PoE Power Supply Mode
As defined in IEEE standard, PSEs provide power for PDs and are classified into MidSpan (the PoE module is installed out of the device) and Endpoint (the PoE module is integrated to device) PSEs. The Endpoint PSE is compatible with 10Base-T, 100Base-TX, and 1000Base-T interfaces. The Endpoint PSE is more widely used than the Midspan PSE.
Endpoint Networking Overview
Endpoint PSEs can work in Alternative A (line pair 1/2 and line pair 3/6) and Alternative B (Line pair 4/5 and line pair 7/8) power supply modes according to different copper line pairs.
- Alternative A mode: Power is transmitted over pairs of lines that transmit data.
The PSE supplies power to PDs using twisted pairs 1/2 and 3/6. The DC power and data frequency do not interfere with each other. Twisted pair 1/2 forms the positive (negative) pole while twisted pair 3/6 forms the negative (positive) pole.
10BASE-T and 100BASE-TX interfaces use twisted pairs 1/2 and 3/6 to transmit data while the 1000BASE-T interface use four twisted pairs to transmit data.
Figure 24-3 10BASE-T and 100BASE-TX interfaces using the alternative A power supply modeFigure 24-4 1000BASE-T using the alternative A power supply mode - Alternative B mode: Power is transmitted over idle pairs of lines.
The PSE supplies power to PDs using twisted pairs 4/5 and 7/8. Twisted pair 4/5 forms the positive pole while twisted pair 7/8 forms the negative pole.
Figure 24-5 10BASE-T and 100BASE-TX interfaces using the alternative B power supply modeFigure 24-6 1000BASE-T using the alternative B power supply mode
Generally, a standard PD supports the two modes, whereas the PSE only needs to support one mode.
