Power over Ethernet and Powered Devices

What PoE is

Power over Ethernet (PoE) carries both electrical power and Ethernet data over one twisted-pair cable. The switch (or injector) is the power sourcing equipment (PSE); the device receiving power — an access point, IP phone, camera — is the powered device (PD). PoE eliminates a separate AC adapter at each endpoint, which is why it dominates AP, phone, and camera deployments.

The IEEE standards and their power levels

Know these numbers for the exam. PSE output is higher than what the PD receives because the cable dissipates some power.

A cheap fixed-power injector or an older 802.3af port may not satisfy a modern 802.3at/bt access point that needs more than 12.95 W — the device may boot in a reduced-power mode, disable a radio, or fail to power at all.

The switch power budget

Each PoE switch has a total power budget (for example, a switch with 370 W of PoE budget cannot fully power 48 ports each demanding 30 W = 1,440 W). When the budget is exhausted, the switch denies power to additional PDs even though the data link works. Always ask: does this port support PoE, at what type, and is there budget left?

A PoE troubleshooting checklist

Work from the physical layer upward:

1. Port capability — Is the specific port PoE-enabled and at the needed type (af/at/bt)?
2. Power budget — Is there enough remaining budget on the switch for one more PD?
3. Device requirement — What power class does the PD request? A high-power PD on a low-power port stays dark.
4. Cable condition — A damaged or out-of-spec cable can break power delivery while data limps along (or vice versa).
5. Link state — Is there an Ethernet link at all? Power and link are separate conditions.

Power up does not equal data works

A PD that powers on has not proven its data path. An IP phone whose screen lights up may still fail to pass traffic if its VLAN, DHCP, or pass-through port is wrong. A camera may power but not reach its recorder because of a VLAN or firewall rule. Always separate the two symptoms: does it have power? does it have working data connectivity?

Common traps

• Trap: assuming PoE is needed on every RJ-45 port. Most ports carry data only; PoE is a switch/port feature.
• Trap: calling PoE a wireless or laptop-charging technology. It is wired, between PSE and PD, and laptops are generally not PDs.
• Trap: declaring a phone or camera healthy because it powered on. Power proves nothing about VLANs, link, DHCP, or pass-through.
• Trap: ignoring the power budget. A switch can run out of PoE watts long before it runs out of ports.

How power gets negotiated

PoE is not a dumb voltage feed. Before applying full power, the PSE performs detection — it places a small voltage on the line to confirm a valid PD signature resistor is present, so it never energizes a legacy non-PoE device and damages it. It then performs classification, where the PD signals its power class (Class 0-4 under af/at, extended classes under bt), letting the switch reserve the right amount from its budget. Only then does the PSE apply operating power. This is why a damaged cable or a non-compliant injector can leave a perfectly good camera dark: if detection or classification fails, the switch withholds power by design.

Injectors and midspans

When a switch port does not provide PoE, a PoE injector (a midspan device) can add power to the cable between the switch and the PD. Injectors are fixed to a particular standard, so an 802.3af injector will not satisfy an 802.3at access point that needs 25.5 W at the device. The reverse — a higher-power injector feeding a lower-power device — is generally safe because negotiation limits delivery to what the PD requests. Always match the injector's type to the PD's requirement, and remember the same 100-meter copper channel limit still applies including the injector run.

Reading a powered-device symptom

Consider an access point that powers on, shows an Ethernet link, but serves no clients. Walk the layers: PoE detection succeeded (it has power), Layer 1-2 link is up (link LED green), so the failure is higher — perhaps the AP is on the wrong VLAN, cannot reach its wireless LAN controller, has no IP from DHCP, or is awaiting configuration. Contrast that with an AP that is completely dark: there, suspect PoE first — port capability, budget, class mismatch, or cable. Separating the power question from the data question is the single most valuable habit on PoE tickets, and it is exactly what the CCST scenario questions reward.