Short answer: twisted-pair copper carries electrical signals and can supply Power over Ethernet (PoE). Fiber carries light and suits longer links or electrical isolation. A transceiver must match both its host port and the link at the other end. PoE needs enough power per port and across the whole switch.
Start with network layers and full-duplex Ethernet.
A stage rig needs more than matching plugs
Setting up a concert stage means checking signal cables, adapters, run length, and the electrical supply. A microphone connector doesn't tell you whether it belongs in a speaker output, and enough sockets don't guarantee enough power. Ethernet has the same separate questions: 1000BASE-T describes a signaling technology; Cat6A describes cabling; SFP+ describes a pluggable interface family; 802.3at describes PoE capabilities.
Choose the medium for the entire run
| Link example | Typical medium and reach | What to verify |
|---|---|---|
| 1000BASE-T | Four-pair Cat5e or better, up to a 100 m compliant channel | All four pairs, terminations, and channel certification |
| 10GBASE-T | Cat6A, up to a 100 m compliant channel | Some copper SFP+ modules have shorter limits; follow the module specification |
| 10GBASE-SR | 850 nm multimode fiber; 300 m on OM3 or 400 m on OM4 for the cited modules | Fiber grade, connector loss, polarity, and compatible SR optics |
| 10GBASE-LR | 1310 nm single-mode fiber; nominal reach up to 10 km | Optical power budget and receiver limits, not only distance |
| Direct-attach copper (DAC) | Short twinax assemblies between supported ports, often within a rack | Exact length, passive/active support, port speed, and vendor matrix |
A conventional structured-cabling channel includes patch cords, not just the cable inside the wall. A common design allows 90 m of permanent link plus 10 m of patching, subject to the applicable cabling rules. Cat6 can carry 10GBASE-T over shorter qualified runs; don't treat it as a blanket 100 m guarantee. Likewise, 2.5GBASE-T and 5GBASE-T depend on supported ports and a suitable installed channel.
On stage, a long signal run may need a different medium rather than another adapter. Fiber avoids electrical signal coupling between endpoints, but it still needs correctly rated optics and clean connectors. Fiber doesn't deliver ordinary PoE; a remote fiber-connected access point needs local power or a separate powered copper segment.
Match both ends of an optical link
Small Form-factor Pluggable (SFP) modules commonly serve 1 Gbit/s links; SFP+ commonly serves 10 Gbit/s and SFP28 commonly serves 25 Gbit/s. These names don't promise that a port accepts every lower-speed module. Check the exact switch model, software, supported speed, and transceiver matrix, just as a stage adapter must suit the equipment behind its socket.
- Match signaling and fiber: an SR optic and an LR optic aren't a compatible pair simply because both use LC connectors.
- Check transmit/receive paths: ordinary duplex fiber needs Tx at one end connected to Rx at the other. Single-fiber bidirectional optics require complementary wavelength pairs, not two identical ends.
- Check connector type and polish: LC and MPO describe different connectors. Don't mate APC and UPC ends or assume cable color proves the installed fiber grade.
- Check optical levels: digital optical monitoring (DOM) reports transmit/receive power, often in
dBm. Compare it with the module's limits. Too little light and receiver overload are both problems.
For a synthetic budget example, a transmitter minimum of -3 dBm and receiver sensitivity of -10 dBm leave 7 dB before margin. If connectors, splices, and fiber consume 4 dB, 3 dB remains. Also check maximum transmit power against receiver overload; this subtraction alone doesn't certify a link. Never look into a fiber end. Use approved inspection and cleaning equipment with the link made safe.
PoE is a negotiated budget
The PSE (power sourcing equipment) is a switch or injector; the PD (powered device) is an AP, phone, or camera. Standards-based PoE detects a compatible PD before applying operating power. Classification and, where supported, LLDP help determine the allocation. A passive injector may apply power without IEEE detection, so it isn't an interchangeable substitute.
| IEEE PoE type | Maximum PSE output | PD power at highest class |
|---|---|---|
| Type 1 (802.3af) | 15.4 W | 12.95 W |
| Type 2 (802.3at) | 30 W | 25.5 W |
| Type 3 (802.3bt) | 60 W | 51 W |
| Type 4 (802.3bt) | 90 W | 71.3 W |
These are type ceilings and highest-class PD figures, not a promise that every port or PD supports them. Cable losses explain the PSE/PD difference. Higher-power four-pair PoE also makes cable quality, bundle temperature, and installation limits important. Like the stage's electrical distribution, count the required allocations, not just the sockets.
Worked example: eight access points
Suppose a switch has a 370 W available PoE budget, and each of eight AP ports reserves 30 W at the PSE. Each AP must also fit within the PD power available from that class. The switch reserves 240 W, leaving 130 W before other devices and your chosen reserve.
| AP count | PSE allocation per AP (W) | Reserved (W) | PoE budget (W) | Remaining (W) |
|---|---|---|---|---|
| 8 | 30 | 240 | 370 | 130 |
Check the budget after a power-supply failure too. If the remaining supply supports only 185 W of PoE, it cannot maintain all eight 30 W reservations. A redundant supply isn't automatically enough for full-load redundancy. Vendor power-priority policy determines which ports remain powered.
Find the failing part
| Symptom | Check | Interpretation |
|---|---|---|
| Copper link falls to 100 Mbit/s | Negotiated speed at both ends; cable pair test; known-good patch cord | Pair faults are one possibility because 1000BASE-T uses all four pairs; configuration can also limit speed |
| Fiber link stays down | Host support, SR/LR match, polarity, DOM, connector cleanliness | Separate an unsupported module from an optical-path fault |
| CRC/FCS errors increase | Compare counter deltas and test the physical channel | Corrupted frames suggest a link problem, unlike ordinary queue discards |
| AP boots with radios disabled | PD-required power, granted class, LLDP negotiation, switch budget | Some APs reduce features under low power; behavior is model-specific |
Record both port IDs, both optic part numbers, fiber grade, patch path, and power allocation. A link light confirms a physical connection, not the correct VLAN, IP address, or application reachability.
Related topics
Continue with Ethernet's FCS, capacity versus goodput, and link aggregation.
Sources
- IEEE 802.3 Ethernet working group and 802.3bt four-pair PoE project.
- Cisco 10GBASE SFP+ data sheet: specific reach, optical limits, and platform-compatibility caveats.
- Ethernet Alliance: getting started with PoE certification.