Written by Miloslav Bien, Thursday 4 June 2026

Accurate time is easy to overlook – until something breaks. For industrial cellular routers deployed in remote sites or critical infrastructure, a synchronized clock is a hard dependency: without it, TLS handshakes fail, VPN tunnels reject authentication due to timestamp skew, syslog data becomes useless for forensic analysis, and time desynchronization across devices can corrupt SCADA/IIoT data integrity.

Time synchronization sources

So where does a router actually get its time from? The answer depends on the hardware - most basic models may support NTP only, while cellular routers add NITZ synchronization, and models with a GNSS module can use satellite time as well.

NTP (Network Time Protocol) is the most accurate option, typically within several milliseconds. It uses a hierarchical stratum model and queries upstream servers over UDP port 123. The main limitation: if the WAN link is down, the router falls back to its local oscillator, which drifts.

Cellular network time (NITZ) is broadcast by LTE/5G base stations with typical accuracy of up to several seconds. Less precise than NTP, but available whenever a cellular connection exists – independent of WAN reachability.

GNSS-based synchronization uses satellite signals to determine time on-site without any network connectivity. Typical accuracy is around one second – sufficient for general clock correction, but not recommended for high-precision requirements.

Configuration on Advantech ICR routers

On Advantech ICR routers, all three sources are configured under Configuration → Services → NTP. The full set of options is available on the ICR-4233, which integrates both an LTE modem and a GNSS receiver.

When Synchronize clock with remote NTP server is enabled, the router queries up to three configured servers – anywhere on the network, not necessarily on the internet. Synchronize clock with cellular network uses NITZ as a fallback when no NTP server is reachable. On equipped models, Synchronize clock with GNSS adds satellite time as an additional source. Enable local NTP server turns the router into a time source for the local network – routers can be chained, though each hop adds a stratum level and reduces precision. For full configuration details, refer to the Configuration Manual.

How are multiple sources prioritized?

When more than one synchronization source is enabled, the router does not simply follow a fixed priority order. Instead, it evaluates source quality continuously using the stratum system: GNSS time is treated as stratum 10, cellular network time (NITZ) as stratum 11, and any remote NTP server carries its own stratum inherited from upstream. Lower stratum generally means higher quality – but stability matters too. An intermittent GNSS signal at stratum 10 may be considered less reliable than a consistent NITZ source at stratum 11. The router's internal algorithms weigh both stratum and signal regularity when selecting the active time source.

Most router models also include a battery-backed real-time clock, which preserves the last known time across restarts. After a reboot, the router immediately serves the stored time to local clients and updates it once synchronization with an external source is re-established.

Recommended deployment approach

Use remote NTP as the primary time source, with NITZ as a fallback during WAN unavailability. GNSS is an option where internet connectivity is unavailable altogether, but for high-precision requirements a reachable NTP server is always preferred. Monitoring NTP peer reachability, stratum, and offset should be part of standard device health checks.