> you need to pay for some special services

maybe not… as i said it is possible to open a session if mobile sends some data to server, and server gets that data from mob_ip:rand_port, so we can use that session to send data from server to mobile…

> buffer_size = maximum_latency*average_rate

i used something similar in my project.

Once again, thank you for your posts.

> his way we need an open port in one side of connection

I think this is an operator issue. In Brazil, some operators provides real IPs and do not block incoming connections to your phone. I did a test here with TIM/Brazil and I could connect to the phone. But I know that we have some operators that block incoming connections and provide just IPs reserved for internal networking. In this case, you need to pay for some special services. Please, check this info with your mobile operator.

> Another problem in using UDP is that we have to manually control all traffic to UDP port.

UDP issues… Possible suggestions: increase receiver buffer according to the equation:

buffer_size = maximum_latency*average_rate

But it is necessary to use setsockopt (option SO_RCVBUF in SOL_SOCKET group). I need to check if this option is available for PyS60.

Your posts a really informative and nice.
I would like to discuss something about UDP. Because the protocol is not controlled, we face few problems, for example: we have an application in mobile phone, which is waiting for some data from server with real IP address. The problem is that we don’t have open ports in mobile phone while connecting through 3G or GPRS. I managed to use it by sending something from mobile phone first, and then using that connection with given random port in server. This way we need an open port in one side of connection. Maybe You have some suggestions about that.
Another problem in using UDP is that we have to manually control all traffic to UDP port. It is necessary to limit the traffic or there will be many lost packets.

Thanks for Your posts!