Detection Mechanism
The Cisco prestandard implementation of the detection mechanism differs from the IEEE 802.3af:
• Cisco prestandard. Injects an alternating current (AC) signal on one pair of the CAT5 cable and checks whether the PES returned this current on another pair
• IEEE 802.3af. Applies a direct current (DC) voltage between two pairs of the CAT5 cable and checks whether some current flows
Figure 8-2 shows the Cisco prestandard detection mechanism. A fast link pulse (FLP), such as a low-frequency, low-intensity AC, is injected on the CAT5 cable's transmit pair. If the station is a powered device, it has a low-pass filter that allows this FLP to come back on the receive pair to the detection mechanism.
A low-pass filter is required, or else the high-frequency signal of Ethernet frames passes back and forth between the transmitting and receiving pairs. This crosstalk interferes with normal Ethernet traffic.
Introduction to PoE 137
Figure 8-2 Cisco Prestandard Detection Mechanism
Switch PES
Figure 8-2 Cisco Prestandard Detection Mechanism
Importance of the Detection Mechanism
Without such a detection mechanism working at the physical layer, there would be a risk of applying a voltage to an intermediary device between the switch and the PES. For example, if a hub is inserted between the switch and the PES, there is no way to detect the nonintelligent hub at Layer 2 because the hub transparently passes frames between the switch and the PES. Therefore, both of them believe that they are directly connected, and the switch applies power to the hub.
With a detection mechanism working at the physical layer (such as the Cisco prestandard or IEEE 802.3af), the hub prevents the switch from detecting the PES; therefore, the switch does not apply power to the hub. This prevents all damages to the switch.
The IEEE 802.3af detection mechanism is different than the Cisco prestandard because it does not inject an AC on one pair, but rather, it measures whether DC can flow among two pairs of the CAT5 cable. Figure 8-3 illustrates that a low-intensity DC is transmitted to the attached station by applying a voltage between the transmit and receive pairs. In the case of an IEEE 802.3af PES, there's a resistance of 22 kilo ohm (kQ). Hence, a current flows. (Actually, the measured resistance must be anything between 19 kQ and 26.5 kQ.) This signals that the attached station is a powered device that requires power.
Figure 8-3 IEEE 802.3af Detection Mechanism
Switch
Current Flows. There is a PES!
Current Flows. There is a PES!
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After it detects the presence of a PES, the switch can also use a similar mechanism to detect the class of the PES, such as its power consumption being either 4.0, 7.0, or 15.4 W.
NOTE All Cisco IP phones and switches that support the new IEEE 802.3af also support the Cisco prestandard implementation.
Moreover, when using the Cisco prestandard method, IP phones further use Cisco Discovery Protocol (CDP) to fine-tune their power requirement to a lower level. (The goal is to allow more devices on the switch.)
Continue reading here: Defending Against Burning Attacks
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