1 Introduction
1 Introduction
PoE (Power over Ethernet) is a technology that provides electrical power to network devices through network cables. PoE technology can transmit power and data signals simultaneously, eliminating the need for additional power cables. The principle of PoE technology is to add DC power to Ethernet cables, allowing network devices to receive power directly through network cables.
- PSE (Power Sourcing Equipment): PSE devices are network equipment that support PoE technology and serve as a core component of the PoE power supply system. PSE typically comes in two forms: PoE power injectors and PoE switches. Their primary function is to transmit power and data signals through Ethernet cables and provide power to Powered Devices (PD).
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PD (Powered Device): PD devices are network equipment that require electrical power in a PoE power system, such as IP phones, cameras, wireless access points, etc. PD devices receive power from PSE devices through Ethernet cables and communicate data with PSE devices.
To standardize and promote the development of PoE technology and resolve compatibility issues between power sourcing and powered devices from different manufacturers, the IEEE Standards Committee has released three PoE standards: IEEE 802.3af (PoE), IEEE 802.3at (PoE+), and IEEE 802.3bt (PoE++). PoE++ is further divided into two categories (Type 3 and Type 4) based on power requirements.
Catergory | PoE | PoE+ | PoE++ | PoE++ |
IEEE Standard | IEEE 802.3af | IEEE 802.3at | IEEE 802.3bt | IEEE 802.3bt |
PoE Type | Type 1 | Type 2 | Type 3 | Type 4 |
PSE Port Power | 15.4W | 30W | 60W | 90W |
PSE Voltage Range | 44-57V | 50-57V | 50-57V | 52-57V |
PD Max Power | 12.95W | 25.5W | 51W | 71W |
PD Voltage Range | 37-57V | 42.5-57V | 42.5-57V | 41.1-57V |
Max Current | 350mA | 600mA | 1.2A | 1.73A |
The following diagram lists supported PD devices based on the four PoE categories:
2 Purpose
2 Purpose
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Cost Efficiency: By eliminating the need for separate power outlets, PoE reduces the cost and complexity of electrical engineering.
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Easy Deployment: PoE device installation is more convenient and rapid, especially in locations that are difficult to access or lack power sources.
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Reliability: PoE technology provides centralized power management, facilitating maintenance and troubleshooting.
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Flexibility: PoE allows flexible network layout adjustments without concerns about power source locations, effectively handling high-load situations.
3 PoE Power Supply Methods
3 PoE Power Supply Methods
PoE interface remote power supply is divided into Signal and Spare modes (Alternative A and Alternative B modes) based on the power supply wire pairs used:
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Signal Power Mode: PSE uses wire pairs (1/2, 3/6) that carry data transmission to simultaneously transmit data and DC power to the PD. Wire pair 1/2 forms the negative pole and wire pair 3/6 forms the positive pole, or vice versa. Since DC power and data frequencies do not interfere with each other, current and data can be transmitted simultaneously on the same wire pair. 10BASE-T and 100BASE-TX interfaces use wire pairs 1/2 and 3/6 for data transmission, while 1000BASE-T interfaces use all wire pairs for data transmission.
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Spare Power Mode: PSE uses wire pairs (4/5, 7/8) to transmit DC power to the PD, where wire pair 4/5 forms the positive pole and wire pair 7/8 forms the negative pole.
PSE may support only one power supply mode, but PD must support both power receiving modes. Asterfusion devices supporting IEEE 802.3af (PoE) and IEEE 802.3at (PoE+) support Signal power mode, where data and power are both transmitted through wire pairs 1/2 and 3/6. Devices supporting IEEE 802.3bt (PoE++) support both Signal and Spare power modes, using all 8 wires to transmit power.
4 PoE Power Negotiation
4 PoE Power Negotiation
When the PSE device powers up and the PD device connects to the PSE device via network cable, power negotiation begins between PSE and PD:
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PD Detection: PSE periodically outputs current-limited low voltage at ports to detect PD device presence. If a specific resistance value is detected, it indicates that the cable terminal is connected to a powered device supporting IEEE 802.3af or IEEE 802.3at standards (resistance value between 19kΩ-26.5kΩ, typical low voltage 2.7V-10.1V, detection cycle 2 seconds).
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Power Capability Negotiation: PSE classifies the PD and negotiates power supply capacity. Power capability negotiation can be achieved not only through analyzing resistance sent by PSE and PD, but also through Link Layer Discovery Protocol (LLDP) for discovering and advertising power capabilities.
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Power Initiation: During the startup period (typically less than 15μs), PSE begins supplying power to PD from low voltage until providing 48V DC voltage.
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Normal Power Supply: After voltage reaches 48V, PSE provides stable and reliable 48V DC power to PD, with PD power consumption not exceeding PSE maximum output power.
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Power Disconnection: During power supply, PSE continuously monitors PD current input. When PD current consumption drops below minimum value or current surges (e.g., device disconnection, PD device power consumption overload, short circuit, exceeding PSE power supply load), PSE disconnects power and repeats the detection process.
4.1 Power Capability Negotiation via LLDP Protocol
4.1 Power Capability Negotiation via LLDP Protocol
Asterfusion PoE switches (as PSE) support LLDP-based power negotiation mechanisms. When connected PD (powered devices) also support LLDP-Med or related extensions, both parties will prioritize fine-grained power capability negotiation through this protocol. If PD does not support LLDP negotiation, the switch automatically falls back to physical layer classification (Hardware Classification) mechanisms based on IEEE 802.3af/at/bt standards for power negotiation.
IEEE 802.3 defines optional TLV (Type Length Value): Power via MDI (Media Dependent Interface) TLV. LLDP packets encapsulate Power via MDI TLV to discover and advertise MDI power capabilities. When PSE detects PD, PSE and PD periodically send LLDP packets to each other containing defined TLV fields. Each side sends its information to the other, and the recipient records the information contained in the packet, achieving information exchange.
The Power via MDI TLV packet defined by IEEE 802.3 consists of a 2-byte TLV packet header and a 12-byte TLV information field, with the packet format shown below:
The meanings of Power via MDI and Extended Power via MDI fields are as follows:
Power via MDI Field Meanings:
Field | Function and Meaning |
MDI power support | Bit 0: Port type 1: PSE port interface, 0: PD port interface |
Bit 1: PSE device MDI power support 1: Support, 0: Not support | |
Bit 2: PSE device MDI power status 1: Enabled, 0: Not enabled | |
Bit 3: PSE device power pair control capability 1: Pair selection controllable, 0: Pair selection not controllable | |
Bite4-7: Reserved | |
PSE power pair | Power class values: 1: Alternative A power mode 2: Alternative B power mode |
Power class | Power class values: 0: Class 0, reference power 15.4W 1: Class 1, reference power 4W 2: Class 2, reference power 7W 3: Class 3, reference power 15.4W 4: Class 4, reference power 30W 5: Class 5, reference power 45W 6: Class 6, reference power 60W 7: Class 7, reference power 60W 8: Class 8, reference power 60W |
Extended Power via MDI Field Meanings:
Field | Function and Meaning |
Power Type | Power type indication 11: PD not supporting IEEE 802.3at standard 10: PSE not supporting IEEE 802.3at standard 01: PD supporting IEEE 802.3at standard 00: PSE supporting IEEE 802.3at standard |
Power Source | Power source indication: PD 11: PSE and local, 10: Reserved, 01: PSE PSE 11: Reserved, 10: Backup power, 01: Primary power |
Power Priority | Interface power priority 11: Lowest priority 10: Medium-high priority 01: Highest priority Note: This field includes 4 bits, with the upper two bits reserved for system use |
Power Value | Contains PD requested power value and PSE allocated power value fields. When PoE power is sufficient, both are equal. Field value ranges from 1-255, with exchange power = 0.1 x decimal value of this field. For example. When sending a value of 255, it indicates an exchange power of 25.5W |
5 PoE Policies
5 PoE Policies
5.1 Power Supply
5.1 Power Supply
Simply put, configuring maximum PoE power supply is like installing an “intelligent circuit breaker” for the entire switch and each port, preventing power system overload.
Asterfusion PoE switches support configuring maximum power supply for the entire system and each access port, allowing users to allocate power according to connected device requirements.
5.2 Power Priority
5.2 Power Priority
Configure interface PoE power priority to ensure critical network devices remain powered and achieve rational power allocation. If total connected device power exceeds maximum system power supply, ports with lower power priority will be disconnected.
5.3 Power Scheduling
5.3 Power Scheduling
To improve energy efficiency and achieve automated management, time-based power policies can be configured for PoE ports connecting wireless APs, IP phones, and other terminal devices. This feature automatically cuts power during non-working hours and restores it before business hours, eliminating unnecessary energy consumption and manual operations.
5.4 Keepalive Detection
5.4 Keepalive Detection
PoE keepalive detection functionality periodically sends ping commands. If PD (powered device) does not respond, it restarts the PD device by cycling PoE power (turning off then on).
Problems Solved: Terminal devices in networks (such as IP cameras, wireless APs, IP phones, etc.) may enter “frozen” states due to software bugs, excessive load, temperature changes, or other unknown reasons. The device remains powered (PoE light is on) but network functionality is paralyzed (cannot ping, service interruption).
Traditional Approach: Requires maintenance personnel to manually discover faults, travel to device locations or find corresponding switch ports, and perform physical restarts (unplugging/plugging network or power cables). This process is time and labor-intensive, especially when devices are installed in hard-to-reach locations (such as elevated cameras).
PoE Keepalive Detection Solution: System automatically completes the “detect-judge-repair” process.
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Detection: Switch periodically sends ping commands to devices.
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Judgment: After multiple consecutive non-responses, device failure is determined.
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Repair: Remote PoE power cycling (turning off then on) simulates “manual unplugging/plugging” operations for device soft restart.
Result: Significantly reduces Mean Time to Recovery (MTTR) from hours/days to minutes without manual intervention, achieving a “self-healing network.”
6 Configuration Methods
6 Configuration Methods
6.1 Configuration Commands
6.1 Configuration Commands
Operation | Command | Description |
Enable interface PoE power function | poe enable | - |
Configure PoE function delay startup time | poe-delay <time> | Value range: 0-3600, unit: seconds(s) |
Configure the system maximum PoE power supply | poe max-power {60w|30w|15w|30w-at} | Different hardware models have varying configurable system and interface PoE power limit ranges. Refer to the device's help information for specific configurable power values |
Configure PoE power priority | poe priority {low|high|critical} | When the device's total power supply reaches the limit, interfaces with lower PoE power priority will be disconnected based on their priority levels |
Configure PoE power stop time range | time-range <name> from <date-stop> to <date-start> | Default configuration provides 24-hour power supply |
Configure PoE power stop time | time-range <name> <date-stop> to <date-start> <day> | name: Policy name date-start: Power start time, format: xx:xx date-stop: Power stop time, format: xx:xx day: Policy repeat mode options: Mon/Tue/Wed/Thu/Fri/Sat/Sun - specific day of week daily -repeat daily working-day - all working days off-day - all non-working days |
Apply PoE power policy to specific interface | poe power-off time-range <name> | name: Power policy name |
Enable device PD compatibility | poe legacy-detect enable | Enables PD device compatibility detection functionality, allowing the device to detect and supply power to PD devices that do not comply with 802.3af or 802.3at standards. If PD device compatibility detection is not enabled, the device will not recognize non-standard PD devices and cannot supply power to them |
Initialize PoE function | poe reinitialize | If the device was not PoE-initialized at the factory, this command can be used for manual initialization. After executing this command, no device restart is required, but all PoE enablement will be automatically disabled and re-enabled after initialization is complete |
Enable interface PoE PD keepalive detection | poe pd alive-check enable | - |
Configure PD device IP address | poe pd alive-check ip <ip address> | <ip address>: e.g., 192.168.1.100 (must be in same subnet as interface) |
Configure the delay detection time after PoE enable/restart | poe pd alive-check delay <seconds> | <seconds>: 5-300 (default 60) |
Configure ping detection interval | poe pd alive-check interval <seconds> | <seconds>: 10-300 (default 30) |
Configure the maximum threshold for action execution | poe pd alive-check err-threshold <time> | <times>: 1-10 (default 3) |
Configure action after detection failure | poe pd alive-check action <action> | <action>: -alarm: Send log alarm + write stateDB (default) -reboot: Restart PoE port |
Configure PoE off→on interval during restart | poe pd alive-check reboot-interval <seconds> | <seconds>: 1-180 (default 10) |
6.2 Typical Configuration Examples
6.2 Typical Configuration Examples
6.2.1 Network Requirements
6.2.1 Network Requirements
In this network, switch PoE power schedule: stop power supply from 20:00 to 08:00 next day on working days, stop power supply all day on non-working days. Eth1 uses all default configurations; Eth2, Eth3, Eth4 configured with 20s delay startup, 30W maximum power supply, high power priority, and compatibility detection enabled; Eth4, Eth5 have keepalive detection enabled.
6.2.2 Configuration Steps
6.2.2 Configuration Steps