Power over Ethernet POE

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Demystifying POE

Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. POE treats each pair as a single conductor, and can use either the two data pairs or the two spare pairs to carry electrical current.

Power over Ethernet is injected onto the cable at a voltage between 44 and 57 volts DC, and typically 48 volts is used. This relatively high voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe.

This voltage is safe for users, but it can still damage equipment that has not been designed to receive POE. Therefore, before a POE switch or midspan (known as a PSE, for power sourcing equipment) can enable power to a connected IP camera or other equipment (known as a PD, for powered device), it must perform a signature detection process.

Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Once this signature has been detected, the PSE knows that higher voltages can be safely applied.

Classification follows the signature detection stage, and is an optional process. If a PD displays a classification signature, it lets the PSE know how much power it requires to operate, as one of three power classes. This means that PSEs with a limited total power budget can allocate it effectively.

POE power classes are as follows
POE Power Class 1 2 3
PSE Power available 4.0W 7.0W 15.4W
Max device power 3.84W 6.49W 12.95W

POE Wikipedia

Comparison of PoE parameters
Property 802.3af (802.3at Type 1) "PoE" 802.3at Type 2 "PoE+" 802.3bt Type 3 "4PPoE"/"PoE++" 802.3bt Type 4 "4PPoE"/"PoE++"
Power available at PD 12.95 W 25.50 W 51 W 71 W
Maximum power delivered by PSE 15.40 W 30.0 W 60 W 100 W
Voltage range (at PSE) 44.0–57.0 V 50.0–57.0 V 50.0–57.0 V 52.0–57.0 V
Voltage range (at PD) 37.0–57.0 V 42.5–57.0 V 42.5–57.0 V 41.1–57.0 V
Maximum current Imax 350 mA 600 mA 600 mA 960 mA
Maximum cable resistance per pairset 20 Ω 12.5 Ω 12.5 Ω 12.5 Ω
Power management Three power class levels (1-3) negotiated by signature Four power class levels (1-4) negotiated by signature or 0.1 W steps negotiated by LLDP Six power class levels (1-6) negotiated by signature or 0.1 W steps negotiated by LLDP<ref>IEEE 802.3bt 145.3.1 PD Type definitions</ref> Eight power class levels (1-8) negotiated by signature or 0.1 W steps negotiated by LLDP
Derating of maximum cable ambient operating temperature None 5C with one mode (two pairs) active 10C with more than half of bundled cables pairs at Imax 10C with temperature planning required
Supported cabling Category 3 and Category 5 Category 5 Category 5 Category 5
Supported modes Mode A (endspan), Mode B (midspan) Mode A, Mode B Mode A, Mode B, 4-pair Mode 4-pair Mode Mandatory

Pinouts

802.3af/at standards A and B from the power sourcing equipment perspective (MDI-X)
Pins at switch T568A color T568B color 10/100 mode B,
DC on spares
10/100 mode A,
mixed DC & data
1000 (1 gigabit) mode B,
DC & bi-data
1000 (1 gigabit) mode A,
DC & bi-data
Pin 1 Wire white green stripe.svg
White/green stripe
Wire white orange stripe.svg
White/orange stripe
Rx + Rx + DC + TxRx A + TxRx A + DC +
Pin 2 Wire green.svg
Green solid
Wire orange.svg
Orange solid
Rx − Rx − DC + TxRx A − TxRx A − DC +
Pin 3 Wire white orange stripe.svg
White/orange stripe
Wire white green stripe.svg
White/green stripe
Tx + Tx + DC − TxRx B + TxRx B + DC −
Pin 4 Wire blue.svg
Blue solid
Wire blue.svg
Blue solid
DC + Unused TxRx C + DC + TxRx C +
Pin 5 Wire white blue stripe.svg
White/blue stripe
Wire white blue stripe.svg
White/blue stripe
DC + Unused TxRx C − DC + TxRx C −
Pin 6 Wire orange.svg
Orange solid
Wire green.svg
Green solid
Tx − Tx − DC − TxRx B − TxRx B − DC −
Pin 7 Wire white brown stripe.svg
White/brown stripe
Wire white brown stripe.svg
White/brown stripe
DC − Unused TxRx D + DC − TxRx D +
Pin 8 Wire brown.svg
Brown solid
Wire brown.svg
Brown solid
DC − Unused TxRx D − DC − TxRx D −