The Three Step Guide to successfully Implementing Power over Ethernet
For a successful implementation of Power over Ethernet (PoE) there are three key steps to follow:
- Equipment selection
- Cable certification
- Installation and troubleshooting
However, before we look at these three steps in more detail, let’s take a quick look back at what PoE is, how it works and standardisation.
About twenty years ago, the idea came about combining power and data communications through a single cable, so eliminating (in most cases) the need for additional AC outlets to be installed near the devices and therefore reducing the cost associated with installing additional electrical requirements (cable / conduit / mains sockets / labour).
So Power over Ethernet (PoE) was born and involves electrical power being passed along with data through a twisted pair cable and over the years a huge range of devices that source and consume power and data through the same cable have become available from IP camera, wireless access points, access control, lighting controllers, industrial control and remote point of sale equipment.
There are three parts that make up a PoE circuit:
- Power Sourcing Equipment (PSE) – this puts power over the cable and is typically a switch or a mid-span injector where the switch is not capable of supplying power.
- Cable – Carries both data and power (IEEE standards for PoE specify 2 or 4 pair twisted cabling system).
- Powered Device (PD) – which is the device being powered over the cable
This can be illustrated in the diagram:
One important aspect of PoE is that the power is only applied to the line by the power source (PSE) once it has been requested by the powered device (PD), which makes the PoE considerably safer than having a typical AC powered device constantly powered.
The first standard was adopted in 2003 and provided a power source of up to 15.4W and over the years amendments have increased this to 90W as illustrated below:So the advantages of PoE can be seen as:
- Reduced costs of additional cabling
- Safer voltages (typically 43 to 57Vdc) so not so strict safety standards
- Devices don’t need an additional power source, so one less failure point
- Power is only applied to the line when the device requests it (IEEE Standard)
Now we can look at the three steps to implementation:
- Equipment Selection – Issues with Standardization
Power over Ethernet provides great opportunities, however, some caution is needed when selecting your equipment. PoE is not itself registered and although there are published IEEE standards, this itself leads to different configurations (two or four pair cable) and vendors have created additional terms such as PoE+ and PoE++ as well as Cisco’s universal PoE (UPOE) which all fit within the IEEE standards. However, there are vendors creating other implementations outside of the standard, such as “power always on” and different power levels, all adding to the confusion.
To try and overcome this and enhance interoperability, the Ethernet Alliance, a consortium of manufacturers representing providers of ninety percent of PSE switching equipment, has announced a PoE Certification Program. This provides a methodology for certifying their products for interoperability with other IEEE-802.3-based PoE solutions. Certification can be carried out by manufactures or third parties, where approved equipment (PSE or PD) can be labelled with EA approval marks and this hopefully simplifies the compatibility selection for the end user.
2. Cable Certification – not all cables are the same!
Power over Ethernet was designed to run over standard category twisted pair cables, however having both power and data signals together means some additional requirements on cables (and therefore the choice of cable to use) are needed:
- Cable resistance needs to be low, if it’s too high then power will be dissipated between the power source (PSE) and device (PD) and may then be insufficient power to operate the device correctly.
- PoE is transmitted by applying a common-mode voltage on either two or four pairs and therefore the current needs to be split evenly across the conductors, so the DC resistance of each conductor must be equal (balanced) as any differences will mean an unbalance which can distort the data signals, which causes bit errors, retransmits and possible failure of the link.
- With the different implementations, it’s not just the DC resistance on each pair, but the unbalance across multiple pairs needs to also be taken into account and this could also impact the data transmission.
Due to these points, the IEEE has included requirements for measurements and testing of both the loop resistance and resistance unbalance within the 802.3 standard.
As well as the IEEE, the Telecommunications Industry Association (TIA - is accredited by the American Nationals Standards Institute to develop voluntary industry standards) also recognises the importance of resistance measurement and has included them within the standard ANSI/TIA 568.2-D (with field testing document TIA-1152-A), however these are optional and not mandatory tests.
A suitable tester to perform these critical resistance measurements is the Fluke Networks DSX CableAnalyzer™ Series (146-2853), so ensuring that the cable plant you deploy will perform in two- and four-pair PoE applications.
3. Installation & Troubleshooting
In an ideal world knowing the capacity of the Power Sourcing Equipment and the Powered Device would make installation and troubleshooting simple, however generally technicians may not have that information (especially if adding to the network) and may have a few other issues, such as:
- Working at a distance from the PSE
- No access to the PSE
- Which cable goes to the PD
- Time wasted locating devices and cabling
The solution to these issues can be found with the MicroScanner PoE from Fluke Networks (185-3021)Simply plug into the cable and if connected to a PSE, it will then show the class (0-8) of power available on the link as shown in the video:
Other key features for the MicroScanner PoE include:
- Identify the speed of the port up to 10 Gbps (a slow port could limit the device's performance)
- Identify if the cable has been damaged and the length to failure
- Unplugged or misrouted cables (can act as a tone source)
So as this article has described, the three keep steps are, pick the right equipment, certify the cable is capable for the job and ensuring the technician can install and troubleshoot easily.
- Fluke Networks MicroScanner PoE (185-3021)
- Fluke Networks MicroScanner PoE Kit (185-3022)
- Fluke Networks MicroScanner PoE Wiremap Adaptor (185-3023)
- Fluke Networks MicroScanner Remote Identifier for PoE (185-3024)
- Fluke Networks DSX 600 Cable Analyzer (146-2853)