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.
What is PoE?
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 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’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’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’s 802.3BT is a pre-standard and is also known as ultra PoE. Click here to learn how PoE will 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.