PoE and EX-series Switches Overview
Power over Ethernet (PoE) is the implementation of IEEE 802.3af, allowing both data and electric power to pass over a copper Ethernet LAN cable. This technology allows VoIP telephones, wireless access points, video cameras, and point-of-sale devices to safely receive power from the same access ports that are used to connect personal computers to the network.
This topic covers:
PoE and Power Supply Units in EX-series Switches
EX-series switch models provide either 8, 24, or 48 PoE ports. The total number of PoE ports for an EX-series switch can be extended by inserting additional PoE cards.
Power supply units with three different power capacities are available for use with the EX-series switches:
- 320-W power supply unit: Supports 8 ports of PoE power at 15.4 W per port, plus system power.
- 600-W power supply unit: Supports 24 ports of PoE power at 15.4 W per port, plus system power.
- 930-W power supply unit: Supports 48 ports of PoE power at 15.4 W per port, plus system power.
All 802.3af-compliant powered devices require no more than 12.95 watts. Thus, if you follow the recommended guidelines for selecting power supply units to support the number of PoE ports, the switch should be able to supply power to all connected powered devices. If you install a higher capacity power supply unit on a switch model that has only 8 PoE ports, it does not extend PoE capabilities to the non-PoE ports.
Power Management Mode
The power management mode is used to determine the number of interfaces that can be provided with power. There are two modes of power management:
- Static—In this mode the power allocated for each interface can be configured.
- Class—In this mode the power allocation for interfaces is decided based on the class of powered device connected.
Classes of Powered Devices
A powered device is classified based on the maximum power that it draws across all input voltages and operational modes. The most common class is 0, in which the switch allows a maximum draw of 15.4 W per port. The switch provides 15.4 W at the port in order to guarantee enough power to run a device, after accounting for line loss. For example, 15.4 W - power loss (16%) = 12.95 W. Table 1 lists the classes of powered devices and associated power levels.
Table 1: Class of Powered Device and Power Levels
Global and Specific PoE Parameters
All EX-series switches with PoE ports have a PoE controller. The PoE controller keeps track of the switch's power consumption and distributes the available power to individual PoE ports. You can set the PoE controller to reserve a limited amount of power (up to 19 W) to handle a power spike. The default is that no power is kept on reserve.
The factory default configuration creates a PoE interface for all the PoE ports on the switch. You can specify maximum power, priority, and telemetries for each PoE interface.
- maximum-power—This setting defaults to 15.4 W. If you follow the recommended guidelines for the installed power supply unit (see Table 1), the switch should be able to provide sufficient power for all PoE ports using the default power setting.
- priority—This setting defaults to low. If a port is set as high priority and a situation arises where there is not sufficient power for all the PoE ports, the available power is directed to the higher priority port(s). If the switch needs to shut down powered devices because a power supply fails and there is insufficient power, low priority devices are shut before high priority powered devices. Thus, security cameras, emergency phones, and other high priority phones should be set to high priority.
- telemetries—This setting allows you to monitor per port PoE power consumption. It is not included in the default PoE configuration.