SIM900D GSM/ GPRS Modem TTL is a high performing modem that facilitates easy and low cost way to connect to any micro-controller. It is easy to connect micro-controller to any 2G GSM/GPRS network so that messages, sms and emails can be sent out from the web without a wired connection. Ideal for M2M and telemetry applications, this product range fit diverse industrial applications
If you want to connect your microcontroller board with this GSM modem just connect controller RX-TX line of controller directly with modem's RX-TX line .No special level converter required.
SIM900 AT Commands