You don't understand. If the cell is only serving you, your figures are correct. In real usage, and more so if everybody tries to use voip, umts (or hsdpa, which really is umts with a better modem) degrades. You won't get that upload.

What is so difficult to understand? Radio is simple: you have one medium and it has a maximum capacity. Whatever tricks you use, when the capacity is exhausted, that's it.


I am not comparing codecs, I am explaining the fundamental problem telcos have with sip over air. It is not a technological problem, it is a marketing problem. It is a bit like the famous "tragedy of the commons": when customers pay 10 times less per calls, some save 90% (which is not really a problem) and some think they can phone for 10 times as long. The system is not financially tenable in the second case, especially if these users chose wasteful codecs (out of ignorance usually).

You don't have that problem with wires, because it is not a shared medium. When some users bog down their connection running p2p clients like mad or having skype relaying the traffic of half the planet, it is not a problem for the rest of the users. You have a problem with data over air, because when one user abuses the cell, everybody's connection goes dripping. And building additional cells costs money.

It works when the cell is not overloaded. When the cell gets busy, latency degrades. I have used sip and skype over umts without much problem. With hsdpa, I can even use google video talk.

I don't think that latency would be much different between umts and hsdpa. It's the same system, really. hsdpa just uses a more efficient modulation.