The Critical Weakness of Internet-Only Smart Security
Smart security systems that rely solely on IP notification (Wi-Fi, fibre) have a critical vulnerability: if the router loses power, the fibre line fails, or an intruder cuts the phone/internet line before entering, all alerts go silent. Cellular backup solves this by sending alerts over a cellular network independent of the ISP.
Cellular Modules for Backup Communicators
SIM800L (2G GSM/GPRS): 80–150 THB, supports SMS and calls on 2G networks still active in Thailand. Alert-only (no meaningful data speed). Controlled by Arduino/ESP32 via AT commands.
SIM7600E (4G LTE-A): 800–1,500 THB, supports 4G LTE data + SMS + voice calls on True/DTAC/AIS 4G networks. Data speed up to 150 Mbps — can send camera snapshots via 4G.
A9G (GSM + GPS): 200–350 THB, combines GSM and GPS in one module — useful when location tracking is also required.
Cellular Backup Architecture in Home Assistant
Hardware: SIM7600E module connected to Raspberry Pi via USB; AIS/True SIM card (1GB/month data package ~100 THB); UPS for router + Pi to prevent power-cut attacks.
Software: Gammu (Python library) or HA SMS integration via notify.sms. Fallback logic: if IP notification times out after 10 seconds, automatically switch to SMS/call.
Automation example: trigger on alarm_triggered = ON, condition internet_status = offline, action: send SMS alert with ALARM! message to two mobile numbers.
Dual-Path Notification: IP + Cellular Simultaneously
For high-security setups, send alerts on both paths simultaneously — LINE OA (IP) + SMS (cellular). If LINE fails (no data signal), SMS still arrives because SMS uses circuit-switched technology requiring no data connection.
Protection Against GSM Jammers
Some intruders use GSM jammers to block cellular signals. Mitigate with: a multi-band SIM7600 operating on multiple frequencies (700/850/900/1800/2100/2600 MHz) — harder to jam across all bands simultaneously; a local siren that activates without needing to send any alert; an alarm panel with tamper detection that triggers on cover removal or sensor disconnection.
SIM Card Management for IoT
AIS NB-IoT SIM: suited to backup systems sending occasional SMS — low cost, pay-as-you-go. True Move H IoT SIM: annual package pricing, low cost, good for backup systems also needing 4G data. Choose a SIM with a long active-without-top-up life to avoid service interruption.
ROI and Cost
SIM7600E module: 800–1,500 THB. SIM card + 1-year data package: 300–600 THB. Total year-one cost: 1,100–2,100 THB — a fraction of home asset values. Subsequent years: only the SIM package renewal at 300–600 THB/year.