understanding poe

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Understanding POE

By Peter Brissette

GSP America

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What is POE?

• Power over Ethernet is defined across a single network link. The three basic components of a PoE connection are:

• The equipment that delivers power to the cable (often referred to as a PSE, which stands for power sourcing equipment)

• The device which receives its power from the cable (also known as a powered device, or PD)

• The cable itself

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Examples

• Typical PDs include IP cameras, wireless access points (APs), etc., and the PSE would normally be a PoEenabled network switch or a midspan power injector, patched in to add PoE capability to a non PoE network switch channel or similar.

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Power Classification

• As a part of the start-up process when a PoEconnection is made, the PD can “advertise” its power class, which is an indication of how much

electrical power it requires to operate.

Wireless AP12.95 watts15.4 watts3, 4, or 0

IP video camera

6.49 watts7 watts2

VoIP Telephone

3.84 watts4 watts1

Example PDMax. PD PowerMin PSE PowerClass

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Power Classification

• Note also that these figures are upper limits to power. Power cannot be “forced”down the cable – a surprisingly common misconception.

• The PD simply presents a load to the cable and draws as much current as it needs.

• Most PoE-powered devices will draw a fixed level of power.

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Power management

• Two Kinds of POE Switches

• There is the kind that operates with a

“guarantee per port” and the kind that operates with a “total power budget”.

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Power Management

• Guarantee per port means that you can

connect as many Class 3 or 0 devices and

the switch will be able to power them.

(15.4 watts each)

– More expensive

– Bigger

– Better in controlled environment

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Power Management

• Total Budget Switch – Can only power as many devices as it has power to spare

• Example: 4 port switch with total of 30 watts.

• 4 Class 2 Devices (4 x 7.5 watts = 30 watts) – this is OK

• 2 Class 3 Devices (2 x 15 watts = 30 watts) this is OK as well but is the maximum it could power

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Built-in Safety Measures

• Signature detection. No voltage can be applied to the cable until a compatible PD has been detected. The classification can be detected before the device is powered up.

• Current limiting is employed to protect the PSE from overload and to quickly disable malfunctioning PDs.

• Polarity protection provided by the bridges in the PDspower input circuitry means it can safely receive power regardless of the configuration and polarity of the voltage on the cable.

• Automatic disconnection of devices is performed when the monitored current falls below a minimum level.

• Low voltages are used to protect installers from electric shock hazards. The voltage used by PoE (up to 57 volts) is high enough to be efficient but low enough to be safe.

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Distance Concerns

300 FT300 FT

POE Switch

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Distance Concerns

• Because of the DC low voltage POE is limited to about 300 feet.

• That can be expanded by products from a Company like Veracity USA.

• With their Outreach devices can extend the distance another 300 feet with each device.

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Advantages

• Lower cost of installation. Only 1 cable for

power and data

• Easier to maintain. Central power supply that will typically have UPS backup

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Closing

• Any questions: email

peter@gspamerica.com

• 1-800-298-0470

• Connect with me on LinkedIn

• www.linkedin/in/peterbrissette