Become a Power over Ethernet (PoE) Expert in Just 5 MinutesDelia Hernandez
Ethernet is the lifeline of local area networks (LAN) but many network administrators choose to take their networks one step further and incorporate Power over Ethernet (PoE) to eliminate the hassle of connecting equipment to a power supply. PoE helps manage cable clutter and is an appealing feature to add to networks needing an upgrade.
However, network administrators need to remain aware that PoE equipment does not come in a one size fits all model. Power Sourcing Equipment (PSE) such as switches or hubs, needs to meet the power requirements of Powered Devices (PDs). PDs includes equipment such as IP surveillance cameras, Voice over IP (VoIP), and Wireless Access Points (WAPs).
The two most relevant PoE standards denoting power output are 802.3af and 802.3at. Whereas the legacy 802.3af standard provides 15.4 watts of power, the newer 802.3at standard, also known as PoE+, provides up to 30 Watts of power.
Determining PoE Budget
Power Sourcing equipment (PSE) such as switches or hubs, come with a predetermined Total PoE Budget. Powered Devices, such as VoIPs and IP cameras, come with a variety of different wattage demands and the sum total must be less than the PSE’s Total PoE Budget.
For example, this 8-Port 802.3at PoE Desktop Gigabit Switch has a Total PoE Power Budget of 130 Watts and is 802.3at compliant. If used at full capacity, the maximum yield per port would be 130 watts/ 8 Ports or 16.25 watts per port, rounded down to the nearest standard (in this case the 802.3af standard). Because 802.3af PDs only draw 15.4 Watts of power, the remaining power will remain unused and will not overpower PDs.
Even though the device has four ports, network administrators must not make the mistake of assuming that the switch can be filled at full capacity with 802.3at devices. To calculate how many 802.3at devices the unit supports, simply divide the Total PoE Budget (130 Watts) by 30 Watts .
Calculating the max amount of 802.3af devices is just as simple. Simply divide the Total PoE Budget (130 Watts) by 15.4 Watts.
As manufacturers continue to exploit the utility of PoE, another non-standardized option is available for network administrators seeking to install PoE with greater output. UltraPoE or PoE++, among other unofficial terms,
delivers up to 60 watts of power using the same 802.3at standard. Ultra PoE is delivered by using the simultaneous transmission of Mode A and Mode B. For another five-minute crash course on Mode A and Mode B, click here.
Ultra PoE is ideal for IP surveillance cameras that require more throughput or a variety of other equipment such as computer workstations, LCD displays, and biomedical equipment.
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