Power over Ethernet (PoE)

1. Introduction to Power over Ethernet

What is PoE?

Power over Ethernet (PoE) is a technology that enables the transmission of both data and electrical power over a single Ethernet cable. This simplifies installations by reducing the need for separate power and data cables. Common use cases include powering devices like VoIP phones, IP cameras, and wireless access points.

Benefits of PoE

Benefits of PoE in Networking

  • Cost Efficiency: Eliminates the need for dedicated power outlets for devices.
  • Simplified Cabling: Combines power and data into a single cable, reducing cable clutter.
  • Flexibility: Devices can be placed in optimal locations without worrying about proximity to power outlets.
  • Centralized Power Management: Network administrators can manage power delivery from a central switch, enabling features like remote rebooting.

2. PoE Standards

IEEE 802.3af (PoE)

The IEEE 802.3af standard, introduced in 2003, provides up to 15.4W of DC power per port from the Power Sourcing Equipment (PSE). Due to cable losses, a maximum of 12.95W is available at the Powered Device (PD). This standard is ideal for low-power devices like basic VoIP phones and small access points.

IEEE 802.3at (PoE+)

Introduced in 2009, IEEE 802.3at (PoE+) delivers up to 30W of DC power per port from the PSE, with 25.5W available at the PD. This higher power output supports devices with greater energy demands, such as cameras with pan/tilt/zoom (PTZ) features.

IEEE 802.3bt (UPoE and UPoE+)

The IEEE 802.3bt standard extends power delivery even further:

  • UPoE: Supplies up to 60W per port using all four wire pairs in the Ethernet cable, suitable for video conferencing systems or larger wireless access points.
  • UPoE+: Supplies up to 100W per port, enabling power delivery to devices like interactive displays and advanced IoT systems.

3. Components of PoE

Components of PoE

Power Sourcing Equipment (PSE)

PSE devices are responsible for delivering power over Ethernet. Examples include PoE-capable network switches and PoE injectors. They negotiate the amount of power required by the connected device and ensure efficient delivery.

Powered Devices (PD)

PDs are the devices that receive power from the PSE. Common examples include VoIP phones, security cameras, and wireless access points. These devices must comply with PoE standards to operate seamlessly with PoE-enabled switches.

4. Practical Example

How PoE Powers a VoIP Phone

A VoIP phone connected to a PoE-enabled switch receives both power and data through the Ethernet cable. The switch detects the phone as a powered device using the PoE handshake mechanism and supplies the appropriate power level based on the phone’s requirements. This eliminates the need for an external power adapter, streamlining the setup.

How PoE Powers a VoIP Phone

Verifying Power Delivery

To confirm that the VoIP phone is receiving power, use the following CLI command on a Cisco switch:

Switch# show power inline
Available: 370.0(w)  Used: 15.0(w)  Remaining: 355.0(w)

Interface  Admin  Oper       Power   Device              Class  Max
                            (Watts)  
---------  ------  ---------- ------- ------------------- -----  ----
Fa0/0      auto    on         10.0    IP Phone 7960       3      15.4
Fa0/1      auto    off        0.0     n/a                 n/a    15.4
Fa0/2      auto    off        0.0     n/a                 n/a    15.4
Fa0/3      auto    off        0.0     n/a                 n/a    15.4
Fa0/4      auto    off        0.0     n/a                 n/a    15.4
Fa0/5      auto    off        0.0     n/a                 n/a    15.4
Fa0/6      auto    off        0.0     n/a                 n/a    15.4
Fa0/7      auto    off        0.0     n/a                 n/a    15.4
Fa0/8      auto    off        0.0     n/a                 n/a    15.4
Fa0/9      auto    off        0.0     n/a                 n/a    15.4
DHCP