Hi Abhijit,
Hope today finds you well buddy! I hope Michael doesn't mind me weighing in on this. I'm sure he will add some excellent info as well!
Here is some related info (note that all competitors now use 802.3af);
This describes the differences between the older Cisco Inline Power (proprietary) also known as Cisco Pre-Standard PoE and the Standards based 802.3af. All Cisco IP Phone require power either via a PoE switch or local/desktop power (even the 7941G :);
Cisco Inline Power and IEEE 802.3af
Cisco launched Cisco Inline Power in March 2000 and has shipped more than 16 million inline power capable ports on the Catalyst switches. This innovation was quickly recognized within the industry and the IEEE started work to standardize Power over Ethernet implementations such that multi-vendor interoperability was enabled. With the ratification of IEEE802.3af, as with other Cisco innovations, Cisco will support both IEEE 802.3af and prestandard Power over Ethernet concurrently. Cisco has also extended prestandard power management extensions using Cisco Discovery Protocol negotiation to Cisco IEEE 802.3af compliant devices to further optimize PSE power management.
Applying Power over Ethernet requires that the device type be resolved to ensure that power is not applied to nonpower capable devices. To prevent unfortunate mishaps and to reduce the burden of network administration, Cisco and the IEEE devised mechanisms whereby the switch is able to determine whether a powered device or a nonpowered device is attached to a port. However, the phone detection mechanisms used by the Cisco prestandard Power over Ethernet implementation and IEEE802.3af are different in that the Cisco prestandard Power over Ethernet implementation uses AC powered device detection and IEEE 802.3af uses DC powered device detection. DC detection differs from AC detection in that AC detection transmits a low frequency AC signal and expects the same signal to be received back on the receive pair. DC detect applies a DC Current and detects the presence of a powered device by measuring the load applied by the powered device. It should be noted that Cisco IEEE 802.3af compliant devices support prestandard and IEE 802.3af detection mechanisms.
Using a Cisco inline power capable switch or Power Sourcing Equipment (PSE), the switch port will send discovery signals on active and inactive Ethernet ports to detect whether a powered device is present or not. It should be remembered that the powered device will not be powered at this time and therefore cannot bring the link up. It is therefore necessary to transmit the discovery signal on a continuous basis as a powered device may be plugged into the port at anytime.
Within a Cisco prestandard powered device, a low pass filter that is connected between the powered devices receive and transmit pairs allows the low frequency discovery signal to loop back to the PSE. A low pass filter is used as it allows the phone discovery signal to loop back to the PSE, but prevents 10/100 or 1000Mbps frames from passing between the receive and transmit pairs. Once the PSE detects that a powered device is attached to the port, the Cisco PSE will apply power to the port.
By contrast, the IEEE 802.3af-2003 standard uses a different powered device detection technique that uses DC detection to determine whether a powered device is attached and to which power classification the device belongs. An IEEE 802.3af-2003 PSE achieves this by applying a DC voltage between the transmit and receive pairs and measuring either the received current (Amps) or voltage (V) received.
continued on next post......
