SMPPSim is a testing utility which mimics the behavior of an SMPP based SMSC. SMPP stands for Short Message Peer to Peer Protocol. SMSCs (Short Message Service Centers) from Logica Aldiscon support this protocol, as a means of allowing external (to the GSM network) applications to submit and receive SMS messages to / from the SMSC. Some other SMSC vendors also support SMPP. If you are intending to implement support for SMS text messaging using the SMPP protocol in your application, you will need some way of testing your application. Ultimately of course, you should aim to use a real SMSC for testing before going live. Hopefully the network operator you are working with can supply a test SMSC for this purpose. 