Power Over Ethernet

What is PoE - Power Over Ethernet?

Power over Ethernet simplifies the deployment of networking devices allowing for system integrators or network administrators to install powered devices at any location. PoE simultaneously transmits data and power using a standard Ethernet cable and eliminates the expense of installing additional expensive circuitry.

PoE Benefits Include:

  • Power and data using one cable
  • Simplified deployment
  • Connectivity at nearly any location
  • Eliminates the expense of additional circuitry

Powered Devices (PD) Classification

PDs can be categorized into “classes” depending on a unit’s power requirements. Classification ensures that powered devices receive the appropriate level of power. This prevents a PD from drawing more power than it needs, allowing for the allocation of unused power to other PDs. Devices with class assignments that require low power draw will not generate as much heat and will utilize smaller cooling systems.

Devices that do not have a specific class assignment will default to Class 0 (.44-12.95 watts). A PSE will classify a PD by transmitting power to the connected PD using the positive pairs, and measuring the amount power loss on the negative pair.

IEEE 802.3af

IEEE’s 802.3af standard, finalized in 2003, specifies up to 15.4 W of DC Power (minimum 44V DC and 350mA) to each device. Powered devices receive only 12.95 W of power due to power dissipation.

IEEE 802.3at

IEEE’s 802.3af standard also known as PoE+ or PoE plus, finalized in 2009, specifies 30 watts of DC power. Powered devices receive only 25.5 Watts due to power dissipation.

IEEE 802.3BT

IEEE’s 802.3BT is a pre-standard and is also known as Ultra PoE. The high power PoE standard will simultaneously transmit data and power using all data pairs. It is expected that the standard will be able to deliver between 60-100 watts of power. The final standard is expected to receive ratification in 2017.

As manufacturers continue to exploit the utility of PoE, another non-standardized option is available for network administrators seeking to install PoE with greater output. UltraPoE or PoE++, among other unofficial terms, delivers up to 60 watts of power using the same 802.3at standard. Ultra PoE is delivered by using the simultaneous transmission of Mode A and Mode B. Ultra PoE is ideal for IP surveillance cameras that require more throughput or a variety of other equipment such as computer workstations, LCD displays, and biomedical equipment.

Basics

Ethernet is the lifeline of local area networks (LAN) but many network administrators choose to take their networks one step further and incorporate Power over Ethernet (PoE) to eliminate the hassle of connecting equipment to a power supply. PoE helps manage cable clutter and is an appealing feature to add to networks needing an upgrade.

The two most relevant PoE standards denoting power output are 802.3af and 802.3at. Whereas the legacy 802.3af standard provides 15.4 watts of power, the newer 802.3at standard, also known as PoE+, provides up to 30 Watts of power.

Determining PoE Budget

Power Sourcing equipment (PSE) such as switches or hubs, come with a predetermined Total PoE Budget. Powered Devices, such as VoIPs and IP cameras, come with a variety of different wattage demands and the sum total must be less than the PSE’s Total PoE Budget.

For example, this 8-Port 802.3at PoE Desktop Gigabit Switch has a Total PoE Power Budget of 130 Watts and is 802.3at compliant. If used at full capacity, the maximum yield per port would be 130 watts/ 8 Ports or 16.25 watts per port, rounded down to the nearest standard (in this case the 802.3af standard). Because 802.3af PDs only draw 15.4 Watts of power, the remaining power will remain unused and will not overpower PDs.

Even though the device has four ports, network administrators must not make the mistake of assuming that the switch can be filled at full capacity with 802.3at devices. To calculate how many 802.3at devices the unit supports, simply divide the Total PoE Budget (130 Watts) by 30 Watts. Calculating the max amount of 802.3af devices is just as simple. Simply divide the Total PoE Budget (130 Watts) by 15.4 Watts.

Compatible vs Compliant PoE Devices

Compliant PoE devices and compatible PoE devices do not hold up to the same 802.3af/at standard. Misunderstanding the difference between compliant and compatible devices can lead to interoperable connections.

Powered Devices labeled as “compliant” fulfill IEEE’s strict requirements of supporting both Mode A and Mode B power modes. 802.3af/at compatible Powered Devices on the other hand, usually only have the ability to provide power using Mode B.

Power pin outs for compatible powered devices are vendor-specific. While 802.3af/at compatible powered devices can operate with compliant power sourcing equipment, there is no guarantee. If you find yourself in the market for an 802.3af/at compatible devices, ensure to check the product’s data sheet or contact your vendor to ensure that your prospective device will be compatible with your existing Power Sourcing Equipment.

Please make a note that compliant Power Sourcing Equipment can either support Mode A or Mode B, but IEEE doesn’t require compliant devices to support both power modes. Power Sourcing Equipment can essentially support one mode and still be considered compliant.

Mode A vs. Mode B

Alternative A, also known as Mode A, will use the data pairs of an Ethernet link to deliver power. Data Pairs include pins 1,2 and 3,6. PSEs using Mode A will supply a positive voltage to pins 1 and 2. Alternative B, also known as Mode B, will use the spare pairs to deliver power. Spare Pairs include pins 4,5 and 7,8. Please refer to our infographic to see a visual representation of Mode A and Mode B.

Endspan PSEs vs. Midspan PSEs

An endspan PSE will directly connect and supply PoE power to a PD such as a PoE switch. Midpoint PSE’s will oftentimes serve as the intermediary devices between a non-PoE capable PSE with a PoE-capable powered device. Examples of Midspan PoE’s include power injectors or power hubs.

Endspan PSE’s provide power on the data pairs, also known as Mode A. Midspan PSEs provide power using the spare pairs, also known as Mode B.

Power Sourcing Equipment vs. Powered Devices

Powered Devices (PD) refer to the group of networking units such as IP cameras, VoIP phones, or WAPs that rely on Power Sourcing Equipment to operate. Power Sourcing Equipment (PSE) such as switches, hubs, and injectors, provides power to Powered Devices.

PoE Compatibility

Source: https://www.versatek.com/blog/compatible-vs-compliant-poe-devices-mode-a-vs-mode-b/
https://www.versatek.com/blog/become-a-power-over-ethernet-poe-expert-in-just-5-minutes/