You can use one of various existing commercial SMPP applications such as Diafaan SMS Server (FAQ: How to connect Diafaan SMPP server), ActiveXperts SMS Server (FAQ: How to connect ActiveXperts SMPP server), Amarrelo SMS Manager, Inetlab.SMPP or Logica OpenSMPP (FAQ: How to connect Logica SMPP server). Many others can be easily found using for ex. Google.
Alternatively you can implement it by your own to your own proprietary application. Detailed specifications of the SMPP protocol ver. 3.4 are available for ex. here if needed.
SMPP v 3.4 is without the specifaction of default character set. If you keep the default value then please use ASCII encoding.
3) Gateway's AT commands details