The physical layer 12 min read
Copper, Fiber, SFPs, and PoE
Understand what the link lights are sitting on: copper categories, multimode and single-mode fiber, transceivers, optical diagnostics, and PoE budgets.
- Choose the appropriate copper or fiber medium for a link.
- Interpret OM and OS fiber designations without treating them as port speeds.
- Explain the roles of SFP modules and DOM diagnostics.
- Distinguish PoE type, class, source output, and device power.
The medium defines the first boundary
Copper Ethernet carries electrical signals and may also carry power. Fiber carries light, is immune to electromagnetic interference, supports longer distances, and electrically isolates the connected equipment. Neither is universally better; the application, distance, bandwidth, environment, and service strategy decide.
A 1000BASE-T copper link is normally designed around a 100-meter channel limit. Higher speeds may impose stricter cabling requirements. Fiber distance depends on the Ethernet optic, wavelength, fiber type, connector loss, splices, and the complete optical budget—not the jacket label alone.
OM and OS are fiber categories
OM3, OM4, and OM5 are laser-optimized multimode fiber categories. OS2 is the common indoor/outdoor single-mode category. These labels describe the fiber's performance; they do not assign one fixed data rate.
OM3 commonly supports 10GbE to 300 meters and 40/100GbE to roughly 100 meters using common parallel optics. OM4 extends those typical distances to about 400 meters for 10GbE and 150 meters for common 40/100GbE parallel links. OS2 can support Ethernet links over kilometers when paired with the appropriate single-mode optics.
Always verify the exact Ethernet standard and optic datasheet. A fiber category tells you what the glass can support; the transceivers define the actual link.
SFPs and DOM
An SFP-family module converts the switch's electrical interface into the signaling required by copper or fiber. The module must match the port, speed, wavelength, fiber type, connector, and optic at the opposite end.
DOM means Digital Optical Monitoring, also called DDM by some manufacturers. A DOM-capable optic can report transmit power, receive power, temperature, supply voltage, and laser bias current. Receive power is especially useful: it turns 'the fiber seems weird' into an actual measured optical level that can be compared with the module's specified operating range.
PoE is negotiated power
Power over Ethernet uses a PSE, such as a switch, to detect and classify a PD, such as a camera or touchscreen, before applying normal operating power. The switch's maximum output is higher than the guaranteed power available at the endpoint because the cable consumes some energy.
The four modern PoE types reach approximately 15.4 W, 30 W, 60 W, and 90 W at the PSE. The commonly quoted guaranteed PD values are 12.95 W, 25.5 W, 51 W, and 71.3 W. Device class provides a more detailed power request inside those types. A switch also has a total chassis power budget, so enough capable ports do not guarantee enough simultaneous wattage.
Put it in an AV system.
Remote AV rack
Use fiber between buildings or distant IDFs for reach, EMI immunity, and electrical isolation. Record fiber type, strand count, connector, optic standard, and estimated loss on the drawings.
PoE-heavy room
Cameras, touchpanels, speakers, and encoders may all link successfully while the switch lacks enough aggregate PoE budget. Schedule each device's expected draw and preserve headroom.
- MMF / SMF
- Multimode fiber / single-mode fiber.
- OM / OS
- Optical multimode and optical single-mode fiber categories.
- SFP
- Small Form-factor Pluggable transceiver.
- DOM/DDM
- Digital optical diagnostics reported by a compatible transceiver.
- PSE / PD
- Power sourcing equipment and powered device in a PoE system.
Read one real port
Open a managed switch and inspect one active fiber port and one PoE port. Record negotiated speed, optic type, receive power if DOM is available, PoE class, actual draw, and the switch's remaining PoE budget.
Three quick questions.
01Does OM4 mean the link automatically runs at 100 Gbps?
No. OM4 describes the fiber. The port and matched transceivers establish the Ethernet speed.
02What DOM value is most immediately useful for a questionable fiber link?
Received optical power, compared with the optic's supported range.
03Why is a 90 W PoE switch port not the same as 90 W at the device?
Some power is lost in the cable, so the guaranteed power at the PD is lower.
Practical example: Twelve powered devices do not equal “any 12-port PoE switch”
Suppose a lab design budgets four PoE+ devices at 30 W per switch port and eight PoE devices at 15.4 W per port:
- Four × 30 W = 120 W
- Eight × 15.4 W = 123.2 W
- Planned switch-side allocation = 243.2 W
- An illustrative 25% design reserve adds 60.8 W, giving a 304 W target.
That reserve is a planning choice, not an IEEE requirement. Check the real device classes, LLDP negotiation and switch allocation method; measured idle draw is not a safe substitute for required maximum allocation.
There are two separate checks: can each port supply the required PoE type, and can the installed power supplies support the total budget? A switch with enough ports can still fail either check. If redundant power matters, check the available PoE budget with one supply failed too.
PoE+ “30 W” is the switch-side figure; up to 25.5 W is available at the powered device under the standard's cable-loss allowance. Do not mix switch-side and device-side wattages in the same budget.
Source: Cisco PoE power requirements reference. The device counts and reserve are illustrative BMT arithmetic, not a product recommendation.