Power Over Ethernet - Standard Implementation

Standard Implementation

Standards-based power over Ethernet is implemented following the specifications in IEEE 802.3af-2003 (which was later incorporated as clause 33 into IEEE 802.3-2005) or the 2009 update, IEEE 802.3at. A phantom power technique is used to allow the powered pairs to also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in common mode as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply may be inverted by crossover cables; the powered device must operate with either pair: spare pairs 4–5 and 7–8 or data pairs 1–2 and 3–6. Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a diode bridge.

Standard PoE parameters and comparison
Property 802.3af (802.3at Type 1) 802.3at Type 2
Power available at PD 12.95 W 25.50 W
Maximum power delivered by PSE 15.40 W 34.20 W
Voltage range (at PSE) 44.0–57.0 V 50.0–57.0 V
Voltage range (at PD) 37.0–57.0 V 42.5–57.0 V
Maximum current 350 mA 600 mA per mode
Maximum cable resistance 20 Ω (Category 3) 12.5 Ω (Category 5)
Power management Three power class levels negotiated at initial connection Four power class levels negotiated at initial connection or 0.1 W steps negotiated continuously
Derating of maximum cable ambient operating temperature None 5°C with one mode (two pairs) active
Supported cabling Category 3 and Category 5 Category 5
Supported modes Mode A (endspan), Mode B (midspan) Mode A, Mode B

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Read more about this topic:  Power Over Ethernet

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