PoE (Power over Ethernet) allows single cable to transport both power and data to the remote IP devices. It benefits the application that power source is absent in the field. Where the cable goes, the power source follows.
The T568B and T568A are the two color codes used for wiring the RJ45 connector. Both of the standards work no difference for PoE pinout. The 12-36 pinouts are taken for data in the fast network. It leaves 45-78 two spare pairs which can be used for power transporting. This is one way to send the power over Ethernet cable.
The typical products we can see taking 45-78 pinout for power is the passive PoE injector. The passive injector simply combines 45 twisted cables in one and 78 twisted as another. The low voltage power can be sent over these two combined twisted pairs. The passive PoE injector doesn’t have any protection for the power so it is the primitive PoE solution. The voltage drop will turn
into serious issues when the cable is longer than 20 meters.
The IEEE 802.3 active PoE injector also takes 45-78 for power. Instead of simply adding power to the twisted pair, the PoE injector has a protection mechanism to guarantee the power is being sent to the right place. The PD signature has two steps to verify and classify whether the remote ip device is IEEE802.3 compliant products. Otherwise the injector will not release the power at all.
Now the PoE switch – (similar as PoE injector) usually takes 12-36 data pairs to send the power. You may confuse why the power can go over a data pair. The network is moving to gigabit which needs all 4 twisted pairs for data. There are no spare twisted pairs in the gigabit network. One of the components named network transformer can separate the power from the data easily. The 12-36 becomes the preferred pinout in IEEE 802.3 standard products.
In the next generation PoE IEEE 802.3bt, all four twisted pairs are occupied to send the power as well as data. This new standard is capable of transporting 95W power over single Ethernet cables . It benefits for the high power demanding application such as cellular base stations, access control with door locks, televisions and interactive displays, Point of Sales (POS) and information kiosks, etc.
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