Learn About Power over Ethernet (PoE)
Power over Ethernet (PoE) Terminology
Last modified by Microchip on 2025/06/26 12:33
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Terminology
Here is a list of helpful terms associated with the Power over Ethernet (PoE) protocol:
| Term | Definition |
|---|---|
| 100BASE-T | An Ethernet standard for Local Area Networks (LANs) that supports data transfer rates of 100 Mbps over twisted-pair cabling. |
| 10BASE-T | An Ethernet standard for LANs that supports data transfer rates of 10 Mbps over twisted-pair cabling. |
| 2-Pair Power Over Data or Phantom Feeding | A method of delivering up to 30W (Class 4) of power over the same pair set used for data transmission in Ethernet cabling. |
| 2-Pair Power Over Spare | A method of delivering up to 30W (Class 4) of power over the pair set not used for data transmission in Ethernet cabling. |
| 4-Pair Power | The use of all four pairs of Ethernet cabling for power delivery. Classes 5-8 must be 4-pair power. Lower Classes 1-4 may be 2- or 4-pair power. |
| 4PID | The Power Sourcing Equipment (PSE) determines whether a connected PD is a candidate to receive power on both pairsets, before applying the power on both pairsets. |
| Active Clamp | A circuit technique used in power converters to improve efficiency and reduce voltage stress on components. |
| Alternative A | A PoE power delivery method where power is supplied over the data pairs (1, 2, 3, and 6) of the Ethernet cable. It is synonymous with Mode A. |
| Alternative B | A PoE power delivery method where power is supplied over the spare pairs (4, 5, 7, and 8) of the Ethernet cable. It is synonymous with Mode B. |
| Assigned Class | The cass that the PSE gives (assigns) to the PD. It represents the amount of power that the PSE allocates for the PD and it also becomes the maximum power limit for the PD. The assigned class is determined based on the requested class and the number of class events that the PSE produces. The assigned class may be lower than the requested class, which is to power that the PD requests during the classification process. |
| AutoClass | An optional feature in the IEEE® 802.3bt™ standard where the PD puts itself in a maximum power mode and PSE will physically measure this maximum power (which includes loss in the cable). The PSE will allocate the measured power, not allocated power, potentially saving energy or allowing more devices to be connected. |
| Auxiliary Power (wall adapter) | An external power source used to provide power to a PD in lieu of PoE. |
| Category 5e Ethernet Cable | Enhanced Category 5 cable that supports higher data transfer rates and improved performance over standard Category 5 cable. |
| Category 6a Ethernet Cable | Augmented Category 6 cable that supports higher data transfer rates and improved performance over standard Category 6 cable. |
| Channel length | The total length of the Ethernet cabling channel, including patch cords and connectors. |
| Channel resistance | The total electrical resistance of the Ethernet cabling channel, affecting power delivery efficiency. |
| Class (1 - 8) | Both the PSE and PD are designed to be of a particular class. PSE class is the maximum amount of power a port can deliver and PD class is the maximum amount of power the PD is designed to receive. |
| Class Probing | A method for the PSE to determine the PD’s requested class, without the intent to allocate power or power up the PD. The PSE generates at least 3 class events to determine whether a dual-signature PD is 4-pair capable and above Class 4 for each pairset. After determining the PD's requested class, the PSE generates a classification reset and a single class event to power up the PD. |
| Class Signature | A current drawn by the PD during a class event. Five class signatures are defined, numbered 0 through 4, each corresponding with a certain range of current draw. |
| Class Signature A | The class signature for a Powered Device (PD) using Alternative A power delivery. |
| Class Signature B | The class signature for a PD using Alternative B power delivery. |
| Classification | The process by which a PD communicates its power requirements to the PSE and the PSE communicates back to the PD how much power it will receive. This process also allows PSEs and PDs to determine the Type of the device to which they are connected, referred to as mutual identification. |
| Classification Current | The current drawn by a PD during the classification process to indicate its power class. |
| Classification Current Resistors | Resistors used in a PD to set the classification current. |
| Classification Events (1 - 5) | A voltage produced by the PSE for a certain amount of time. They simultaneously allows the PSE to discover the requested class of the PD (by measuring what class signature the PD produces), as well as granting a certain amount of power to the PD. Consecutive class events determine what the assigned class is. |
| Classification Reset | Used to reset the PD (make it forget any preceding class events) by reducing the PSE voltage below a specified voltage for minimum time. |
| Classification Signature | See Class Signature. |
| Class_Sig_A | The first two class events. |
| Class_Sig_B | Class events from the 3rd class event. |
| Connection Check | If the PSE is configured to apply four-pair power, a connection check is performed to see if the PD is a single-signature PD or a dual-signature PD. This process is only specified in the IEEE 802.3bt standard. |
Conversation Mode | When the PSE requests to provide lower power to the PD. |
| Data Link Layer (DLL) Classification | After Physical Layer Classification allocates power to the PD, the PSE and PD communicate using the Link Layer Discovery Protocol (LLDP) to optimize allocated power, potentially allowing the PSE to recover power. May also be used to allow the PD to recover power lost during power demotion. It is optional for PSEs to support DLL classification. |
| Data Terminal Equipment (DTE) | Devices such as computers and network switches that connect to a network and communicate over Ethernet. |
| Detection | The process by which the PSE identifies the presence of a valid PD that is capable of receiving PoE power is connected to the port. |
| DLL Classification | The PSE and PD communicate using the LLDP after the PD has received operating power to fine-tune power allocation. DLL classification has finer power resolution (1/10W) and the ability for the PSE and PD to participate in dynamic power allocation. All PDs, except for class 0-3 PDs, are required to support DLL classification. It is optional for a PSE to support DLL (i.e., midspans do not support DLL). |
| Dual-Signature (DS) PD (DSPD) | A PD where a separate and independent PD controller is connected to Mode A and Mode B. Classification, Detector and Power of each controller are independent of the other. |
| Effective Resistance | Found by taking the difference of the two measurements the resistive slope can be determined, which is used for detection evaluation. |
| Endspan PSE | PSE is integrated into network switches or routers that provide PoE power and data directly from the network port. |
| Event | See Classification Events. |
| Extender Power | When channel length and resistance are not known, the power that the PD is allowed is the available power based on the maximum channel resistance. For example, a Class 8 PSE supplies 90W, but the PD can consume only up to 71W. 19W is reserved in the PSE budget to be dissipated in the cables. When channel info or channel resistance is known, the PD is allowed to consume higher power. For example, if the channel length is ~10m, PD can consume up to 90W minus 2W, which is the actual cable allocation as defined by the IEEE 802.3bt. |
| Fingers | See Classification Events. |
| First Class Long Event (LCE) | A class event generated by the PSE to indicate if it is a Type 3 or Type 4 PSE, during the LCE the PSE and PD determine if both support AutoClass. |
| Flyback DC-DC Converter | A type of power converter used in PoE systems to convert and regulate voltage levels. |
| Forward DC-DC Converter | A type of power converter used in PoE systems to convert and regulate voltage levels with higher efficiency. |
| HDBaseT 2.0 | A standard for transmitting ultra-high-definition video, audio, Ethernet, and power over a single Ethernet cable. |
| HDBaseT 3.0 | An enhanced version of the HDBaseT standard with improved performance and additional features. |
| IEEE Standard 802.3 af | The original PoE standard, where the PSE provides up to 15.4 watts of power over Ethernet. |
| IEEE Standard 802.3 at | An updated PoE standard, also known as PoE+, where the PSE provides up to 30 watts of power. |
| IEEE Standard 802.3 bt | The latest PoE standard where the PSE provides up to 60 watts (Type 3) or 90 watts (Type 4) of power. |
| Inrush | The first phase of delivering power to the PD is called inrush. During inrush the PSE actively limits the amount of current that flows. The purpose of this is to prevent excessive inrush current from flowing when the PD starts up. |
| Internet Protocol (IP) | The primary protocol for transmitting data over the Internet and other networks. |
| IPORT | Combined current consumed by PD on both pairsets. |
| IPort-2P | The current on a given pairset (either Alternative A or Alternative B). |
| IPort-2P-other | The other pairset current. For example, if we evaluate the current on Alternative B, then IPort-2P is the current on Alternative B and IPort-2P-other is the current on Alternative A. |
| Link Layer Discovery Protocol (LLDP) | A network protocol used by devices to advertise their identity and capabilities on a local network. The protocol is based on Type/Length/Value (TLVs). |
| Link Section | A segment of the Ethernet cabling channel between two network devices. |
| Long Class Event | An extended first classification event (88 to 105ms) indicates the PSE is either Type 3 or 4. For comparison, Type 1 first classification event is 6 to 75ms and Type 2 is 6 to 30ms. It also indicates to the PD whether short Maintain Power Signatures (MPSs) and/or AutoClass are supported. |
| LTPoE++ | A proprietary PoE standard that provides up to 90 watts of power over Ethernet. |
| Maintain Power Signature (MPS) | The minimum PD power consumption that will keep the PSE port active. Examples include an LED Light that is off or an on-hook IP phone. |
| Mark Event | A reduced voltage produced by the PSE that serves to demarcate between class events or the last class event before power-up. |
| Mark Event Current | The current the PD draws during a mark event. |
| Media Dependent Interface (MDI) | The physical interface between a network device and the Ethernet cabling, i.e., the Ethernet port. |
| Midspan PSE, Power Hub or PoE Injector | A device that adds PoE power to an existing Ethernet connection, typically used when the network switch does not support PoE. |
| Mindi Simulator | A simulation tool used to model and analyze power delivery and performance in PoE systems. |
| Mode A | A PoE power delivery method where power is supplied over the data pairs (1, 2, 3, and 6) of the Ethernet cable. It is synonymous with Alternative A. |
| Mode B | A PoE power delivery method where power is supplied over the spare pairs (4, 5, 7, and 8) of the Ethernet cable. It is synonymous with Alternative B. |
| Mutual Identification | The process devices use to discover each other’s type. The PSE discovers the PD's type via its requested class. The PD discovers the PSE's type by measuring the length of the first class event. This sequence permits the PD to determine whether it can employ the short MPS feature. |
| Non-isolated Boost DC-DC Converter | A type of power converter that increases voltage levels without electrical isolation between input and output. |
| Non-isolated Buck DC-DC Converter | A type of power converter that decreases voltage levels without electrical isolation between input and output. |
| Pairset | Refers to two pairs of the Ethernet cable, each alternative is a pairset (A or B). |
| PCLASS | The power class of a PD as determined by the classification process. |
| PCLASS -2P | The power class of a PD using two pairs of Ethernet cabling for power delivery. |
| PD Class | The power classification of a PD is based on its power requirements. |
| PD Detection Resistor | A resistor used in a PD to indicate its presence to the PSE during the detection process. |
| Physical layer classification | Physical layer classification is performed after detection prior to power up. It is a handshake procedure between the PSE and PD where the PSE applies a series of voltage levels or fingers (called class events) and the PD responds with a pre-determined current level (called the class signature). |
| PoE | Generalized term for a technology that allows Ethernet cables to carry electrical power in addition to data. This term may also refer to the IEEE 802.3af standard, which defines Type 1 PSEs capable of delivering 15.4W of 2-pair power. |
| PoE Handshake | The initial communication process between the PSE and the PD to establish a PoE connection. |
| PoE+ | Refers to the IEEE 802.3at standard, which defines Type 2 PSEs capable of delivering 30W of 2-pair power. |
| PoE++ | Refers to the IEEE 802.3bt standard, which defines Type 3 PSEs capable of delivering 60W of 4-pair power and Type 4 PSEs capable of delivering 90W of 4-pair power. |
| Power Budget | The maximum amount of power that can be delivered over an Ethernet cable to power PoE-PDs, measured in watts. |
| Power Classification | An optional step that follows signature detection. If a PD provides one of eight possible classification signatures, the PSE knows how much power the device requires. If no classification signature is provided, the PD is considered Class 0, and 12.95W is allocated. |
| Power Demotion | When a PSE assigns a lower Class than what the PD requests by limiting the number of class events produced or by using the AutoClass feature. This term is not used in the standard. |
| Power Interface (PI) | The PI is the physical point where the cabling connects to the PSE or to the PD. In essence, it is where the two modular connectors touch. All specifications in the standard apply at the PI, anything inside the PSE or PD is considered implementation-specific. |
| Power over HDBaseT (POH) | A standard from the HDBaseT™ Alliance, published 2011 and based on IEEE 802.3at allows for 95W over 4-pair. |
| Power Removal | Power may be removed for a number of reasons, for example the PD is disconnected from the PSE, the PSE no longer has power available, the PD draws excess power, or there is a short circuit in the cable. |
| Power Sourcing Equipment (PSE) | Devices such as network switches or PoE injectors that provide power to PDs over Ethernet cabling. |
| Power Up Sequence | The sequence of events that occur when a PD is powered on by the PSE. It includes:
|
| Power Up | The process of supplying power to a PD over Ethernet. |
| Power via Media Dependent Interface (MDI) | The delivery of power over the Ethernet cabling interface. |
| Powered Device (PD) | A device that receives power over Ethernet from PSE. |
| Powered Device Controller | A component within a PD that manages power delivery and communication with the PSE. |
| Powering | This is the operational phase. The PSE monitors the current/power draw and disconnects the PD in case it exceeds a set limit. The PSE also checks if the PD becomes disconnected, in which case, power is removed from the port. |
| Power-up | The process of supplying power to a PD over Ethernet. |
| PPD | The power class of a PD as determined by the classification process. |
| PPSE | The power class of the PSE as determined by the classification process. |
| Pre-Detection | The initial phase of the PoE detection process where the PSE checks to see if something is connected to the port. |
| PSE Class | The power classification of the PSE based on its power delivery capabilities. |
| PSE Polarity | The electrical polarity of the power supplied by the PSE to the PD. |
| RDETECT | The detection resistance used by a PD to indicate if is a device that is capable of receiving power to the PSE. |
| Rectifier Bridge | A circuit used in PDs to convert AC power from the Ethernet cable to DC power. |
| Requested Class | The class that the PD advertises during physical layer classification. It represents the amount of power that the PD wants from the PSE. The requested class is determined by the class signatures that the PD produces when exposed to class events. |
| Safety Extra Low Voltage (SELV) | A standard for electrical safety in low voltage systems. |
| Short Maintain Power Signature (MPS) | The minimum current that the PD must draw to indicted to the PSE that it is still present and should not be disconnected. |
| Single-Signature Powered Device (SSPD) | A PD where both Mode A and Mode B are connected to the same PD controller. Detection and cassification via Mode A or Mode B will be identical. |
| Synchronous Rectification | A technique used in power converters to improve efficiency by replacing diodes with actively controlled switches. |
| Type | Refers to a PSE or PD with common characteristics regarding power capabilities, classification and various performance features. |
| Type 1 | The original PoE standard (IEEE 802.3af) where the PSE provides up to 15.4 watts of 2-pair power. |
| Type 2 | The PoE+ standard (IEEE 802.3at) where the PSE provides up to 30 watts of 2-pair power . |
| Type 3 | The PoE standard (IEEE 802.3bt) where the PSE provides up to 60 watts of 4-pair power. |
| Type 4 | The PoE standard (IEEE 802.3bt) where the PSE provides up to 90 watts of 4-pair power. |
| Universal Power over Ethernet (UPOE) | A Cisco® proprietary PoE standard that delivers up to 60 watts of power to the PD. |
| Unshielded Foiled Twisted Pair (U/FTP) | A type of Ethernet cable with individual pairs shielded by foil to reduce crosstalk and electromagnetic interference. |
| VAUX Bias | The auxiliary voltage used to power certain components within a PD. |
| VOFFSET | During the Detection phase, the PSE will generate approximately a 2.7V to 10V voltage ramp and repeatedly measure PD resistance. The PD's 25K Ω detection resistor usually sits behind a rectifier. To account for this, VOFFSET of approximately 1.9V is added to the criteria range the PSE will use to determine whether a valid PD detection signature was detected. |
| VPSE | The voltage supplied by the PSE to the PD. |
| Wall Adapter Priority | A feature that allows a PD to prioritize power from an external wall adapter over PoE power. |
| Wireless Access Points | Network devices that provide wireless connectivity to client devices and can be powered using PoE. |
Learn More
- Power over Ethernet (PoE), Power Delivery (PD) and DC-DC Design Considerations
- Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Front End Using PD702x0 and PD701x0 ICs
- Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Using PD702x1 and PD701x1 ICs
- Implementing Auxiliary Power in PoE
- Power over Ethernet (PoE) Solutions
