Physical mediums - fiber and copper
In one line
The physical layer sets the distance, bandwidth, and resilience ceiling that every higher layer inherits - and cabling outlives the electronics plugged into it.
Copper
Twisted pair (Cat5e/6/6A), ~100 m channel limit, carries PoE / PoE+ / PoE++ for powering APs/phones/cameras, but is susceptible to EMI and won't reach high speeds at distance. DAC (twinax) is short-reach copper for the rack: passive (~1-3 m) or active (to ~7 m+) - cheap, low-power, no optics.
Fiber
- MMF (OM3/OM4/OM5) - shorter reach, cheaper optics; good inside a building/DC.
- SMF - long reach, more expensive optics, effectively future-proof for the backbone.
- Connector geometry: duplex LC for serial, parallel MPO for 40/100/400G and breakout (e.g., 1x40G -> 4x10G).
- Transceivers (SR/LR/ER) set reach; the medium dictates the optic - see Optics and cabling selection for the AI/DC-specific DAC/AEC/AOC tradeoffs.
Design drivers
Distance, the bandwidth roadmap, PoE needs, EMI and path diversity, and structured-cabling lifecycle - because fiber plant outlives several generations of switches, backbones favour SMF.
IPv6 / dual-stack note
L1 is address-family agnostic; the medium choice is identical for v4 and v6.
Related
Spaced repetition
What is the practical channel-length limit for balanced twisted-pair copper (Cat6A)?
~100 m.
Choose MMF vs SMF: MMF is [...]; SMF is [...].
Choose MMF vs SMF: MMF is shorter reach with cheaper optics; SMF is long reach, pricier optics, future-proof for backbones.
The short-reach copper cabling used inside a rack (passive ~1-3 m / active to ~7 m+) is [...].
The short-reach copper cabling used inside a rack (passive ~1-3 m / active to ~7 m+) is DAC (twinax).
Sources
- Cisco Press, Optical Network Design and Implementation; structured-cabling standards (TIA-568).