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Mobile VOIP Calculation (for PlatinumTel / ptel)

Update on 2014-01-29

It looks like I was off by a penny for the fee. It’s really 1¢/minute, not 2¢/minute. This means that the overall calculation ends up being about 3.23¢/minute, not 4.23¢/minute. This is decently below the 5¢/minute for PlatinumTel. No idea if it performs well (in terms of latency and drop-outs).

Below is the calculation I did for myself around September of last year:

I switched to PlatinumTel on a pay-as-you-go plan. They are a T-Mobile MVNO. I like ’em because they are pretty cheap (5 cents per minute, 2 cents per text), and also they have a pay-as-you-go data option (10 cents a megabyte). The pay-go data isn’t cheap, but we’re usually on WiFi, and it’s just nice to be able to get data if you’re in a pinch. (As far as I know, other pay-go operators don’t allow pay-go data option.)

I’ve been very happy with them. Their coverage is the same as T-Mobile, but (I’m guessing) they don’t support roaming.

From my calcs, the 10c/MB data rate even with an efficient vocoder (G.729) breaks even with the 5c/minute. And the latency isn’t that great. It may end up being cheaper, since I hear (no pun intended) that voice calls have a lot of silence, so you end up being ahead with the VOIP option.

Here are the grueling details of the calculation:

I installed CSIPSimple for my VOIP app. I use for my VOIP provider at 2 cents per minute. (I also use them at home with an Obihai box.) I also bought a G.729 license for CSIPSimple (I think the code is here, but that doesn’t really give you a license). G.729 is supposed to be 8 kbit/s. However, with TCP/IP and Ethernet overhead (for small packets), this balloons to 31 kbps. So, for a minute of conversation, I need 31kbit/s*60s = 1.872 Mbit = 228 kB. This minute then costs 228 kB * 10c/1024kB = 2.23 cents. I pay 2 cents per minute to to route my calls, so it ends up being 4.23 cents.

I found the latency to be worth the extra .87 cents per minute. Call me extravagant.

(Note I assumed 1 MB = 1024 kB; I’m not sure if ptel means 10c/MB or 10c/1000kB.)

What’s a shame is how Ethernet and TCP/IP balloons the amount of traffic required for such small packets. I have no idea how things are going to fare for VOLTE given this egregious overhead. Perhaps there’s a better transport, but it sort of defeats the purpose of VOIP to use anything other than TCP/IP. (The Cisco page talks of some alternatives to Ethernet such as cRTP which I’m not familiar with.) Of course, none of this is a problem for carriers. They true cost for data isn’t 10¢/MB. Also, LTE has its own MAC, so it maybe packs things in better than Ethernet. For sure, sending a periodic number of LTE resource blocks (RB’s) would be highly optimal for a voice coder.

I have verified that from what PlatinumTel charges me, the 31 kbps is correct. Perhaps the Android stack is based on an Ethernet MAC that is then encapsulated for 3G or HSPA.

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