Introduction

Power over Ethernet (PoE) technology has revolutionized the way we deploy network devices by allowing both data and power to be delivered over the same Ethernet cable. This innovation has not only simplified installation processes but also opened avenues for powering devices like IP cameras, VoIP phones, and wireless access points. To fully understand how PoE operates, it is essential to explore the network cables involved, particularly how different wires in 100Mbps (Fast Ethernet) and 1000Mbps (Gigabit Ethernet) cables contribute to PoE functionality.

Ethernet Cable Standards

Before diving into the specifics of wire utilization in PoE systems, it is important to familiarize ourselves with Ethernet cable categories:

Cat 5e: Supports 100Mbps and is typically used for Fast Ethernet. It can also support 1000Mbps (Gigabit Ethernet) for short distances.
Cat 6: Better performance over longer distances, supports 1000Mbps (Gigabit Ethernet) up to 100 meters with improved crosstalk performance.
Cat 6a and Cat 7: Designed for higher frequencies, providing better shielding and supporting higher speeds and PoE applications.

Wire Configuration

Category 5e and 6 cables typically use four twisted pairs of wires. Each pair consists of two wires:

Pair 1: White/Blue and Blue Pair 2: White/Orange and Orange Pair 3: White/Green and Green Pair 4: White/Brown and Brown

PoE Standards

The IEEE has defined several standards for Power over Ethernet:

IEEE 802.3af: Delivers up to 15.4 Watts of power (12.95 Watts guaranteed at the powered device) and requires at least 2 pairs of wires.
IEEE 802.3at (PoE+): Increases power delivery capability to 30 Watts (25.5 Watts guaranteed), also using at least 2 pairs of wires.
IEEE 802.3bt (PoE++): Introduces higher power levels, supporting up to 60 Watts (Type 3) and 100 Watts (Type 4). This standard can utilize all 4 pairs of wires.

Wires Utilization in Different Ethernet Cables

100Mbps (Cat 5/5e)

In traditional 100Mbps networks, PoE devices primarily utilize the first two pairs of wires, designated as:
Pair 1 (Pins 1 and 2): Transmits data.
Pair 2 (Pins 3 and 6): Transmits data and carries power under IEEE 802.3af and 802.3at standards.

The other pairs (Pair 3 and Pair 4) are not typically used for data transmission in 100Mbps connections. Instead, they can be reserved for auxiliary functions or, in certain PoE applications, can carry additional power (if the infrastructure supports it).

PoE Standards

The IEEE has defined several standards for Power over Ethernet:

IEEE 802.3af: Delivers up to 15.4 Watts of power (12.95 Watts guaranteed at the powered device) and requires at least 2 pairs of wires.
IEEE 802.3at (PoE+): Increases power delivery capability to 30 Watts (25.5 Watts guaranteed), also using at least 2 pairs of wires.
IEEE 802.3bt (PoE++): Introduces higher power levels, supporting up to 60 Watts (Type 3) and 100 Watts (Type 4). This standard can utilize all 4 pairs of wires.

1000Mbps (Gigabit Ethernet, Cat 5e and above)

At 1000Mbps, also known as Gigabit Ethernet, PoE implementations change slightly. All four pairs of wires are used for data transmission.
Pair 1 (Pins 1 and 2): Transmits data.
In terms of PoE power delivery, all 4 pairs can be utilized, especially for higher power standards (IEEE 802.3at and IEEE 802.3bt):

For IEEE 802.3af, power delivery typically uses Pair 1 and Pair 2.
For IEEE 802.3at, power can still use just these two pairs or all four pairs for optimal efficiency.
For IEEE 802.3bt, all four pairs (with Type 3 and Type 4) can distribute power simultaneously, allowing for a more powerful PoE solution.