Foundry Configuration Guide for the FESX, FSX, and FWSX
6 - 4 © Foundry Networks, Inc. December 2005
Autodiscovery
POE autodiscovery is a detection mechanism that identifies whether or not an installed device is 802.3af
compatible. When you plug a device into an Ethernet port that is capable of providing in-line power, the
autodiscovery mechanism detects whether or not the device requires power and how much power is needed. The
autodiscovery mechanism also has a disconnect protection mechanism that shuts down the power once a
powered device has been disconnected from the network or when a faulty powered device has been detected.
This feature enables safe installation and prevents high-voltage damage to equipment.
POE autodiscovery is achieved by periodically transmitting current or test voltages that can detect when a
powered device is attached to the network. When an 802.3af compatible device is plugged into a POE port, the
powered device reflects test voltage back to the power sourcing device (the Foundry device), ultimately causing
the power to be switched ON. Non-compatible 802.3af devices do not reflect test voltage back to the power
sourcing device.
Power Class
Different power classes determine the amount of power a POE powered device receives. When a valid powered
device is detected, the Foundry POE device performs power classification by inducing a specific voltage and
measuring the current consumption of the powered device. Depending on the measured current, the Foundry
device assigns the appropriate class to the powered device. Powered devices that do not support classification
are assigned a class of 0 (zero). Table 6.3 shows the different power classes and their respective power
consumption needs.
Power Specifications
The actual implementation of the 802.3af standard limits power to 15.4W (44V to 57V) from the power sourcing
device. This is in compliance with safety standards and existing wiring limitations. Though limited by the 802.3af
standard, 15.4 watts of power is ample, as most powered devices consume an average of 5 to 12 watts of power.
IP phones, wireless LAN access points, and network surveillance cameras each consume an average of 3.5 to 9
watts of power.
The FSX’s 48-volt power supply (part number SX-POE-AC-PWR) provides power to the POE daughter card, and
ultimately to POE power-consuming devices. The number of POE power-consuming devices that one 48-volt
power supply can support depends on the number of watts required by each power-consuming device. Each 48-
volt power supply can provide 1080 watts of power, and each POE port supports a maximum of 15.4 watts of
power per POE power-consuming device. For example, if each POE power-consuming device attached to the
FSX consumes 10 watts of power, one 48-volt supply will power up to 108 POE ports. You can install a second
48-volt supply for additional POE power. Power supply specifications are covered in the Foundry FastIron X-
Series Chassis Hardware Installation Guide and in the Foundry FastIron Stackable Hardware Installation Guide.
CAUTION: The SX-POE-AC-PWR power supply is designed exclusively for use with the FSX POE devices.
The power supply produces extensive power to support 802.3af applications. Installing the power supply in a
device other than the FSX POE will cause extensive damage to your equipment.
Table 6.2: Power Classes for Powered Devices
Class Usage Power (Watts)
0 default 15.4
1 optional 4
2 optional 7
3 optional 15.4
4 future class 0
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