Standard implementation Power over Ethernet

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powering devices

three modes, a, b, , 4-pair available. mode delivers power on data pairs of 100base-tx or 10base-t. mode b delivers power on spare pairs. 4-pair delivers power on 4 pairs. poe can used on 1000base-t ethernet, in case there no spare pairs , power delivered using phantom technique.


mode has 2 alternate configurations (mdi , mdi-x), using same pairs different polarities. in mode a, pins 1 , 2 (pair #2 in t568b wiring) form 1 side of 48 v dc, , pins 3 , 6 (pair #3 in t568b) form other side. these same 2 pairs used data transmission in 10base-t , 100base-tx, allowing provision of both power , data on 2 pairs in such networks. free polarity allows poe accommodate crossover cables, patch cables , auto mdi-x.


in mode b, pins 4–5 (pair #1 in both t568a , t568b) form 1 side of dc supply , pins 7–8 (pair #4 in both t568a , t568b) provide return; these spare pairs in 10base-t , 100base-tx. mode b, therefore, requires 4-pair cable.


the pse (power sourcing equipment), not pd (powered device), decides whether power mode or b shall used. pds implement mode or mode b disallowed standard.


the pse can implement mode or b or both. pd indicates standards-compliant placing 25 kΩ resistor between powered pairs. if pse detects resistance high or low (including short circuit), no power applied. protects devices not support poe. optional power class feature allows pd indicate power requirements changing sense resistance @ higher voltages. stay powered, pd must continuously use 5–10 ma @ least 60 ms no more 400 ms since last use or else unpowered pse.


there 2 types of pses: endspans , midspans. endspans (commonly called poe switches) ethernet switches include power on ethernet transmission circuitry. midspans power injectors stand between regular ethernet switch , powered device, injecting power without affecting data.


endspans used on new installations or when switch has replaced other reasons (such moving 10/100 mbit/s 1 gbit/s or adding security protocols), makes convenient add poe capability. midspans used when there no desire replace , configure new ethernet switch, , poe needs added network.



ieee 802.3at capable devices referred type 2 . 802.3at pse may use layer2 communication signal 802.3at capability.



class 4 can used ieee 802.3at (type 2) devices, requiring valid class 2 , mark 2 currents power stages. 802.3af device presenting class 4 current considered non-compliant and, instead, treated class 0 device.


configuration via ethernet layer 2 lldp

the setup phases follows:



pse (provider) tests pd (consumer) physically using 802.3af phase class 3.

pse powers pd.


pd sends pse: m pd, max power = x, max power requested = x.
pse sends pd: m pse, max power allowed = x.

pd may use amount of power specified pse.



the rules power negotiation are:



pd shall never request more power physical 802.3af class
pd shall never draw more max power advertised pse
pse may deny pd drawing more power max allowed pse
pse shall not reduce power allocated pd in use
pse may request reduced power, via conservation mode




^ ieee 802.3at-2009, clause 33.1.1c
^ koussalya balasubramanian; david abramson (may 2014). base line text ieee 802.3 bt (pdf). retrieved 2017-04-02. 
^ ieee 802.3at-2009 table 33-11
^ ieee 802.3at-2009 table 33-18
^ ieee 802.3bt table 145-1
^ ieee 802.3at-2009 table 33-1
^ ieee 802.3at-2009 33.1.4 type 1 , type 2 system parameters
^ ieee 802.3bt 145.3.1 pd type definitions
^ ieee 802.3bt 145.1.3.1 cabling requirements
^ herbold, jacob; dwelley, dave (27 october 2003), banish wall warts power on ethernet , electronic design, 51 (24): 61, archived original on 2005-03-20 
^ ieee 802.3-2008, section 2, table 33-12
^ ieee 802.3at-2009, table 33-18
^ ltc4278 ieee 802.3at pd synchronous no-opto flyback controller , 12v aux support (pdf).  2010-01-11 cds.linear.com
^ ieee 802.3-2005, section 2, table 33-3
^ ieee 802.3-2008, section 2, clause 33.3.4
^ lldp / lldp-med proposal poe plus (2006-09-15) (pdf). 2010-01-10






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