SMPP: submit_sm and deliver_sm

Question

What is the difference between submit_sm and deliver_sm operations in the SMPP protocol?
When to use each one?

Answer 1

submit_sm and deliver_sm are both methods to transfer SMS between an SMPP client (usually some application or ESME) and an SMPP server (usually some SMS gateway or SMSC).

They differ just in the direction of the SMS flow.

When an application is sending SMS to the server it typically uses the submit_sm method and when the server is sending SMS or DLR to the application, it uses the deliver_sm method.

This is the default usage, but there are also implementations out there, which can reverse the behaviour per configuration.

The default is nicely explained on https://smpp.org/.