PoE on the Switch
A PoE switch is PSE (Power Sourcing Equipment) that delivers electrical power to connected PD (Powered Devices) over existing copper cables without interfering with the network traffic, updating the physical network or modifying the network infrastructure.
PoE Features
PoE provides the following features:
Power over Ethernet can be used in any enterprise network that deploys relatively low-powered devices connected to the Ethernet LAN, such as:
PoE Operation
PoE implements in the following stages:
- Detection--Sends special pulses on the copper cable. When a PoE device is located at the other end, that device responds to these pulses.
- Classification--Negotiation between the Power Sourcing Equipment (PSE) and the Powered Device (PD) commences after the Detection stage. During negotiation, the PD specifies its class, which is the amount of maximum power that the PD consumes.
- Power Consumption--After the classification stage completes, the PSE provides power to the PD. If the PD supports PoE, but without classification, it is assumed to be class 0 (the maximum). If a PD tries to consume more power than permitted by the standard, the PSE stops supplying power to the port.
PoE supports two modes:
- Port Limit--The maximum power the switch agrees to supply is limited to the value the system administrator configures, regardless of the Classification result.
- Class Power Limit--The maximum power the switch agrees to supply is determined by the results of the Classification stage. This means that it is set as per the Client's request.
PoE Configuration Considerations
There are two factors to consider in the PoE feature:
You can decide the following:
- Maximum power a PSE is allowed to supply to a PD
- During device operation, to change the mode from Class Power Limit to Port Limit and vice versa. The power values per port that were configured for the Port Limit mode are retained.
- Maximum port limit allowed as a per-port numerical limit in mW (Port Limit mode).
- To generate a trap when a PD tries to consume too much and at what percent of the maximum power this trap is generated.
The PoE-specific hardware automatically detects the PD class and its power limit according to the class of the device connected to each specific port (Class Limit mode).
If at any time during the connectivity an attached PD requires more power from the switch than the configured allocation allows (no matter if the switch is in Class Limit or Port Limit mode), the switch does the following: