# Which OT devices does passive monitoring miss?

Devices that do not talk on a segment you mirror. That includes devices that are silent most of the time, such as a spare controller or a unit that only answers a master you do not see; devices on serial fieldbuses such as Modbus RTU or Profibus; devices behind an unmanaged switch whose traffic never reaches the mirrored link; equipment that talks only over its own cellular router; and remote sites without a sensor. Passive monitoring also sees less inside encrypted protocols, and it cannot read a firmware version that the device does not announce. Close those gaps with a walk-down, the engineering project files and, where the plant accepts it, a carefully planned query with the vendor's own tools during a maintenance window. NetSenX is passive only, so its network map shows what communicates on monitored segments, and nothing more.

FAQ: [OT and IoT](https://netsenx.com/en/faq/ot) · https://netsenx.com/en/faq/which-ot-devices-does-passive-monitoring-miss

## Read next

- https://netsenx.com/en/ot-network-monitoring
- https://netsenx.com/en/compare

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