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Different methods used on EX platform to determine presence of PoE device (PD).

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Article ID: KB27710 KB Last Updated: 25 Jun 2013Version: 2.0
Summary:

This article provides information about the different methods used on EX platform to determine presence of PoE device (PD).

Symptoms:

When a PoE (Power over Ethernet) devices connected to EX switch (PoE model), it draws power from the switch.

It can be observed that, some PoE devices (IP Phone, Access Points) function with EX4200 but do not power up with EX2200.

This can occur when the PoE device (PD) doesn’t fulfill the required method used to detect the presence of a PoE device.

Cause:
 
Solution:

The EX3200/EX4200 platforms use a different method for disconnect compared to the EX2200.


Methods used by EX4200/EX3200 & EX2200 to determine the presence of PoE Device:
 

Platform              

POE Controller

Method used

EX3200 / EX4200        

63K

AC Disconnect

EX2200 / EX2200-12T
EX3300 / EX4200PX /EX6200

69K

DC Disconnect

* EX4200 (Latte) and EX4200PX (Amaretto) have different POE Controllers and support different disconnect modes.

In AC Disconnect method the impedance level is measured to identify the PD's (PoE device) presence.

In DC Disconnect method the current drawn is measured to identify the PD's presence.


If the PoE device is connected to an EX4200, the delay in phone drawing current after detection and classification does not lead to disconnect because the EX4200 determines the loss of any other PoE device by measuring the level of ‘electric impedance’ at the port.

However, the EX2200 uses current drawn by ‘PoE device’ to determine the presence of the phone. Hence, when a PoE device (PD) doesn't draw the current or draws current which is below threshold level after start-up, the switch assumes that the phone is no longer connected.

The EX2200's hardware is not capable of supporting ‘AC disconnect’ method and moreover, the ‘DC disconnect’ method is a more robust since it helps to save more power.

This is the reason why some PoE devices work with EX4200 and not with EX2200.

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