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Cisco prestandard PoE and 802.3af

Hi team,

Could you tell me the key standardized points of 802.3af compared to prestandard PoE ?

And which competitors now have standardized their PoE products followinging 802.3af ?



Hall of Fame Super Red

Re: Cisco prestandard PoE and 802.3af

Hi Gia,

Here is some related info (note that all competitors now use 802.3af);

This describes the differences between the older Cisco Inline Power (proprietary) and the Standards based 802.3af;

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......

Hall of Fame Super Red

Re: Cisco prestandard PoE and 802.3af

....from previous

Once a powered device has been discovered, an IEEE 802.3af PSE may optionally perform powered device classification by applying a DC voltage and current to the port. If the powered device supports optional power classification it will apply a load to the line to indicate to the PSE the classification the device requires by attenuating the DC voltage. The PSE then determines the powered device's classification using the Volt-Amp (VA) slope returned by the powered device's signature. If the powered device does not support classification, the powered device is assigned to Class 0, the default class.

Once the PSE has detected the powered devices IEEE 802.3af class, the PSE can manage the power allocation by subtracting the powered device's class maximum value from the overall power budget. If the value exceeds the available budget, power is not applied to the port. If the power is within the budget, power can be applied. The semantics of managing power budgets are vendor implementation specific.

From this excellent White Paper;

This doc is also very good;

Power over Ethernet (PoE) Power Requirements FAQ

Hope this helps and best of luck!