What is Power over Ethernet?
What is Power over Ethernet?Date of publication: 22-08-2024 🕒 4 min read
The PoE (Power over Ethernet) technology provides both data transfer and power supply for a device over a single Ethernet cable, i.e. a twisted-pair cable. The fact that only one cable is used for data transfer and power supply facilitates more efficient network equipment management. What is more, it helps reach places where it would not be possible to power devices using standard methods.
PoE – principle of operation
An Ethernet cable contains eight conductors in four twisted pairs. Depending on the power supply mode and operating standard, in PoE the current is sent via various pairs of conductors within a twisted-pair cable. In order to build a PoE system, the following basic components are required:
- Ethernet cable (twisted-pair cable) – usually Cat5, Cat5e, Cat6 or newer categories of cables are used.
- PSE Power Sourcing Equipment) provides power supply to network equipment via a twisted-pair cable. It initiates and manages the process of supplying power to Powered Devices (PD). The main types of PSE devices include PoE switches (network switches with a built-in PoE function) and PoE injectors (devices providing power supply over an Ethernet cable when the network switch is not compliant with the PoE). Injectors are connected between the switch and PD.
PoE power supply modules and splitters
- PDs ( Powered Devices) are receiving devices supplied via an Ethernet cable and PSE devices. Examples of PDs include IP cameras, VoIP phones, sensors and IoT devices.
PoE standards
The Power over Ethernet (PoE) technology encompasses several power supply types specifying the amount of power that can be supplied to PDs (Powered Devices) over an Ethernet cable and a PSE device. The main types of PoE power supply include:
- 802.3af standard (PoE, type 1) – this standard makes it possible to supply up to 15.4 W per an Ethernet port in a PSE device, which translates into 12.95 W of minimum power in PD. Power can be supplied via two conductor pairs (pair 1-2 and 3-6) in a min. Cat3 Ethernet cable. It handles two modes, i.e. A (endspan) and B (midspan).
- 802.3at standard (PoE+, type 2) – this standard makes it possible to supply up to 30 W per an Ethernet port in a PSE device, which translates into 25.5 W of minimum power in PD. Power is supplied in the same manner as in the 802.3af standard, i.e. via two conductor pairs. A min. Cat5 Ethernet cable is required. It handles two modes, i.e. A (endspan) and B (midspan).
- 802.3bt standard (PoE ++) – it is the latest PoE power supply standard which provides more diverse power supply options. It comes in two variants:
a. 802.3bt standard (PoE ++, type 3) – this standard makes it possible to supply up to 60 W per an Ethernet port in a PSE device, which translates into 51 W of minimum power in PD. It operates both in the A and B mode, and also supplies devices in the 4-pair mode using all Ethernet cable conductors. A min. Cat5 Ethernet cable is required.
b. 802.3bt standard (PoE ++, type 4) – this standard makes it possible to supply up to 100 W per an Ethernet port in an PSE device, which translates into 71 W of minimum power in PD. It supplies devices only in the 4-pair mode. A min. Cat5 Ethernet cable is required.
PoE operation modes
Taking into account the principle of operation, there are a few major operation modes available in a Power over Ethernet (PoE) system:
- Mode A – also referred as the Endspan, in which power supply and data are sent over the same Ethernet cable conductor pairs (conductor pairs 1-2 and 3-6).
- Mode B – also referred to as the Midspan, in which power supply and data are sent over separate conductors, which makes it possible to separate data signals from power supply signals (conductor pairs 4-5 and 7-8 are used to supply power).
- 4-Pair Mode – in which all four conductor pairs are used to supply power, which makes it possible to increase the amount of power available for PD.
Passive PoE
Passive PoE is a form of supplying power over an Ethernet cable, which does not use the standard negotiation protocols for power supply that are applicable to PoE standards such as 802.3af, 802.3at or 802.3bt. With passive PoE, power is delivered at a fixed level, regardless of the needs of the device, meaning there is no automatic power detection and negotiation. Disregarding standard negotiation procedures means that there is a risk of too much or too little power being delivered to PD, which may lead to equipment damage or malfunction. Passive PoE is sometimes used in selected cases in which a simple and low-cost solution is required to supply power to devices that do not require dynamic power adjustment. However, due to the potential risks and lack of standard compliance, caution and thorough understanding of the device requirements and PoE specifications are highly recommended before this solution can be used successfully.
PoE advantages
The main advantages of using Power over Ethernet (PoE) systems include:
- elimination of additional power supply cabling – PoE eliminates the need to use separate power supply cables for devices, which greatly simplifies the network installation operations;
- simplicity and affordability – a PoE system may prove to be cost-effective, as it eliminates the necessity to install electric sockets and additional power supply cabling;
- international compatibility – PoE is compliant with international standards, which ensures compatibility among devices supplied by various manufacturers;
- power supply management – PoE facilitates remote control of device power supply over PoE switches, which makes it possible to monitor and control multiple devices from a single site. As a result, it streamlines operations such as troubleshooting and energy consumption optimisation.
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