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PoE

What Device can use for PoE?

  • Today there are nearly 200 types of device which are PoE enabled including:
  • IP Security Cameras
  • Network routers
  • A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (These ports on the mini-switch do not themselves provide PoE.) (In most modern VoIP phones a one-port switch is embedded to which a local workstation can be installed using another VLAN that the voice-VLAN used by the phone itself)
  • Network webcams
  • Network Intercom / Paging / Public address systems and hallway speaker amplifiers
  • VoIP phones
  • Wall clocks in rooms and hallways, with time set using Network Time Protocol
  • Wireless access points
  • Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs.
  • Industrial devices (sensors, controllers, meters etc.)
  • Access control and Help-points (intercoms, entry cards, keyless entry, etc.)
  • Lighting controllers
  • Remote Point of Sale (POS) kiosks

What is the IEEE 802.3 ?

The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) to each device. Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.

The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power.

Power sourcing equipment
Power sourcing equipment (PSE) is a device such as a switch that provides ("sources") power on the Ethernet cable. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers 25.50 W.

When the device is a switch, it's called an endspan. Otherwise, if it's an intermediary device between a non PoE capable switch and a PoE device, it's called a midspan. An external PoE injector is a midspan device

Powered device
A powered device (PD) is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP cameras.

Many powered devices have an auxiliary power connector for an optional, external, power supply. Depending on the PD design, some, none, or all power can be supplied from the auxiliary port, with the auxiliary port sometimes acting as backup power in case of PoE supplied power failure.

Standard PoE parameters and comparison
Property 802.3af (802.3at Type 1) 802.3at Type 2
Power available at PD 12.95 W 25.50 W
Maximum power delivered by PSE 15.40 W 34.20 W
Voltage range (at PSE) 44.0每57.0 V 50.0每57.0 V
Voltage range (at PD) 37.0每57.0 V 42.5每57.0 V
Maximum current 350 mA 600 mA per mode
Maximum cable resistance 20 次 (Category 3) 12.5 次 (Category 5)
Power management Three power class levels negotiated at initial connection Four power class levels negotiated at initial connection or 0.1 W steps negotiated continuously
Derating of maximum cable ambient operating temperature None 5∼C with one mode (two pairs) active
Supported cabling Category 3 and Category 5 Category 5
Supported modes Mode A (endspan), Mode B (midspan) Mode A, Mode B

Stages of powering up a PoE link
Stage Action
Volts specified
[V]
802.3af 802.3at
Detection PSE detects if the PD has the correct signature resistance of 19–26.5 kΩ 2.7–10.0
Classification PSE detects resistor indicating power range 14.5–20.5
Mark 1 Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load. 7–10
Class 2 PSE outputs classification voltage again to indicate 802.3at capability 14.5–20.5
Mark 2 Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load. 7–10
Startup Startup voltage > 42 > 42
Normal operation Supply power to device 37–57 42.5–57
The setup phases are as follows:PSE (provider) tests PD (consumer) physically using 802.3af phase class 3.
PSE powers up PD.
PD sends to PSE: I'm a PD, max power = X, max power requested = X.
PSE sends to PD: I'm a PSE , max power allowed = X.
PD may now use the amount of power as specified by the PSE.
The rules for this power negotiation are:PD shall never request more power than physical 802.3af class
PD shall never draw more than max power advertised by PSE
PSE may deny any PD drawing more power than max allowed by PSE
PSE shall not reduce power allocated to PD, that is in use
PSE may request reduced power, via conservation mode[24]:10

802.3af Standards A and B
PINS on Switch
10/100 DC on Spares
(mode B)
10/100 Mixed DC & Data
(mode A)
1000 (1 Gigabit) DC & Bi-Data
(mode B)
1000 (1 Gigabit) DC & Bi-Data
(mode A)
Pin 1
Rx +
Rx + DC +
TxRx A +
TxRx A + DC +
Pin 2
Rx -
Rx -  DC +
TxRx A -
TxRx A -  DC +
Pin 3
Tx +
Tx + DC -
TxRx B +
TxRx B + DC -
Pin 4
DC +
unused
TxRx C + DC +
TxRx C +
Pin 5
DC +
unused
TxRx C - DC +
TxRx C -
Pin 6
Tx -
Tx - DC -
TxRx B -
TxRx B - DC -
Pin 7
DC -
unused
TxRx D + DC -
TxRx D +
Pin 8
DC -
unused
TxRx D - DC -
TxRx D -
 
     
 
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