SMS API example

I am a newbie to SMS and am currently working on a project which
involves developing an SMS API to interface to some applications using
simple text/ASCII. The SMS gateway which would allow me to do the
testing has SMPP 3.4 installed.

Can anybody advice me on the best way to do this. I have been looking
at the open source Perl API written by Sampo Kellomaki (symlabs).

However, I am not sure how to handle incoming requests (from SMSC),
process them and forward to the application and sub-sequently process
the application result which needs to be sent to the SMSC.
Can anyone help me get a basic idea on how to do this ?

Any help will be greatly appreciated !

Yamuna