Ventajas de utilizar controladores de gestión de la alimentación PoE en equipo de fuente de alimentación.
Información básica sobre Power over Ethernet (PoE)
Es importante comprender algunos aspectos fundamentales de Power over Ethernet IEEE 802.3af (PoE hasta 15.4W) 802.3at (PoE+ hasta 30W), y 802.3bt (Hi-PoE hasta 100W).
Los cables Ethernet (CAT 5e y meyor) contienen cuatro pares trenzados. En Ethernet 10 y 100BASE-TX, solo se utilizan dos de estos pares para distribuir datos. Gigabit Ethernet (1000Base-T) requiere el uso de los cuatro pares.
Teniendo en cuenta que se necesitan dos conductores para proporcionar alimentación a través de un cable, el estándar PoE trata cada par como un conductor. Puede utilizar los dos pares de datos (lo que se conoce como alimentación fantasma o alternativa A) o los dos pares libres (alternativa B) para transportar la corriente eléctrica. Dado que Gigabit requiere 4 pares de datos, PoE en enlaces Gigabit transmitirá alimentación a través de dos de los pares de datos (alimentación fantasma).
La potencia se inyecta en el cable Ethernet a una tensión de 44-57 voltios de DC (50-57 voltios para PoE+ y Hi-PoE). Esto permite que la alimentación se transfiera por el cable de un modo eficiente, cumpliendo al mismo tiempo los requisitos de baja tensión de la norma MBTS (Muy baja tensión de seguridad).
Norma Muy baja tensión de seguridad
La norma MBTS especifica el proceso que debe seguirse para aplicar potencia de forma segura a un cable de red. Se explican las fases de detección de firma, clasificación, arranque, funcionamiento y desconexión.
Detección de firma
Aunque 44 y 57 voltios de DC son seguros para los usuarios, pueden dañar equipos que no se hayan diseñado para recibir alimentación PoE. Por lo tanto, según la norma IEEE, antes de que el equipo de fuente de alimentación (PSE) pueda proporcionar alimentación a un dispositivo alimentado (PD) conectado (como una cámara IP o un punto de acceso inalámbrico), debe llevar a cabo una comprobación de capacidades para garantizar que el dispositivo Ethernet pueda recibir alimentación. Esto tiene lugar a través de un proceso de detección de firma.
Signature detection is performed by injecting a very small voltage on the link to detect a characteristic signature of IEEE-compatible PDs. This is presented with a 25k ohm resistance. Once detected, the PSE knows that higher voltages can be safely applied. A device not supporting this standard will not present this signature and voltage will not be applied. This means that on a remote switch with multiple PoE PSE ports, the connection of various types of Ethernet equipment can be safely done with compliant and non-compliant PoE PDs. It is important to note that Passive PoE injectors do not perform this check and will always apply power on the cable. These devices have visual warnings on the ports to not connect non-PoE complaint devices, however, that does not guarantee that staff at remote sites will not inadvertently connect a device to the wrong port.
Classification
Classification follows the signature detection stage and determines the amount of power for which a device is rated. If a PD displays a classification signature, it lets the PSE know how much power it requires to operate. This means that PSEs with a limited total power budget can allocate it effectively. PoE power classes and types are as follows:
| IEEE PoE Specifications for Cat5e @100m | ||||||||
|---|---|---|---|---|---|---|---|---|
| Standard / Type | 802.3af Type 1 |
802.3at Type 2 |
802.3bt Type 3 |
802.3bt Type 4 |
||||
| Name / Year | PoE / 2003 | PoE+ / 2009 | Hi-PoE / 2018 | Hi-PoE / 2018 | ||||
| PSE Input Voltage (Min) | 44V/50V | 50V | 52V | |||||
| Supported PD Class | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| PSE Output Power to Single-Signature PD | 4W | 6.7W | 14W | 30W | 45W | 60W | 75W | 90W - 100W |
| Single-Signature PD Input Power (Min) | 3.84W | 6.49W | 13W | 25.5W | 40W | 51W | 62W | 71.3W |
| PSE Output Power to Dual-Signature PD (per pairset) | 4W | 6.7W | 14W | 30W | 45W | -- | -- | -- |
| Dual-Signature PD Input Power (pairset / total) | 3.84W / 7.68W |
6.49W / 12.98W |
13W / 26W |
25.5W / 51W |
35.6W / 71.2W |
-- | -- | -- |
| Ethernet Pairs | 2-Pair | 2-Pair or 4-Pair |
4-Pair | |||||
The table above represents the power available at the PSE. It is important to note that because of cable resistance, voltage drops along the length of the cable occur and power is lost resulting in less power being available to the remote PD. As an example, a PSE detecting a Class 3 device will provide up to 15.4 watts. However on a full 100 meter cable, no more than 12.95 watts will be available to the remote PD.
If a PD does not display a classification signature, a class 0 level of power is used.
Start-up
Following the detection and classification of a newly connected device, voltage is applied in a controlled manner so that unwanted noise is not injected into the cable eventually reaching the full negotiated PoE power level required.
Operation Monitoring
Once full voltage is applied the PSE continues to monitor how much electrical current it is being delivered to the PD and will cut the power to the cable if too much, or not enough, power is drawn. This protects the PSE and PD against overload and ensures that PoE is disconnected from the cable if the PD is unplugged.
Disconnect
If, during the operation, the cable is disconnected the PSE will remove the voltage from the cable within 15 to 20ms. This ensures that voltage is removed in the event that a non-PoE compliant device is then connected.
Smart PoE Management Controller
Properly designed PoE PSEs will have PoE Management Controllers built into the product which meet the IEEE 802.3af/at/bt standards. This ensures that:
- both PoE and non-PoE complaint devices can be safely connected;
- unwanted noise is not injected on the network cable during power up;
- current draw is constantly being monitored for protection of both the PSE and PD;
- voltage is removed immediately upon disconnect enabling a non-PoE device to be connected if desired.
Identifying a non-compliant PSE is easily determined through the visual warnings displayed on the product‘s PoE port(s) or user guide.
Perle PoE PSE's with built-in Power Management Controllers
Perle PoE Ethernet Extenders are the only VDSL- based Ethernet Extenders on the market that have a built-in PSE controller that is fully compliant with PoE standards. Competitive PoE Ethernet Extender products operate as a simple passive power injector and will always apply power to RJ45 port pins which may result in damage if non-PoE compliant Ethernet devices are accidently attached.
