A Guide to Understanding PoE Technology

Power over Ethernet (PoE) technology is comparatively simple to deploy; however, it is also rife with terms that can be confusing and may take a minute to understand. The purpose of this article is to explain several PoE terms that are often unclear.

PoE PSE vs. PoE PD

PoE PSEs (power sourcing equipment) are the devices that provide power (electricity). Examples of PoE PSEs are PoE switches, PoE extenders, PoE injectors, and PoE media converters.

PoE PDs (powered devices) are gadgets that are powered up by the PSE. A few examples of common PoE PDs include IP cameras, VoIP phones, and wireless access points (WAPs).

For clarification: A PoE switch is a PSE that delivers electrical power to an IP camera, which is a PD.

Mode A vs. Mode B

PoE delivers power by two different methods: Mode A and Mode B. To fully understand this, here are some terms you should know:

  • Pin: An electrical connector. There are eight metal strips (pins) on the end of an Ethernet cable.
  • Pinout: This is the diagram that spells out which pins need to connect to function correctly. Mode A and Mode B use different pinouts, which we will explain further below.
  • RJ45: The registered jack-45 is commonly referred to as an RJ45, a jack-45,  or a data jack. It is the eight-pin connector found on the end of an Ethernet cable.
  • Ethernet cable: This is the wire commonly used to carry data and power on a local area network (LAN).

Mode A injects power onto an Ethernet cable on pins 1,2,3, and 6, while Mode B injects power on 4,5,7, and 8. There is not much difference in how these modes perform, but it is important to know that 24-volt PoE devices will require Mode B. Rule of thumb for 48-volt devices is to use Mode A. Both Mode A and Mode B work well as long as they are the correct mode for the task. 

Endspan PSEs vs. Midspan PSEs

A PoE endspan PSE (such as a switch) is the primary power device in a network. It is called an endspan device (or endpoint) because it is located at the end of the cable.

Midspan PSEs are intermediary devices (such as PoE injectors and PoE hubs) positioned between a non-PoE PSE (such as a regular switch) and PoE PDs in a network.

PoE-Compliant Devices vs. PoE-Compatible Devices

PoE-compliant devices adhere to the strict Institute of Electrical and Electronics Engineers (IEEE) 802.3aft/at/bt standards, and as such, should support both Mode A and Mode B power modes. However, IEEE compliance standards do not specifically require the support of both modes. As a result, some PoE-compliant PSEs only use one mode. Therefore, when purchasing a PoE-compliant PSE, it is vital to know if it supports Mode A, Mode B, or both. 

On the other hand, PoE-compatible devices do NOT stick to IEEE standards. This does not mean they are not a reliable alternative. However, it is essential to note that PoE-compatible devices are vendor specific and which mode (or modes) they support varies from manufacturer to manufacturer. Therefore, it is necessary to check all specification data sheets to determine that any PoE-compatible units to be purchased will meet the power mode used by your network.

Frequently Asked Questions About PoE

What is PoE?

PoE (Power over Ethernet) is a technology that carries both power and data over a single Ethernet cable. This is distinguished from traditional electrical wiring that supplies only power and requires a second wire to transmit data. 

What are the different types of PoE?

There are four types of PoE under three IEEE PoE standards. They are as follows:

  1. Type 1 (IEEE 802.3af), also called PoE, has a maximum of 15.4 watts (W) per port.
  2. Type 2 (IEEE 802.3at) is often called PoE+ and has a maximum power of 30W.
  3. Type 3 (IEEE 802.3bt) is known as PoE++ and has a total power of 51W.
  4. Finally, type 4 (also IEEE 802.3bt), often called Hi-PoE, has a maximum power output of 100W.  

What devices use PoE?

Many IoT devices can be powered by PoE. Following is a list of  some of the  common devices that use 30W of power or less:

  • IP, VoIP, and video phones
  • PTZ cameras
  • Remote computer terminal
  • Door access systems

Here are some popular PoE devices that require 31W to 100W of power:

  • LED lights
  • Video conferencing
  • Security card readers
  • Video surveillance cameras

What are the benefits of PoE?

There are several reasons to use PoE technology. PoE is:

  • Cost-effective: With PoE, both power and data are transmitted over only one Ethernet cable, cutting cable expenditures in half. Furthermore, there is no need to hire an expensive electrician to install PoE because of its low wattage.
  • Easy to install: PoE PSEs are plug-and-play, and there will be no need to worry about hooking up to electrical outlets.
  • Safe: PoE has built-in safety measures. The PSE (e.g., a PoE switch) must encounter a “handshake” with the PDs in the network before any power is sent. This handshake ascertains how much power the PD needs. No handshake—no power. In addition, once a connection is made and power is delivered, the PSE continually monitors the power supply. If the electrical current gets too high or too low, the PSE shuts down all power flow, so devices are not damaged.
  • Speedy: PoE can deliver data speeds up to one gigabit per second (Gbps).

What are the limitations of PoE?

PoE technology does have some drawbacks that need to be considered.

  • Distance limitation: PoE has a distance limitation of 100 meters (approximately 328 feet). For longer distances, equipment such as PoE extenders will need to be deployed.
  • High switch costs: PoE switches are significantly more expensive than regular switches.
  • Outages: Because a single PoE connection typically links many devices, they all will go down if one device fails.
  • Power limits: PoE technology supports 15.4W to 100W of power only.

Last Thoughts

The future of PoE technology looks bright. As IoT-connected devices increase, so will PoE solutions. PoE is widely used to support such cutting-edge projects as smart cities, smart grids, and smart buildings. According to Global Market Insights, “Power over Ethernet Solution Market size exceeded USD 700 million in 2018 and is estimated to grow at a CAGR of over 15% from 2019 to 2025.”

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