If it's an intermediary device located on the network cable somewhere between a non PoE switch and a PoE device, it's referred to as a midspan.
One type of midspan is an injector which is an interface placed in series between the network switch and the PD (explained next). A power supply connects to the injector, and sends the power in the direction of the PD. Another type of midspan is a hub which manages both the data as well as injects the power onto the network cable.
A powered device (PD) is the object of the injection such as a electronic access control module, IP Camera, PoE phone or an access point.
Many powered pevices have an integral auxiliary power connector for an optional, external, power supply.
Having multiple ways to power a PoE PD can enable the system designer to provide for continued powered operation even if there is a power failure.
Two PoE Modes, A and B, are available. Mode A delivers phantom power on the data pairs of 100BASE-TX or 10BASE-T. Mode B delivers power on the spare pairs. PoE can also be used on 1000BASE-T Ethernet in which case, there are no spare pairs and all power is delivered using the phantom technique.
Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities.
In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3 and 6 (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 for crossover cables as well as patch cables.
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 "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.
The PSE, not the powered device (PD), decides whether power mode A or B shall be used. PDs must accommodate either mode in order to be compliant with the Standard.
To stay powered, the PD must continuously use 5–10 mA for at least 60 ms with no less than 400 ms since last use or else it will be unpowered by the PSE.
Endspans are normally used on new installations or when the switch has to be replaced for other reasons (such as moving from 10/100 Mbit/s to 1 Gbit/s or adding security protocols), which makes it convenient to add the PoE capability. Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.
PoE is also used in non-Ethernet applications such a UTP video which uses Category 5 style cable topology to route rather than Ethernet topography
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