Power over Ethernet

What is Power over Ethernet?

What are the advantages of PoE Technology?

Cost

Installing PoE devices lowers costs because separate power supplies and cables are not required—only a connection to an Ethernet port on the PSE. Upgrading a standard Ethernet network to support PoE only requires replacing the access switch, the other physical infrastructure can stay the same.

Reliability

Power failures are a leading cause of reliability problems in any network. Battery backup units reduce dependency on continuous power and increase the reliability of active devices. However, this approach can be expensive in a large network, requiring that all active devices connect to a battery backup unit.

By delivering PoE instead, the cost is minimized by connecting only the access switch to the battery backup. During a power outage, the switch and all the connected PoE devices stay powered.

Flexibility

PoE switch interfaces can connect to PoE or non-PoE devices without any restriction. Once a PoE switch is in place, it is possible to connect a PoE or non-PoE device to the Ethernet port. The only limitation is that the total PoE power requirements are within the power budget of the switch.

How much power can PoE supply?

The amount of PoE power that a switch can supply is dependent on two things:

• The maximum PoE power a single port can supply
• The total PoE power available (the power budget)

The first value determines if a powered device is compatible with the port of a specific switch. The second value indicates how many powered devices can connect to the same switch.

PoE Power Classes

The maximum power supplied by a single port depends on the switch's capabilities, as defined by international standards (such as IEEE 802.3at and IEEE 802.3bt). These rules dictate that a switch or injector can't supply more power than the standard allows. The standards define four power level types for individual PoE ports:

Type 1 (called PoE)

Maximum power per port is 15W. (Allowing for losses through the cable, the max power available at the PD is 12.95W)

Type 2 (known as PoE+)

Maximum power available for each port is 30W (or up to 25.50W at the PD)

Type 3 (called 4PPoE, Ultra PoE, UPOE)

Maximum per port power is 60W (which equates to 51W at the PD)

Type 4 (called Ultra PoE, UPOE)

Maximum power available for each port is 100W (with up to 71W available at the PD)

For a reliable PoE solution, the PD's power requirements must match the capabilities of the PSE's port.

What is a Power Budget?

The PoE power budget is the maximum PoE power a PSE can supply to all of its PoE ports. The budget dictates how many connected PDs the switch can power. If the PDs draw too much power from the PSE, it may cause an error condition that restricts power to some PDs.

Some models of Allied Telesis PoE switches have removable power supplies that enable the PoE budget to be varied according to requirements. For example, the x930 series switches have three types of power supply modules that can be "hot-swapped" (i.e. changed while the switch is being used without interrupting traffic, provided another power supply is installed):

• PWR150 is a non-PoE power supply that delivers power for the switch systems only.
• PWR800 is a PoE power supply that delivers system power and an additional 380W of PoE power.
• PWR1200 is a PoE power supply that delivers system power and 740W of PoE power.

So by changing the model and number of power supplies installed in the switch, the PoE budget of the x930 series switches can be adjusted from 380W to a massive 1440W (that's 30W per port on all 48 ports!)

It is important to note that most PSEs use the PoE Type to calculate the power consumption, not the actual PD consumption. This means that even if a PD is consuming only 20W, the PSE assumes that it is a Type 2 device and allocates it 30W from its PoE power budget. (For a solution to this, see PoE Power Budget Override below.)

Smart Power Budget Features

Allied Telesis has been developing PoE products ever since the first standards were published. We've used this experience to create some useful features that make using PoE more flexible and reliable:

PoE Power Budget Override

Instead of allowing the switch to determine the PD power consumption using PoE Type, our switches enable you to specify the maximum power a port can supply. This optimizes the amount of power allocated to each device so the power budget can support more PDs per switch.

Dynamic Power Allocation

Some Allied Telesis switches can calculate their power budgets based on the actual power consumption of the attached PDs. So, the budget reflects the real-time power requirements of the connected PDs, and no manual configuration is required.

PD Priority

If the PDs exceed the PSE power budget, the PSE will remove power from some PDs to protect itself from damage. Our switches allow you to allocate a priority to each of the PDs so you can choose those to sacrifice first to protect your most critical services.