on August 31st, 2011

There is a technology that touches all of us – no matter what area of engineering you are active in and that is of course – good old Ethernet. And one of the biggest and dare, I say useful changes has been Power over Ethernet (PoE) and this note is to clarify what it is and to show you how you can take advantage of it. Being engineering, there is inevitably a twist in the tale of using PoE though, as discussed later.

One of the big frustrations today is having to provide a deluge of power adapters to power the myriad of devices around us. Fortunately, Power over Ethernet (PoE) comes to the rescue for much of this.

I must also candidly admit that I had many difficulties with PoE a few years ago with many (blue chip) vendors supplying products based on the supposedly immutable standard but which were then incompatible, causing much handwringing on my part. But I believe we are now firmly into the maturity phase now.

Why do we need Power over Ethernet ?
There has been rapid growth in Ethernet based devices requiring power. Such as VoIP telephones, cameras, Bluetooth Access points, switches, wireless access points and device servers. And an increasing list in such a wide variety of areas such as smart signs and vending, electronic banking machines, audio and video juke boxes  and so forth. There is even a Power over Ethernet Shaver on the market (not for me, as I am bewhiskered – hiding defects in my character as my mum tells me somewhat grimly) ! These devices all need power to operate. And we all know the frustrations of finding plug-in power supplies nearby to power these devices. As well as the additional mess of wiring everywhere.

The good old IEEE rides to the rescue
The IEEE thus developed the IEEE802.3af standard in 2003 to standardise a system of providing power over Ethernet over the same cabling you use to send the data communications signals (e.g. Category 5E or above). Benefits are endless – mobility of your phones and cameras, increased safety and reliability, security and reduced costs.

The Technology behind PoE
There are two different types of devices:

Powered devices (PDs) -  These accept low voltage power from a Power Sourcing Equipment device over structured Ethernet cable. Powered devices operate at 48Vdc and are classified as Safety Extra Low Voltage devices.

Power Sourcing Equipment (PSE) - These provide the dc power to the powered devices (PDs). They provide up to 12 Watts @48Vdc for each PD. A PSE may be either an endspan device (typically a network switch) or a midspan device providing power to the line. The maximum current supplied by the PSE is 350mA.

Two Alternatives with a dose of common sense
Power is provided over the cabling through two alternatives. In the first alternative power is provided on the same conductors as the data. The second alternative is where the power is carried over a spare pair of wires in the cable. Powered devices can accept power in either format. Only one format can be used. PDs automatically adjust for polarity of the power supply voltage (yayyy….for some common sense here). This is particularly vital for the unpredictability in polarity (e.g. in the cases where a cross over cable is used).

Protect your assets
A fairly obvious requirement is to prevent damage to existing Ethernet equipment. Hence, the PSE runs a discovery process by applying a small current-limited voltage to the cable to check for the presence of a 25k resistor in the remote device. If the resistor is detected; then the full (but current limited) 48V is applied.

Let’s boost the power
Although the most a device can pull through the cable is 12W to 15W; the IEEE is working on a standard to boost this to 50W. One challenge for PoE is in the wiring closets and data center operations where power issues are already stretching things with heating; and thus cooling required of switches / backup power supplies. The actual load a switch can handle can also be challenging.

Now for the problems
Some of the concerns are conditioning of power. How do you protect such an interconnected power network from a lightning strike or a surge ?  Before; everything was run over isolated fiber optic cables. Hence there will be more stress on engineering professionals who understand the power architecture and the risks and mitigation strategies. There are risks with using VoIP telephones (as we who have purchased all know). The costs have fallen but if power fails; that is the end of your telephone system. So more effort has to go into the design of the back end power supplies to have redundant power supplies to avoid the system failing.

Now for the Inevitable Twist to the Tale
One final note. Will PoE be surpassed by the newer technologies ? Speeds of Ethernet are rapidly increasing meaning that Fiber is the only real long term transport mechanism. Plus the mobility and increasing speed of Wireless is making this a very attractive option. What will this do to the use of PoE ?

A Toolkit of suggestions
A few suggestions when implementing PoE.

  • Be aware of the technology and look at any equipment you are using for compatibility
  • The tricky bit isn't really the IT guys but having a competent engineering designer (to consider the power and cabling issues)
  • Check out your proposed competent designer’s clients and ensure they were happy with her work.
  • Define the performance criteria very carefully. Ensure you budget for sufficient power from your switches; now that they may be supplying this power to the various devices.
  • Start small and gradually build your PoE system up with your particular applications

Thanks to www.poweroverethernet.com and B & B Electronics for some interesting reading.

And despite the daily knocks in your work remember Herm Albright’s advice to stay positive:

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.

Yours in engineering learning


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