Quad-band 850/900/1800/1900MHz GPRS multi-slot class 12/10 GPRS mobile station class B Compliant to GSM phase 2/2+ Class 4 (2 W @ 850/900MHz) Class 1 (1 W @ 1800/1900MHz) The SIM800A requires huge power, so use a 12V DC 2 to 3 Amps adapter, any wrong adapter might cause damage also to the modem. On the board there are Tx(D) and Rx(D) pins, near the 5V and GND pins, don't ever connect anything to them or send any command to them, they are not the TTL pins for serial communication, they are for upgrading the firmware, and if you connect them to the microcontroller or computer, the firmware will get damaged and the module will stop working. Please read carefully the marking on the SIM card tray on how to open or close, those are made of brittle plastics and will break if you put too much pressure on them, so be very gentle and careful while removing or inserting SIM Cards. Don't tight the antenna too much, the screw threads might get damaged by excessive or over tightening of antenna. Please don't touch the connections or components and always put the modem on a non conductive base