We’re working on an app that streams a large amount of data out via UDP for GSM-based phones. The idea is to use it while in conjunction with a voice call. If no voice call is present, things are good. However, if a voice call is active, it seems that most of the data we’re sending out simply never makes it, even on HSPA+. If we enable Wifi, then everything is good.
I’m worried that this is a physics problem, rather than a phone one. I ruled out CPU contention by inserting a log statement after we send a chunk of data, and saw that it appeared plenty of times.
Is the 3G radio capable of sending data and voice simultaneously well? We tried a speed test during a voice call and saw a huge drop in bandwidth.
I’ve run into exactly this problem on AT&T’s network in the US. They simply throttle the bandwidth while the call is in progress; whether you’re on 3G or H. To see how much you’re getting throttled try installing speedtest.net’s app and running a speed test during and outside a call.
In the bay area I see data rates as low as 30kbit/s during a call on AT&T’s network. On T-Mobile it’s not throttled so much (same location). A partner in Israel tried the same test and saw only slight throttling during a call — he was still seeing 500+kbit/s using the same phone that we tested on in the US. (The same phone that got several Mbit/s outside a call in the US.)
As the poster above points out, CDMA phones can’t use data concurrently with a phone call at all (but you said GSM explicitly in your post anyway).