PoE is an acronym that stands for “Power over Ethernet.” It is a set of standards and non-standard implementations of transmitting power to devices over an ethernet cable. PoE is intended as a simplification technique. By providing power over an already present data cable, you eliminate the need for a separate power supply. This means you can place a device without worrying about the nearest power socket. It also means that you only have to run a single cable.
Standard Modes
There are three standardized modes of PoE: Alternative A, Alternative B, and 4PPOE. Ethernet cables feature eight connector pins. The wires themselves are organized in twisted pairs. Slower ethernet transmission rates of 10 or 100Mbps, as seen in 10BASE-T and 100BASE-TX, respectively, only require using two of the four twisted pairs to transmit data. Gigabit ethernet, 1000BASE-T involves using all four twisted pairs, as do all higher bandwidth modes.
PoE Alternative A configuration uses two twisted pairs for power transmission. Specifically, it uses the same pins used for data transmission in 10BASE-T and 100BASE-TX. This is similar to phantom power, commonly used in cabled condenser microphones. The actual data transmission works through the method of differential signaling. Power is transmitted using a standard voltage for each pair. This prevents the power delivery from interfering with data signaling.
PoE Alternative B configuration also uses two twisted pairs for power transmission. However, it utilizes the two twisted pairs not used for data transmission in 10BASE-T and 100BASE-TX. The advantage of this mode over Alternative A is the ease of troubleshooting. With each pair only being used for a single function, it is easier to diagnose problems.
4PPoE makes use of all four twisted pairs to transmit power. This doubles the potential power that can be delivered. It utilizes a standard voltage for each pair, meaning that data transmission remains unimpeded.
How Much Power?
There are four “types” of power supply, with each one being added in newer standards. The original allowed for 12.95W of delivered power. Due to power loss in the cable, the transmitting device could transmit 15.4W. Type 2 doubled that to 25.5W received and 30W sent. Type 3 doubles again to 51W received and 60W sent. Up to 100W can be sent, and 71.3W can be obtained in Type 4. Type 1 and two only support the use of two twisted pairs to transmit power. Alternative A, or Alternative B. Type 3, supports two twisted pair modes and 4PPoE. Type 4 requires 4PPoE.
As you might expect, unexpectedly delivering 100W of power down an ethernet cable could present a problem to devices not configured to handle it. To avoid any issues, PoE is not assumed. Instead, it is negotiated. Using a small voltage, the Power Sourcing Equipment or PSE detects the electrical resistance the Powered Device or PD has.
The PD will be configured with a certain amount of standardized resistance based on its needed power. Alternatively, LLDP can be used. LLDP is a link length protocol that operates on Layer 2 of the OSI model. LLDP stands for Link-Layer, Discovery Protocol, and it can be used to negotiate power supply through a small amount of transmitted data.
Conclusion
PoE stands for Power over Ethernet. It is a concept that allows the transmission of electrical power over an ethernet cable. This means that devices can be placed without considering the proximity of a standard power socket. Current standards allow for up to 100W of power to be transmitted. Ethernet works by using differential signaling over twisted pairs of cables. PoE applies a standard voltage to each pair, allowing power transmission without interfering with the differential signaling of data.
Some non-standard implementations are in practical use. Typically, these were implemented before the standards, helping drive standardization by showing a need—for non-standard implementations. However, as you might expect, they do not necessarily offer the same performance characteristics as the standard implementations.
