How Do PoE Switches Supply Power for PoE Powered Devices?

Driven by the need for connectivity and monitoring of smart IoT devices, the PoE switch has evolved into an increasingly efficient method of delivering power and data over the network. This article aims to explain the operational principles and modes of the PoE switch's power supply, as well as the limited distance and maximum voltages associated with PoE switch supply.

Devices in PoE Network

A PoE network consists of two types of devices: power sourcing equipment (PSE) and powered devices (PD). The PSE supplies power to the PD. The PSE device typically takes the form of a PoE switch, while the PD devices encompass IP phones, IP surveillance cameras, wireless LAN access points, PoE lighting, and similar devices.

Working Process of PoE Switch Power Supply

To comprehend the working principles of the PoE switch power supply, let's examine the PoE network switch and PoE IP camera as an example to illustrate how the PoE switch power supply operates. Upon connecting a PoE IP camera to a PoE Ethernet switch, the following working process occurs:

Detection of PDs: Initially, the PoE switch delivers a minimal voltage through the port until it detects that the cable terminal connection corresponds to a PD that supports the IEEE802.3af standard. Typically, a 24.9kΩ resistor is employed in the PD equipment to verify compliance with the IEEE802.3af power supply standard. It is important to note that only active PoE network switches perform this check, while passive PoE network switches or injectors do not. The differences between active and passive PoE switches will be further illustrated here: Active vs. Passive PoE Switch: Which Should We Choose?

Classification of PoE Switch Power Supply Capability: After detecting the PD, the PoE switch supplies a voltage of 15-20V to the PD and determines its specific power requirement by measuring the current. The switch classifies the device into various types: Class 0, 1, 2, 3, 4, 5, 6, 7, and 8, based on the presence of a resistor, and provides the appropriate power accordingly.

Commencement of Power Supply: Once the Power over Ethernet classification is completed, the PSE device initiates power delivery to the PD device, starting from a low voltage that gradually ramps up to the full 48V DC within a configurable startup period (typically less than 15μs).

Normal Power Supply: Once the voltage reaches 48V, the PoE switch reliably and consistently provides a stable 48V DC power output to the PD.

Disconnection of PoE Switch Power Supply: The PoE switch swiftly (usually within 300-400ms) discontinues power supply and re-enters the PD detection procedure under the following circumstances:

• The PD is removed.

• The power consumption of the PD is overloaded or short-circuited.

• The total power consumed by the PDs is out of the power budget of the PoE-powered switch.

In these situations, the switch and PDs are protected, preventing any potential damage to non-PoE devices inadvertently connected to the PoE ports after the PDs are disconnected.

PoE Switch Power Supply Mode

The power supply mode between the PSE and PDs in a PoE switch can be categorized into three distinct modes:

Mode A

In this mode, the PoE switch supplies power to PDs through data pair 1-2 and pair 3-6. Pair 1-2 represents the positive polarity, while pair 3-6 represents the negative polarity.

Mode B

In Mode B, the PoE switch delivers power to PDs via pair 4-5 and pair 7-8. In 10BASE-T and 100BASE-T, these two pairs are not used for data transmission and are referred to as spare pairs in 10/100M PoE applications. Pair 4-5 represents the positive polarity, while pair 7-8 represents the negative polarity.

The primary distinction between Mode A and Mode B lies in the utilization of PINs, as depicted visually in the following diagram:

PSE devices can transmit power in two different modes: Mode A, also known as "endspan," where power is relayed through the data pairs, and Mode B, also known as "midspan," where power is relayed through the spare pairs. Compliant PSE devices are capable of supporting Mode A, Mode B, or both, while compliant PDs can work with both Mode A and Mode B. However, compatible PDs usually support Mode B only. Let's explore the working scenarios between switches and IP cameras based on these two modes.

4-pair Delivery

In this mode, power is delivered using all four pairs. Pairs 1-2 and 4-5 serve as the positive polarities, while pairs 3-6 and 7-8 act as the negative polarities.

The table below illustrates the three modes in two distinct network scenarios:

Please note that the Power over Ethernet (PoE) power supply mode is determined by the Power Sourcing Equipment (PSE). Both PoE switches and PoE injectors can act as PSE devices to provide power and data to Powered Devices (PDs). PoE Ethernet switches, commonly referred to as endspans (or endpoints according to IEEE 802.3af), typically utilize PoE mode A. On the other hand, PoE injectors (also known as midspan devices) function as intermediary devices between non-PoE switches and PDs, supporting only PoE mode B.

PoE Switch Power Supply Distance

PoE can transmit power up to 100 meters from the PSE to the PDs. However, the total length of Ethernet cabling is limited to 100 meters due to signal attenuation specified by Ethernet cabling standards. In the case of PoE switches, the maximum distance for power transmission is typically 100 meters. The actual PoE transmission distance may vary depending on the network cables used. For Cat5e, Cat6, Cat6a, and Cat7 cables, the maximum transmission distance is 100 meters. However, Cat8 cables, despite supporting 25/40 Base-T transmission speeds, have a reduced maximum transmission distance of only 30 meters.

PoE Switch Supply Voltages and Types

According to the IEEE 802.3 standard, PoE switches are categorized into four types, each associated with a specific supply voltage. The table below provides a breakdown of the supply voltage for each PoE switch type.

Conclusion

Power over Ethernet (PoE) technology plays a crucial role in the ongoing digital transformation. Understanding the power supply aspects of PoE switches is essential for ensuring the protection of both PoE and non-PoE devices. Familiarizing oneself with common issues and solutions related to PoE switch connections is also beneficial in preventing unnecessary time and financial resources from being wasted during the deployment of PoE networks. By staying informed and prepared, organizations can fully leverage the advantages of PoE technology and optimize their network infrastructure.