Hello, everyone!
Today, I will continue with PoE technology.
PoE is an abbreviation from Power over Ethernet. This technology allows remote power supply of the device via UTP cable.
First, the devices must support PoE. There are many terminal devices that can be powered via a UTP cable. These are devices: POS machines, cameras, data collectors, VoIP phones, wireless access points (APs), etc.
There is a special standard for PoE, it is IEEE 802.3. The standard was developed when it was realized that the two pairs in the UTP cable were unused. Those two pairs could be used for power. Cat 5 UTP cables or higher category cables are used for PoE technology. PoE uses a 48 V power supply.
PoE standard evolution is showed in the next table.
Table 1. PoE standards
A PoE system is composed of two segments:
PSE (Power Sourcing Equipment) - A device that allows remote powering of other devices that support PoE technology, and
PD (Powered Device) - A device that can be powered via PoE technology.
There are two modes of PSE:
Mid-span PSE. This is an extra device for the remote power supply. It is not part of some device. We can use mid-span PSE between switches, routers and PDs (figure 1, left side), and
End-span PSE. This is a part of some devices (figure 1, right side). This type of mode is used for Huawei ONUs.
Figure 1. Mid-span and end-point PSE
There is the process of how a PD is powered by a PSE. There are five steps:
Detection. PSE checks if there is a PD device, based on resistance and capacitance. At this stage, the output voltage of PSE is 2.8 - 10 V;
Classification. In this phase, PSE determines the power consumption for the PD device, according to the output current. Now, the output voltage of PSE is from 15.5 V to 20.5 V;
Power-up. In this stage, PSE provides – 48 V for PD device;
RTP and power management. PSE does real-time monitoring and power management;
Disconnection. If the PD is turned off, the PSE will turn off the power to the unit. The PD needs to use a minimum of 10 mA of current. If the current value required by the PD is less than the threshold (5 - 10 mA), the PSE will turn off the power.
I will continue to explain this theme in the next article >> UTP cable (5): UTP testing.
Thank you!
