BAS, OT and network coordination in a shared closet
A telecommunications closet is a physical fact: heat, power, grounding, and a patch field. It becomes a political fact when BAS, production OT, and corporate IT land on adjacent RU units with different patch disciplines. The BAS needs deterministic delivery of a few hundred objects. OT may need cyclic I/O. IT needs scanners and identity. Without a written coordination — addressing, QoS, who may span, who furnishes UPS, who keys the door — the last team to patch “wins.” That is not architecture. See OT/IT diagnostics and Division 25.
How the investigation is actually run
Photograph the rack on arrival: RU by RU, power strips, earth bars, patch labels, and the switch that “keeps it simple.” List VLANs and uplinks. Then capture during the known bad window, not during a quiet tour. A BAS outage during an IT discovery scan is a clock problem you can prove. An OT cell that fails when a building workstation broadcasts is a domain problem you can prove. Guessing which team is rude is not a method.
PoE loads that starve a controller, mixed bonding that puts shield current on a BAS trunk, and DHCP on a field subnet are typical closet findings. Each has a different owner. The closet rule set should say what may share a switch, what needs a dedicated path, who keys the door, and who approves a patch. Without that, the last patch wins forever.
Hand the written rule to IT, operations, and the controls contractor in the same meeting. If they will not sign a short contract, you have found the real interface: refusal. Document that too.
Symptoms
A BAS outage during an IT discovery scan. An OT cell that fails when a building workstation broadcasts. PoE cameras starving a controller midspan. Ground loops from mixed bonding practices — EMI and SNR.
Causes
One switch “to keep it simple.” DHCP on a trunk that still has MS/TP thinking it is stable. No owner of the patch schedule. Related: OT/IT industrial Ethernet, BAS network architecture.
Investigation
Photograph the rack. List VLANs, uplinks, power, and earth bars. Capture during the known bad window. The conclusion is a closet rule set: what shares a switch, what gets a dedicated path, who approves a patch. Interface essay: engineering at system interfaces.