Under Cisco's pre-standard scheme, the PSE (switch) will send a fast link pulse (FLP) on the transmit pair. The PD (device) connects the transmit line to the receive line via a low-pass filter.The PSE gets the FLP in return. The PSE will provide a common mode current between pairs 1 and 2, resulting in and default of allocated power. The PD must then provide Ethernet link within to the auto-negotiation mode switch port. A later CDP message with a type-length-value tells the PSE its final power requirement. A discontinuation of link pulses shuts down power.