Editing Power over Ethernet (section)

===Powering devices===
Three modes, ''Mode A'', ''Mode B'', and ''4-pair mode'', are available. (In the standard these are discussed as two Modes, with the term ''4-pair mode'' for both simultaneously.) Mode A delivers power on [[T568A]] and [[T568B]] pairs 2 and 3{{dash}}the data pairs of [[100BASE-TX]] or 10BASE-T. Mode B delivers power on pairs 1 and 4{{dash}}the pairs not used by 100BASE-TX or 10BASE-T. 4-pair mode delivers power using all four pairs. PoE can also be used with 1000BASE-T, 2.5GBASE-T, 5GBASE-T and 10GBASE-T Ethernet, in which case there are no spare pairs and all power is delivered using the ''phantom'' technique.

Mode A has two alternative configurations (MDI and MDI-X), using the same pairs but with different polarities. In Mode A, pins 1 and 2 (pair 3 in T568A wiring, pair 2 in T568B) form one side of the 48&nbsp;V DC, and pins 3 and 6 (pair 2 in T568A, pair 3 in T568B) form the other side. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks. The free polarity allows PoE to accommodate crossover cables, patch cables and [[auto MDI-X]].

In Mode B, pins 4–5 (pair 1 in both T568A and T568B) form one side of the DC supply and pins 7–8 (pair 4 in both T568A and T568B) provide the return; these are the pairs 10BASE-T and 100BASE-TX do not use. Mode B, therefore, requires that all four pairs of the connectors be wired.

The ''Power Sourcing Equipment'' (''PSE''), not the ''Powered Device'' (''PD''), decides whether Mode A or Mode B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.<ref>IEEE 802.3 ''33.3.1 PD PI''</ref> The PSE can implement Mode A, Mode B, or both (''4-pair mode''). A PD indicates that it is standards-compliant by placing a 25&nbsp;kΩ<!-- Ω --> resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support PoE. An optional ''power class'' feature allows the PD to indicate its power requirements by changing the ''sense resistance'' at higher voltages.

To retain power, the PD must use at least 5–10&nbsp;mA for at least 60&nbsp;ms at a time. If the PD goes more than 400&nbsp;ms without meeting this requirement, the PSE will consider the device disconnected and, for safety reasons, remove power.<ref>{{citation |first1=Jacob |last1=Herbold |first2=Dave |last2=Dwelley |date=27 October 2003 |journal=Electronic Design |volume=51 |issue=24 |page=61 |url=http://elecdesign.com/Articles/Index.cfm?ArticleID=5945&pg=3 |archive-url=https://web.archive.org/web/20050320054651/http://elecdesign.com/Articles/Index.cfm?ArticleID=5945&pg=3 |archive-date=2005-03-20 |title=Banish Those "Wall Warts" With Power Over Ethernet}}</ref>

There are two types of PSE: ''Endpoint'' and ''Midspan''. Endpoint devices (commonly PoE switches) are Ethernet networking equipment that includes the power-over-Ethernet transmission circuitry. Midspan devices are ''power injectors'' that stand between a non-PoE Ethernet switch (or one that cannot provide sufficient power) and the powered device, ''injecting'' power without affecting the data. Endpoint devices are normally used in new installations or where the switch has to be replaced for other reasons (such as moving from {{nowrap|[[10/100]] Mbit/s}} to {{nowrap|1 Gbit/s}}), which makes it convenient to add the PoE capability. Midspan PSE can be used e.g. to power a single piece of equipment added to a network that does not provide PoE.

<!-- Only change if you have read and understood the relevant specifications and datasheets! -->
{| class="wikitable"
|+Stages of powering up a PoE link
|-
! rowspan=2 | Stage
! rowspan=2 | Action
! colspan=2 | Volts specified (V)
|-
! 802.3af
! 802.3at
|-
| Detection         || PSE detects if the PD has the correct signature resistance of {{nowrap|19–26.5 kΩ}}.
| colspan="2" style="text-align:center;"| {{nowrap|2.7–10.1}}
|-
| Classification    || PSE detects resistor indicating power range ([[#Power levels available|see below]]).
| colspan="2" style="text-align:center;"| {{nowrap|14.5–20.5}}
|-
| {{nowrap|Mark 1}} || PD signals it is 802.3at-capable. PD presents a {{nowrap|0.25–4 mA}} load.
|align=center| — ||align=center| {{nowrap|7–10}}
|-
| {{nowrap|Class 2}} || PSE outputs classification voltage again to indicate 802.3at capability.
|align=center| — ||align=center| {{nowrap|14.5–20.5}}
|-
| {{nowrap|Mark 2}} || PD signals it is 802.3at-capable. PD presents a {{nowrap|0.25–4 mA}} load.
|align=center| — ||align=center| {{nowrap|7–10}}
|-
| Startup           || PSE supplies startup voltage.<ref name="802.3-2008, section 2, table 33-12">IEEE 802.3-2008, section 2, table 33-12</ref><ref name="802.3at, table 33-18">IEEE 802.3at-2009, table 33-18</ref> ||align=center| {{nowrap|> 42}} ||align=center| {{nowrap|> 42}}<!-- Page 3 -->
|-
| Normal operation || PSE supplies power to device.<ref name="802.3-2008, section 2, table 33-12"/><ref name="802.3at, table 33-18"/> ||align=center| {{nowrap|37–57}} ||align=center| {{nowrap|42.5–57}}<!-- Page 14 -->
|}
IEEE 802.3at-capable devices are also referred to as ''Type 2''. 802.3at PSE may also use [[#Configuration via Ethernet layer 2 LLDP|LLDP communication]] to signal 802.3at capability.<ref name="linear-LTC4278_ds">{{cite web |title=LTC4278 IEEE 802.3at PD with Synchronous No-Opto Flyback Controller and 12V Aux Support |url=http://cds.linear.com/docs/Datasheet/4278fa.pdf |archive-url=https://web.archive.org/web/20110713210954/http://cds.linear.com/docs/Datasheet/4278fa.pdf |archive-date=2011-07-13 |website=cds.linear.com |page=15}}</ref>
{{Anchor|Power levels available}}
{| class="wikitable"
|+Power levels available<ref name="802.3s2t33.3">IEEE 802.3-2018, section 2, table 33-9</ref><ref>IEEE 802.3bt, table 145-26</ref>
|-
! Class !! Usage !! Classification current (mA) !! Power range at PD (W) !! Max power from PSE (W) !! Class description
|-
| 0 || Default  ||  0–5 || 0.44–12.94 || 15.4 || Classification unimplemented
|-
| 1 || Optional ||  8–13 || 0.44–3.84 || 4.00 || Very Low power
|-
| 2 || Optional || 16–21 || 3.84–6.49 || 7.00 || Low power
|-
| 3 || Optional || 25–31 || 6.49–12.95 || 15.4 || Mid power
|-
| 4 || Valid for Type 2 (802.3at) devices,<br />not allowed for 802.3af devices|| 35–45 || 12.95–25.50 || 30 || High power
|-
| 5 || rowspan=2 | Valid for Type 3 (802.3bt) devices || 36–44 & 1–4|| 40 (4-pair)|| 45 ||
|-
| 6 || 36–44 & 9–12|| 51 (4-pair)|| 60 ||
|-
| 7 || rowspan=2 | Valid for Type 4 (802.3bt) devices || 36–44 & 17–20 || 62 (4-pair) || 75 ||
|-
| 8 || 36–44 & 26–30 || 71.3 (4-pair) || 90 ||
|}

Class 4 can only be used by IEEE 802.3at (Type 2) devices, requiring valid Class 2 and Mark 2 currents for the power-up stages. An 802.3af device presenting a Class 4 current is non-compliant and, instead, will be treated as a Class 0 device.<ref name="802.3, section 2, 33.3.4">IEEE 802.3-2008, section 2, clause 33.3.4</ref>{{rp|13}}