What physical constraints shape a campus design before you choose any switches?
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Distance, power and space. Copper twisted pair is limited to a 100 m channel, so every outlet must be within reach of a closet. Closets connect to the main equipment room over fiber, ideally on two diverse paths. Each closet needs power and UPS for the PoE load, cooling for the heat and rack space for growth.
Distance
- Structured cabling allows a 100 m channel for twisted pair: up to 90 m of fixed cable plus patch cords. That decides where the intermediate distribution frames (IDFs, the closets) go.
- Closet to main distribution frame (MDF) runs use fiber. Multimode is fine for short runs inside a building (10GBASE-SR reaches about 300 m on OM3 and 400 m on OM4). Use single-mode for long runs and between buildings.
- Run redundant uplinks through different risers or conduits. Two fibers in one conduit fail together.
Power
- PoE per port at the switch: 802.3af up to 15.4 W, 802.3at (PoE+) 30 W, 802.3bt 60 W (Type 3) or 90 W (Type 4). Newer APs, PTZ cameras and some lighting can need more than PoE+.
- Size the power supplies for total PoE draw plus the switch itself, and decide whether PoE must survive one supply failing.
- UPS runtime decides how long phones, APs and badge readers stay up in a power cut.
Space and cooling
A PoE switch turns much of its draw into heat in the closet, and the powered devices dissipate the rest. Closets need ventilation or cooling, rack space for future switches and a tidy patching layout.
Follow-ups
What if some desks end up 120 m from the nearest closet?
Those runs are out of spec. They may link at a lower speed, show errors or fail certification. Fix it with another closet, a small switch fed by fiber nearer the desks, or a different layout, not by hoping the cable works.
Traps
- Sizing switches by port count and forgetting the PoE budget, so the last cameras or APs do not power up.