind, let's look at what a typical VoIP call would take. In this example, I used Skype, but I assume even a SIP-based call would have a similar profile. I did a 20 second call to our favorite echo123 user, i.e. the Skype Call Testing Service.

In that period of time, Skype generated 385,855 bytes of network traffic, or about 377 kilobytes of data. For a minute of VoIP conversation, that's just a hair over a megabit of data.

Yes, I realize my math may be somewhat off here as I include call setup and tear down packets. Then again, keep in mind Skype does a lot of things that are hard to account for. Are we really account for just voice traffic here?

For the sake of argument, I did the same experiment with Gizmo5, which uses industry-standard SIP for communication. I got roughly 357 kilobytes of data for a minute of call. In this case, I was able to focus only on the voice stream itself--not the call set up, etc.

This is hardly an apples to apples comparison, and you could argue that I could have gotten my numbers more precise. But for the purposes of ballpark comparisons, it's safe to say that as long as you don't talk for hours on end over a mobile Internet connection, you're probably safe with the occasional Skype or Gizmo5 call.

That is, assuming you have an "unlimited" plan. If you pay per megabyte, all bets are off.