Raven XT G2263 speed problem

We’ve got AirLink Raven XT modems and use local mobile operator’s sim cards in them.
problem is that when we make communication between two modems, they communicate at very low speed. file transfer rate is about 400-500 Bytes per second.

We checked with our mobile operator company and they say everything is fine on their side. usually UMTS is activated on sim cards but when you’re in non-coverage area or have no 3G device it uses GPRS. That’s true, we have communication through GPRS but the speed is extremely slow.

I also used other mobile operator’s sim cards but the results are exactly the same. Can it be modem problem? has anybody experienced the problem?

And finally: what is typical uplink/downlink speed of Raven XT G2263-CD?

Thanks in advance.