How do PoE switches handle data and power simultaneously?

PoE (Power over Ethernet) switches are designed to handle both data and power transmission simultaneously over the same Ethernet cable. Here’s a breakdown of how this is achieved:

1. Ethernet Cable Structure

--- Standard Ethernet cables, like Cat5e, Cat6, or Cat6a, consist of eight copper wires twisted into four pairs. For standard data transmission, only two pairs (four wires) are needed. PoE technology takes advantage of the unused pairs to transmit power, or in some configurations, sends both power and data over the same pairs.

2. Power Injection

PoE switches inject power into the Ethernet cable alongside the data signals. Depending on the PoE standard, the power is injected in one of two ways:

--- Mode A (Phantom Powering): Power is transmitted along the same pairs that carry data (pins 1-2 and 3-6).

--- Mode B (Spare Pair Powering): Power is transmitted on the unused pairs (pins 4-5 and 7-8) in 10/100 Mbps Ethernet.

In both cases, the power and data signals are able to coexist without interference, thanks to the separation of their frequencies—power is transmitted as a low-frequency DC current, while data is transmitted as high-frequency signals.

3. Power and Data Separation at the Device

--- At the receiving end (the powered device, or PD), a PoE splitter inside the device separates the power from the data. The Ethernet controller in the device handles the data transmission, while the power supply circuit uses the DC voltage from the Ethernet cable to power the device.

4. Negotiation (Power Classification)

--- PoE switches use a process called power classification to detect whether a connected device is PoE-compatible and determine how much power it needs. This is done using a handshake protocol known as LLDP (Link Layer Discovery Protocol) or a simpler detection mechanism where the switch sends a small voltage through the cable to identify the device's power requirements.

--- Once the power needs are identified, the switch adjusts the power output accordingly, ensuring the appropriate amount of power is supplied without disrupting data flow.

5. PoE Standards

Different PoE standards allow for varying amounts of power to be delivered:

--- IEEE 802.3af (PoE): Up to 15.4W per port.

--- IEEE 802.3at (PoE+): Up to 25.5W per port.

--- IEEE 802.3bt (PoE++): Up to 60W (Type 3) or 100W (Type 4) per port.

6. Power Budget Management

--- A PoE switch manages its total power budget, distributing available power to all connected devices. It monitors how much power each device is drawing and dynamically adjusts to ensure all connected devices receive the power they need while maintaining data transmission.

7. Data Integrity

--- PoE switches are designed to maintain data integrity, ensuring that power transmission doesn’t interfere with data signals. This is achieved by using precise filtering techniques and voltage regulation to prevent power-related noise from affecting data communication.

In summary, PoE switches use intelligent power management and frequency separation techniques to transmit data and power simultaneously over the same Ethernet cable, ensuring efficient, reliable operation for powered devices without data disruption.