Power over Ethernet (PoE)

What Is Power over Ethernet (PoE)?

Power over Ethernet (PoE), also called Power over LAN (PoL) or active Ethernet, is a technology that allows electrical power to be transmitted over Ethernet cables alongside data. This eliminates the need for separate power cables, simplifying the installation and maintenance of networked devices such as IP phones, APs, and IP cameras. PoE enables power sourcing equipment (PSE), such as network switches or PoE injectors, to deliver power to powered devices (PDs) using standard Ethernet cables. There are different PoE standards, such as IEEE 802.3af, IEEE 802.3at (also known as PoE+), and IEEE 802.3bt (also known as PoE++) which offer varying levels of power delivery capabilities.

What motivates the demand for PoE?

The increasing utilization of IP phones, network video surveillance, and wireless Ethernet devices has led to a growing demand for power supply via Ethernet. In many instances, these devices rely on DC power. However, they are often situated outdoors or high above ground level, making access to traditional power outlets challenging. Even when available, installing the necessary AC/DC converters for these devices can pose difficulties. Managing uniform power distribution across numerous terminals within expansive LANs can also be daunting. PoE technology provides a remedy to these issues.

PoE, a wired Ethernet technology, combines the transmission of DC power and data within a single cable. This approach offers centralized power provision for various terminals, including IP phones, wireless access points (APs), portable device chargers, point-of-sale (POS) machines, cameras, and data collectors. PoE has the following advantages:

• Reliability: PoE ensures reliability by powering multiple PDs from a single PSE, simplifying power backup arrangements.

• Deployment simplicity: PoE streamlines deployment by enabling terminals to be powered through Ethernet cables, eliminating the need for separate external power sources.

• Standardization: PoE adheres to international standards, featuring uniform RJ45 power ports across all PoE devices, enabling seamless connectivity with devices from various vendors.

Understanding PoE Operation

As illustrated in the diagram below, a PoE power delivery system comprises a PSE and PDs.

• PSE: A PoE-enabled device, such as a PoE switch, that supplies power to a PD via an Ethernet cable. It performs functions like detection, analysis, and intelligent power management.

• PD: A device that receives power, such as an AP, portable device charger, POS machine, or camera. PDs are categorized as standard or nonstandard depending on their compliance with IEEE standards.

  

Power Supply Mode in PoE

According to IEEE standards, PSEs are categorized into midspans and endpoints. Midspans are installed externally to the device, while endpoints integrate the PoE module internally. PSE endpoints offer two power supply modes: Alternative A (utilizing line pairs 1/2 and 3/6) and Alternative B (utilizing line pairs 4/5 and 7/8).

In Alternative A mode, a PSE supplies power to a PD through data pairs 1/2 and 3/6. Pair 1/2 serves as the negative polarity, while pair 3/6 serves as the positive polarity. Data pairs 1/2 and 3/6 are used for transmitting data in 10BASE-T and 100BASE-TX interfaces, while all four data pairs are utilized for data transmission in 1000BASE-T interfaces. Since DC power and data frequency are independent, both power and data can be transmitted over the same pair.

In Alternative B mode, a PSE delivers power to a PD via spare pairs 4/5 and 7/8. Pair 4/5 functions as the positive polarity, while pair 7/8 serves as the negative polarity.

IEEE standards dictate that the two power supply modes cannot be simultaneously employed by a PSE. Consequently, a PSE can operate in only one mode, while PDs must support both modes.

Power Supply Negotiation in PoE

Upon powering on and establishing connections with PDs, the PSE initiates power supply negotiation, following these steps:

• PD detection: The PSE intermittently sends low-voltage signals with limited current through its interfaces to detect PDs. A resistance within the range of 19 kΩ to 26.5 kΩ indicates the presence of IEEE 802.3af- or 802.3at-compliant PDs. The detection voltages typically range from 2.7 V to 10.1 V, with a detection interval of 2 seconds.

• Power supply capability negotiation: The PSE identifies the types of connected PDs and negotiates power levels by analyzing the detected resistance or exchanging Link Layer Discovery Protocol (LLDP) messages.

• Commencement of power supply: Within less than 15 microseconds, the PSE begins supplying low voltage to the PDs, gradually ramping it up to the full 48 V DC.

• Stable power delivery: Once the voltage reaches 48 V, the PSE ensures the consistent and reliable delivery of 48 V DC power to the PDs. The total PoE power output to all PDs remains within the maximum capacity of the PSE.

• Power disconnection: While supplying power, the PSE continuously monitors the input current of PDs. If it detects a PD's current dropping below the minimum threshold or increasing abruptly, the PSE ceases power delivery to that PD and reverts to the PD detection procedure. This disconnection may occur due to PD disconnection from the PSE, encountering power overload or a short circuit, or exceeding the PSE's power supply capacity.

LLDP Power Capability Negotiation

IEEE 802.3 includes an optional Type-Length-Value (TLV) known as Power via Media Dependent Interface (MDI) TLV. This TLV is embedded within LLDP packets to facilitate the discovery and advertisement of MDI power capabilities. When a PSE detects a PD, they engage in periodic exchange of LLDP Data Units (LLDPDUs) containing the Power via MDI TLV to negotiate power supply parameters.

The Power via MDI TLV consists of a two-byte TLV packet header followed by a 12-byte TLV information field, depicted in the following figure.

The following tables describe fields in Power via MDI and Extended Power via MDI.

PoE Standards: Ensuring Interoperability

The IEEE has released several PoE standards to standardize power delivery over Ethernet cables, ensuring interoperability across various devices from different vendors. Here's a breakdown of the differences between the three main standards:

1. IEEE 802.3af (PoE):

• Ratified in June 2003, IEEE 802.3af defines PoE power supply for devices.

• Specifies the maximum power a PSE can deliver and the power a PD can receive.

• PSEs can provide up to 15.4 W of DC power on each port.

• Due to power dissipation in cables, only 12.95 W is guaranteed to be available at PDs.

2. IEEE 802.3at (PoE+):

• Ratified in October 2009, IEEE 802.3at is backward-compatible with IEEE 802.3af.

• Increases the maximum PoE power output to 25.5 W per port to support a wider variety of terminals.

• Offers higher power delivery capabilities compared to IEEE 802.3af.

3. IEEE 802.3bt (PoE++ or 4PPoE):

• Ratified in September 2018, IEEE 802.3bt further expands power capabilities.

• Introduces two additional power types: Type 3, delivering up to 51 W, and Type 4, delivering up to 71.3 W.

• Supports higher data rates, including 2.5GBASE-T, 5GBASE-T, and 10GBASE-T, facilitating applications like high-performance wireless access points and surveillance cameras.

• Offers significantly higher power delivery compared to IEEE 802.3af and IEEE 802.3at.

In summary, IEEE 802.3af provides basic PoE functionality, IEEE 802.3at enhances power delivery with PoE+, and IEEE 802.3bt further expands power capabilities with PoE++, offering higher power delivery and supporting higher data rates.