Thomas
Lövskog

PoE - Power over Ethernet

CEO Additude Innovation

If Technology doesn’t fix the problem … we need more technology!


Nearly all modern appliances need both power and communication. Power can typically be ...

a) Batteries

b) Locally generated through solar, thermal, antimatter, etc; or

c) Remote through a wall-outlet.

Communication is typically in the same way either ...

a) Wired; or

b) Wireless.

In my humble opinion portable equipment uses battery and wireless communication. Non-portable equipment uses wired power and communications. A laptop could use either power and sometimes a wired connection. A desktop PC connected to a wall-outlet using wireless communications, on the other side, is plain creepy and strange.

For non-portable equipment it would then be great if we only needed one cable for both power and communications. This is where PoE, or Power over Ethernet, comes in.

It is absolutely nothing new to combine power and communication. The building automation system X-10 from the early 70's overlaid the 60Hz 110V AC (American system initially) with a 120kHz carrier, sending databits at a whooping speed of 30 bits per second.
Later in 1992 EIB (European Installation Bus) was developed. It used a twisted pair for data and power. This time DC at 28V and 9600 bps in the communication channel. For lightning control it is sufficient, even on a crowded bus.
With Ethernet we start at 10/100Mbps with the capability of sending 12'ish watt to each device. Huge difference.

Passive or "Homebrew"

Passive or "homebrew" is actually any methodology to transmit power over Ethernet cables, that is not according to the standard. The historical reason for this is that the 10/100 Mbps Ethernet only uses 2 pairs, pairs 1-2 and 6-3. One pair per direction in simplex configuration.

That of course was something that was bound to be used. So a few schemes exists. Either use one pair for both positive and negative potential. Or use one of the pairs both conductors for the positive and the other pair for negative. Any variant and any voltage. 

A drawback is that there is no negotiation between the source and destination. With Passive PoE, the proprietary nature of the power specifics means that it's often wise to use only power injectors or switches specifically designed for the devices that require Passive PoE. The power is "always on", so it's possible to burn out devices if they're not prepared for electrified Ethernet wires, or if the CAT5 cabling is wired incorrectly.

The real deal

The two standards that right now exists are IEEE 802.3af also named Active PoE and IEEE 802.3at or PoE+. The difference is mainly the difference in power supply possible. The former is 15.4W and the later 30W. A few Wats are assumed being lost in the cabling.

There is a lot of types, cases and modes in PoE. The table below tries to summarize the types.

Type Alternate naming Related standard Maximum Power
1 PoE, Active PoE, 2-pair PoE  IEEE 802.3af  15.4W (12.95W at device); Min 44V and 350mA
2 PoE+, PoE Plus  IEEE 802.3at  30W (25.5W at device); Min 50V and 600mA
3  4-pair PoE, 4P PoE, PoE++, UPOE  IEEE 802.3bt  60W (51W at device); Min 50V and 600mA; 4 Pairs
4 higher-power PoE IEEE 802.3bt 100W (71W at device); Min 52V and 960mA; 4 Pairs

The equipment that source power is called PSE or Power Source Equipment. This can be a simple PoE injector which takes a ethernet signal and a DC power and combine these. Typically mode B is used, since that is the absolutely easiest to implement. The other PSE type is a Switch with built in PoE on all or some ports.

The equipment that consumes the power is called PD. Powered Device.

There are two modes on how power is delivered.

Mode A: Power is superimposed as a DC offset on the pairs together with the data. This is available on 10/100 as well as 1000Mbps.

Mode B: Power is transferred on pair 4-5 and 7-8. The data is transferred on 1-2 and 3-6. This is only available on 10/100Mpbs Ethernet.

 

Two additional standards are being worked on. The upcoming IEEE 802.3bt standard, expected to be finished September, 2018 will introduce two additional power types: up to 55 W (Type 3) and up to 90-100 W (Type 4).

There is also a application specific standard called IEEE 802.3bu. This is a PoE variant for the Ethernet standard IEEE 802.3bw from 2015. This in turn is 100Mbps Ethernet with full duplex over a single pair. Mostly intended for industrial and automotive use. In other PoE standards power is transferred between pairs. Here it is transferred in one pair.

It used to be complicated ... not so much anymore

The first PoE PD I worked on was very early and it was a mess. Today you can get a much better device from as an example SiLabs or Maxim. Nearly plug and play. So nowadays there is absolutely now reason to not use this fabulous technology.

Below is the development kit from Silicon Labs for their non-isolated frontend. For a isolated you need a tiny tine more components. Just your ordinary transformer/optocoupler configuration. So, there is no more excuses. Let there be PoE!

 

It is a little like USB used to be. Hard, error prone, difficult to layout, annoying software ... you really wanted to use simple UARTs again. Nowadays it is more like adding a little sauce to the design and pick a few options in the software configuration. Just as it should be.