Understanding Power Over Ethernet (PoE):

Power over Ethernet (PoE) is a technology that passes electric power over twisted-pair Ethernet cable to powered devices (PD), such as wireless access points, IP cameras, and VoIP phones in addition to the data that the cable usually carries.

PoE enables one RJ45 cable to provide both data connection and electric power to Powered Devices instead of having a separate cable for each. 

Am going to use the following PoE terminologies through this article:-

• Powered Devices - PDs == to refer to devices powered by PoE  such as Wireless Access Points, VoIP phones and IP cameras.
• Power Sourcing Equipment - PSE == to refer to devices that send power and data over the Ethernet cable to a connected PD. 

 PSE devices are classified as either mid-span PSE or end-span PSE.

End-span PSE:

The typical end-span PSE device is a PoE Network Switch. Since the switch itself can power connected devices, there is no need for an additional power source between the PoE switch (PSE) and the connected PoE edge device (PD).

Mid-span PSE:

If a non-PoE enabled network switch is to be used with a PoE device, a power source that adds power to the connection is required. 

This device is placed between ("in the middle") the non-PoE network switch and the PoE PD device. 

A very common type of a PoE mid-span PSE device is a PoE-injector.

Note:

Power Over Ethernet(PoE) can transmit Electrical power over an Ethernet cable up to maximum distance(cable length) of 100 meters /328 feet to a PD.

PoE relays power using two different modes known as Mode A and Mode B.

PoE Mode A: Mode A utilizes data pairs 1-2 and 3-6 to deliver power..i.e PoE Mode A uses Ethernet cable pins 1& 2 for positive (+) DC power and pins 3 & 6 for Negative (-) DC power from the PSE (PoE switch). Devices that use PoE mode A are usually end-span PSE devices such as switches.

PoE Mode B: Mode B utilizes spare pairs 4-5 and 7-8 to deliver power..i.e PoE Mode B uses Ethernet cable pins 4 & 5 for positive (+) DC power and pins 7 & 8 for Negative (-) DC power from the PSE (PoE injector/Extenders). 

PoE mode B devices are usually stand alone mid-span PSE devices that accompany non-PoE switches such as PoE injectors and PoE extenders.

Power Over Ethernet (PoE) Extenders/ repeaters:

Although PoE extenders primary function is to extend the maximum distance that Power from a PoE switch or PoE injector can go .i.e beyond 100meters, PoE extenders also repeat the data signal from Ethernet cables and extend the cable distance limitation of Ethernet allowing for data transmission  beyond 100m limitation.

PoE Extenders/repeaters are mid-span PSE "are placed between end-span PSE PoE switch and PD PoE device such as Ray AP, VoIP & IP cameras.

PoE Extenders/repeaters receive power and data from the PoE switch or injector and use some of that power (approximately 4-5 Watts) to power it self and then passes the remaining power to the next PoE device down the line. 
As an example,if a PoE extender is connected to an IEEE 802.3at PoE+ injector that can deliver roughly 25 watts of power to the PoE extender/repeater, you will have approximately 20 – 21 watts available for your connected PoE device (Ray AP,VoIP phone or IP Camera)


PoE Extenders are especially useful in overcoming the power and data transmission limitation of 100 meters in Ethernet data cables and also PoE maximum power transmission distance of 100 meters.
Poe Extenders can be daisy chained to power a PoE device that is 200 meters/656 feet away. This 2x beyond the 100 meters limit of PoE. 

 Table Below shows the power availability to PoE device to the number of PoE extenders used to and distance.

As PoE technology has become rapidly adopted and with the increasing number of  PoE enabled devices becoming the norm, PoE technology has also evolved and improved to accommodate the power requirements of all these devices.

 Starting from the lowest power consuming PoE PDs such as VoIP phones to the highest power consuming devices such as laptops.  

At the time of writing this article, IEEE -compliant PoE switches and PoE-injectors can output power via PoE starting from 12W up to over 70W per port.  

There are currently 3 major IEEE 802 power over Ethernet standards:

1. IEEE 802.3af  commonly known as PoE
   
2. IEEE 802.3at  commonly known as PoE+
3. IEEE 802.3bt  commonly known as PoE++  (UPoE is Cisco's proprietary version of PoE++ type 3)

 The table below shows the 3 different Power over Ethernet types and their respective power input and output:

IEEE 802.3af/at/bt compliant PoE technology is safe. PoE injectors and switches will not damage any equipment, even if the equipment is not designed for PoE applications. Before the PSE sends any power to a connected PD, the PSE initiates a handshake procedure that establishes how much power the connected device requires. This procedure uses low voltage and is harmless to any connected device, PoE or non-PoE. If the handshake is completed, the PoE injector or switch begins sending power, which triggers the PD to start up. If that handshake is not completed for any reason, the PSE never sends any power. It is this built-in feature of all IEEE 802.3af/at/bt-compliant devices that makes PoE technology inherently safe. 

PoE Cabling Type Consideration:

Ethernet (Cu/Al) Cable.

Copper-Clad Aluminum (CCA) cable uses an aluminum (aluminium) core and is coated with copper. It contrasts with regular network cable that has a 100% copper core.

 Simply put, aluminum isn't as good an electrical conductor. It has a higher DC-resistance value than copper, so more power is lost and dissipated as heat (and it only gets worse the longer the cable connection is). Higher heat and the larger power loss on the cable are both major concerns in PoE applications, so we do not recommend using them with PoE injectors or switches.   

For standard networking applications, CCA cables usually get the job done just fine and cost less than 100% copper cable.

 Conclusion: Demand 100% copper network cable from your supplier for PoE applications.

Switch Power Budget:

A PoE switch power budget is the total amount of power output available to the PoE ports of the switch.

Each port located on the PoE switch is capable of producing a certain amount of power per port, denoted in watts. Separately the entire switch has an overall PoE budget. The overall PoE budget for the switch does not equate to the sum of the power supplied to each port. For example, a 24-port switch supplying power up to 30 watts per port does not mean the overall PoE power budget is 720 watts. Both the power output per port and overall PoE budget play an important role in determining the right power requirements for your install. The switch’s datasheet typically denotes both measurements. 

All switches have a PoE Power budget allocated to them and exceeding the budget causes connected devices to fail.

A good example  of a how a switch power budget works can be described as follows.

Take an example of Ray Switch Model RSL2-24P below with the following specs:

Ray Switch Model RSL2-24P

Powering 802.3at PoE+ devices on Ray RSL2-24P switch with 370W PoE Power Budget:

Consider Powering 802.3at PoE+ Ray Access Points  using Ray RSL2-24P Switch with a PoE Power Budget of 370W.

802.3at supplies 30W to the Ray AP. so in this case (Switch Budget ÷ 802.3at PoE+ power requirement.)

To Calculate No. of PoE ports== (370W ÷ 30W) = 12.3 

In this case the maximum number of 802.3at PoE+ ports that can be powered by our switch is 12ports.

Meaning out of the 24Ports on the switch, we can power a maximum of 12 switch ports connected to 802.3at PoE+ Ray Access Points. 

Powering 802.3af PoE devices on Ray RSL2-24P switch with 370W PoE Power Budget:

Consider powering 802.3af PoE AVAYA VoIP Phones using Ray RSL2-24P Switch with a PoE Power Budget of 370W.

802.3af supplies 15.5W to VoIP Phones. so in this case (Switch Budget ÷ 802.3af PoE power requirement.)

To Calculate No. of PoE ports==(370W ÷ 15.5W) = 23.8 

In this case the maximum number of 802.3af PoE ports that can be powered by our switch is 23ports.

Meaning out of the 24Ports on the switch, we can power a maximum of 23 switch ports connected to  802.3af PoE VoIP Phones. 

Thanks

Ray Support