Why should I use SMPP instead of a GSM modem ?

[Leon]

You should use SMPP instead of a GSM modem in the following scenarios:

You are sending more then 500 messages per hour. A GSM modem wil take about 5-10 seconds to send a message, so it will be to slow when you need to send more messages.

You are sending messages to different countries. Using your local provider you will pay much more for SMS to foreign countries, then when using a global SMPP provider.

You want a delivery report for every message.
When sending using GSM, the reference of a message is just 1 byte ( 0-255 ). This means that when some messages are not acknowledged, and you already sent more then 255 messages, there are no unique message references anymore.