how do GPRS communication create for sending/receiving message through GSMmodem in c#

Hi everybody!
I am working a project about " send/ receive message from PC through modem gsm using GPRS".
But, i really dont know how to built this system. So , i search some forum and see a solution using TCP/ip protocol(clien/server).
Please help me to understand about solution and give me some examples (source code) for this system in c#!.
I want know : if using tcp/ip protocol in this solution in c#. What step will i have to do?
I will describe my model :
I have a GSM modem connected with PC through com port .And a mobile ( maybe mobile'position anywhere in my country).Message is sended continuous. So, the price for system is very high if i send SMS in GSM network
In order to improve this problem, i want to write application for sending/receiving message by GPRS communication for this system.
Please help me to find solution for this application.