PoE detection process

There are tow types of PoE.

1. Cisco PreStandard
2. 802.3af

One of the major difference between them is the detection process.
Detection process is used by PoE swtich to understand if the connected device need power provision.

1. Cisco PreStandard -
FLP (Fast Link Pause) sent from PoE switch, PoE device will loop it back to Switch.
In Cisco PreStandard PoE, CDP is used to negotiate the power level. Switch provide minimal power (6.3W) at first, then adjust to the value(up to 10W) derived from the power level negotiation.

2. 802.3af -
Small DC current constantly applied to line, PoE devices have resistor and will returned a specific level of resistence to the line. Non-PoE devices will not be harmed by this current.
The resistence level also determined which Power classes is needed by devices.



Reference:
CCNA Voice by Jeremy Cairo
http://en.wikipedia.org/wiki/Power_over_Ethernet
Troubleshooting Cisco PoE
Cisco Catalyst 4000 inline power solution