PoE Switch Power Supply Process Explained in Detail

Power over Ethernet (PoE) switches has gained immense popularity in recent years. They can transmit data and power through the same Ethernet cable at a time. This data and power transfer capability of these switches make them an ideal choice for small enterprise and home network applications. Although these switches may pass off as any regular switch, there is a well-defined process involved in the working of these switches. Are you intrigued to learn about the PoE switch's power supply process? If yes, this post introduces you to the same and more.

An Overview of PoE Switch and Its Standards

A PoE switch is a power sourcing equipment or PSE that uses cables to deliver data and power simultaneously to powered devices or PDs like VoIP phones and cameras. Theses switches are built to the Institute of Electrical and Electronics Engineers (IEEE) standards.

Currently, there are three PoE standards accepted universally. These standards offer you an idea of the minimum power that a PoE switch can deliver and the maximum power that a PD device connected to it can receive. These switches use the Ethernet cable that meet CATx standards. The cables feature four twisted pairs of wires, of which some standards use two pairs of wire each for data and power transmission, and some use all four pairs for power and data transmission.

• PoE: This switch is built to IEEE 803.3af standards. Each switch port can deliver 15.4W at any time to the connected PD. Some power will be lost during the transmission through the cable. It is designed for the supply current of 10-350mA and supply voltage of 44-57V. The switch uses two-pairs of wire to support PD devices like sensors, VoIP phones, static surveillance cameras, wireless access points, etc.
• PoE+: This switch follows the IEEE 802.at standard and is backward compatible with PoE. A PoE+ switch is designed for current supply of 10-600mA and voltage supply of 50V - 57V. Each port can provide 30W to PD. However, some power will be lost during the transmission, so the available power is 25W. PoE+ switches use two pairs of wire to power biometric sensors, LCD displays, tablets, etc.
• PoE++: This is the most advanced PoE standard used to power modern day video conferencing systems. These switches are further distinguished into Type 3 and Type 4 PoE switches based on their power transmission capacities.
• Type 3: Each switch port can carry up to 60W over a single Ethernet cable. These switches use four pairs for data and power transmission.
• Type 4: Each port of these switches can carry up to 100W over a single Ethernet cable. This makes them suitable for powering devices like TVs and laptops. Each switch uses four wire pairs for power and data transmission.

Although designed for better power and data transmission, these switches are a rarity, and most companies still use PoE++ switches.

PoE Switch Power Supply Process Explained

There are two types of PoE switches used today – an active PoE switch and a passive PoE switch. The active PoE switch negotiates a power supply between the PSE and the PD, whereas the passive PoE switch doesn’t have this feature. The latter type of switches do not adhere to any standard, but it still supplies power at a specific voltage.

Let’s try to understand the PoE switch power supply process using an example of a PoE IP camera connected to the PoE switch.

• The PoE switch releases a small voltage until the port identifies the type of PD connected to it.
• fter detecting the powered device, the PoE switch will apply 15-20V to the PD. The switch identifies PD in any of the eight classes from 0, 1, 2, 3, 4, 5, 6, 7, and 8.
• Next, the switch starts applying current to the PD based on the classification. The power supply is raised from low to high within a few microseconds. The maximum power supplied is 48V.
• Once the 48V supply is attained, the switch will continue to provide a stable power supply.
• The PoE switch will perform detection if any of the following occurs – the PD has high power consumption and is on the verge of short circuit or overloading; the PD power consumption is higher than what PoE switch; or the PD is replaced with a new device or removed from the operation.
• The PoE switch supplies power in either of the three modes – Mode A, Mode B, and Mode C.
• Mode A: The PoE switch uses data pairs 3-6 and 1-2 to supply the power to PD. The data pair 3-6 acts as the negative polarity; however, the data pair 1-2 acts as positive polarity.
• Mode B: The PoE switch uses pairs 7-8 and 4-5 to power the PD device. In this, the pair 7-8 acts as a negative polarity and 4-5 acts as a positive polarity. The two pairs are not utilized for power transfer in 100BASE-T and 10BASE-T. These pairs are known as spare pairs.
• 4-pair Delivery: In 4-pair, the power is delivered through all four pairs. In this, the pair 7-8 and 3-6 are negative polarities, and pair 4-5 and 1-2 are positive polarities.

When the switch relays power through Mode A or data pairs, it is known as endspan, and if Mode B or spare pairs are involved, then it is known as midspan. The mode of power supply is decided by the PoE switch.

PoE Switches are today an integral part of our digital lives. If you have been considering PoE switch for your small enterprise or home network application, ensure to purchase them from trusted manufacturers. It is quite common to find these switches from different brands, however, they all may not work for you. Check the underlying PoE technology and know the power supply demands of the devices connected to these switches before selecting them for your application. If you are unclear about your needs, you can try contacting the manufacturer. An expert manufacturer will always guide you through the selection, installation, and offer post-sales assistance to assure you the best value for investment.

R.W. Tull is the President of VERSITRON. He interfaces daily with current and potential end-users. R.W. works closely with clients to review layout diagrams and drawings in order to ensure that the best fiber optic solution is achieved.