PSTN refers to Public Switch Telephone Network which we usually call as the normal phone line and hard line. The PSTN alarm system refers to the alarm system using the normal phone line. When alarms, the system can call you through the phone line.
GSM refers to Global System of Mobile communication which we usually know as cellular phone. GSM alarm system refers to the alarm system using SIM card of cellular phone. When buy, please pay an attention to the frequency/band. When alarms, the system can call you and send SMS through the SIM card.
What's Dual-net mode?
Dual-net mode refers to that the alarm system can both use the PSTN and GSM network. You can connect a phone line with the alarm system, or insert a SIM card, or both. It can call you through phone line or the SIM card, and send SMS through the SIM card.