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Industry Insights

PoE, Connectivity and Cabling Introduction - Part 1

Thursday, June 23, 2016 | by Gregg Lafontaine

Safely supplying low voltage power over twisted pair communication cabling has existed from the earliest forms of cabled communication.  From the early days of the automated central office supplied ring voltages of 30V + 15V via 1 or 2 pairs to residential or business land line telephones, to today where data signaling voltages of 5V or less over twisted 4 pair structured cabling is found in almost all commercial building communication cabling infrastructure.
 
A common definition of PoE

“Power over Ethernet or PoE technology describes a system to safely deliver DC electrical power, along with data, to remote devices over standard data-com cabling on an Ethernet network.”
 
Goals of Power over Ethernet (PoE)

PoE is a low voltage technology that can power an end device from a central location (telecom closet, communication cabinet) using the same transmission path / cabling and at the same time as the Ethernet data transmissions. PoE eliminates the need to run a separate cabling solely for purpose of powering end devices (examples include: IP phone sets or wireless access points).
 
What standards bodies have impacted PoE ?

The PoE application, and the cabling it is likely to be run on, are primarily impacted by 2 separate standards organizations. IEEE which governs and defines all Ethernet (802.3) applications which PoE is part of, and the TIA through its foundation 568 commercial cabling standard (SCS structured cabling system) and referenced standards defining structured communication cabling design, installation and testing guidelines which have been embraced and followed in the Americas since the late1990s.( A corresponding structured cabling standard is defined by ISO/IEC 11801 for the European and other world markets and it also supports PoE.)

While the PoE application will typically run over structured cabling defined by TIA & ISO/IEC, the PoE standard is controlled entirely by IEEE. The IEEE does solicit input from the cabling standards bodies regarding cabling recommendations or to initiate liaison discussions on joint technology topics and concerns. Several corporate members belong to both standards bodies acting as liaisons.   
 
The first Power over Ethernet (PoE) standard (IEEE 802.3af) was introduced in 2003. This standard targeted applications already served by twisted pair copper cabling. One primary targeted application of interest for PoE was IP telephony.  The IEEE PoE application was targeted for deployment over 4 pair cabling via both the building or campus infrastructure (TIA 568 structured cabling) as well as application specific 4 - pair cabling infrastructure.
 
What are the key elements of PoE as defined by IEEE?

802.3af,  IEEE’s first PoE standard, defined the characteristics of the Powered Device (PD) and the Power Sourcing Equipment (PSE).  

Power-Supplying Equipment (PSE)

Usually located at the originating end( Endpoint PSE, typically supplied by the switch)
 or 
Mid-Span (Mid-Span PSE adding power between the 2 ends of a data circuit)  

 Powered devices (PD’s) are end devices that accept power from the PSE as well as communicate via the LAN interface. These network devices (PDs) may be powered, as well as communicate via the LAN. They receive power & data from a LAN switch (endpoint PSE) or use an inline powering alternative (Midspan PSE) where data come from the switch but power is introduced (midspan) between the switch and PD.
 
What are the currently approved PoE standards?

• IEEE 802.3af PoE • Ratified in 2003 • specifies 15.4W at the PSE, with min of 12.95W available to the PD    
• IEEE 802.3at PoE+ • Ratified in 2009 • specifies 34.2W at the PSE, with min of 25.5W available to the PD    
 
The IEEE 802.3af PoE standard first defined the requirements for PoE  to supply up to 15.4 W of DC power to each device without affecting  10/100/1000 Mbps  Ethernet networks running on 100Ω balanced cabling.  Power was supplied on 2 pairs. You may have heard references to Alternative A and Alternative B regarding which 2 pairs are being used to provide a power path. Alternative A refers to providing power on the same pairs as the 10BaseT and 100BaseT data signal (pairs 2 & 3, RJ45 positions 1,2,3,6 ).  Alternative B refers to the supply of power on the unused 10BaseT and 100BaseT pairs (pairs 1 & 4, RJ45 positions 4,5,7,8 ). = From a cabling perspective the choice of  Alternative A or Alternative B is not of much importance.  Please note, since all 4 pairs are used for 1000BaseT power will always be supplied on data pairs. MidSpans only operate on alternative B.  802.3af PDs can operate with either powering alternative. 

Pending (higher  power) PoE standards

• Proposed IEEE 802.3bt PoE Type 3 standard is in the working group  • specifiying 60W at PSE, with 49 watts at the PD, Doubling IEEE 802.3at • Utilizing all 4 wire pairs • Projected to be ratified by IEEE in 2017
and
• Proposed IEEE 802.3bt PoE Type 4 standard is in the working group •  specifying 100W at PSE  and a maximum current 1000 mA , with 96 watts at the PD • Utilizing all 4 wire pairs • Projected to be ratified by IEEE in 2017

Why is PoE a safe method to deliver power?

The IEEE approved model for PoE is safe for two primary reasons:

  1. The foundation is built around the NEC (National Electric code) thresholds for low voltage cabling, similar to what has been used for years for phone systems.  
  2. Power is only supplied by the PSE onto cabling that has a valid PD connected onto the end following and authorization negotiation.  When a PD is removed or channel continuity is interrupted, power is no longer supplied to that cable channel.

 
When is PoE power on the cabling? 

Power is only supplied by the PSE onto cabling that has a valid PD connected to the end.
To accomplish this, each PoE supported port of a PSE will poll each cabling channel it is connected to.

  • First the PSE applies a very low trace detection voltage onto the attached  cable pairs.
  • A valid PD that requests power will acknowledge this detection voltage with a specific signature response to the PSE.  Along with a valid PoE signature, PDs may optionally present a classification signature, which defines how much wattage the device is requesting.
  • After completion of signature communication process, power is supplied by the PSE as long as that PD is attached / connected and requesting power. When the PD is detached, the PSE will begin the trace voltage polling again.
  • If no valid signature detected, the PSE will not supply power to that line. If a PoE switch (PSE) does not receive a valid PD signature while power is not supplied that port will only send data.
  • For Mid Span PSEs, if no valid signature is detected, the Mid Span will not supply power to that line but data will continue to pass through the Mid Span to support the end device.

This "hand shaking" design controlled by the IEEE standards ensures that power is only supplied when an appropriate device is attached and requesting PoE power. 
 
What are some cost benefits of PoE power vs alternative power ?

PoE allows both data & power to be delivered by the same cabling to a powerable device with the following benefits.

  • An AC power outlet does not need to be cabled to the PD.
  • Use of DC power supports the elimination of AC power and DC transformers at the device location, reducing power infrastructure requirements.
  • PoE power supplied from a central location (e.g. wiring closet) ,offers simplified  and central control of power distribution by the Ethernet switching device.
  • Centralized power sourcing offers the opportunity to provide lower cost uninterrupted back up power supply (UPS).

 
Cabling requirements for IEEE  PoE

IEEE qualification process for PoE standards has been based on the TIA’s  568 commercial cabling standards. This means TIA and ISO category 5e, cat6 and cat6a 100 meter cabling channels would support these PoE applications and target specifications.  This cabling model was considered when IEEE calculated the anticipated attenuation of power over the cabling. 
 
Some Examples:
• The IEEE 802.3af standard specifies that the PSE will supply up to 15.4 W of DC so that at least 12.9 W will be available at the powered device (PD) on channels up to 100 meters.  
• The IEEE 802.3at standard specifies that the PSE will supply up to 34.2W of DC so that at least 25.5W W will be available at the powered device (PD) on channels up to 100 meters.
 
 Summary

“Power over Ethernet or PoE technology describes a system to safely deliver DC electrical power, along with data, to remote devices over standard data-com cabling on an Ethernet network.”
 
There are 2 primary standards bodies impacting PoE:
• IEEE which governs and defines all Ethernet (802.3) applications including PoE.
• TIA or ISO/IEC through commercial cabling standard (SCS structured cabling system) and referenced standards defining the structured communication cabling design, installation and testing guidelines. 
 
The PoE application was designed to run over structured cabling defined by TIA, ISO/IEC.
 
The currently  approved PoE standards powering 2 pairs:
IEEE 802.3af PoE • Ratified in 2003 • 15.4W at the PSE, with min of 12.95W available to the PD    
IEEE 802.3at PoE+ • Ratified in 2009 • 34.2W at the PSE, with min of 25.5W available to the PD   
 
Pending (higher power) PoE standards Projected to be Ratified by IEEE in 2017 powering all 4 pairs:
• Proposed IEEE 802.3bt PoE Type 3 in working group • 60W at PSE • with 49 watts at the PD
• Proposed IEEE 802.3bt PoE Type 4 in working group • 100W at PSE •with 96 watts at the PD