Power Over Ethernet Application Notes

It can cost between $100 and $200 to run an ethernet cable to a wireless access point. But it can costs a further $400 to run an electrical conduit to power that access point.

The alternative is to install Power Sourcing Equipment (PSE) called Endspan Equipment into the ethernet switch, or by adding Midspan Power Insertion Equipment after the ethernet switch. This will combine the data and power at the source to be sent through the ethernet cable. The power from the cable can then be extracted at the endpoint with a Powered Device (PD) to operate the access point.

This solution removes the need to run expensive electrical conduit and will cost considerably less than $400.

Is there a PoE Standard ?

Yes, the Institute of Electrical Electronics Engineers (IEEE) 802.3af Task Force has ratified the technology into the Ethernet standard. The IEEE 802.3af standard covers the interoperability of using PoE products and covers the PSE - Endspan or Midspan, the Powered Device (PD) and the cabling requirements. It is endorsed by all network players that power IP-connected devices, such as voice-over-IP (VoIP) phones, wireless LAN (WLAN) access points (AP) and advanced security devices, over the existing Ethernet cabling infrastructure.

How is the power put onto the ethernet cable ?

The power can be added to the ethernet cable if the switch has a built-in power source in the form of PSE is called Endspan Equipment, as shown below: -

Power Over Ethernet (POE) diagram - Endspan Equipment

Alternatively the power can be added to the cable after the switch by using a PSE called Midspan Equipment, as show below: -

Power Over Ethernet (POE) diagram - Midspan Equipment

Under normal operation the PSE supplies a nominal d.c. voltage of 48V, this is limited to a maximum of 15.4 Watts (at the present time).

How is the power extracted from the cable ?

To extract power from the cable the connected device must be PoE enabled.

How does the PSE know that the connected device must be PoE enabled ?

Before the PSE supplies power, it has to verify that the device connected is a Powered Device (PD). It does this by checking that the PD has a conforming signature. All Silver Telecom PD modules have a built-in signature chip that does that automatically.

Only upon successful completion of a valid signature will the PSE apply power to the cable. This ensures that non Powered Devices are not damaged by the PSE, thus making the system backwards compatible.

What is Power Class ?

This is optional within the IEEE 802.3af specification, that can be used by the PSE to manage the overall power requirements of the system. Below is a table that shows the various power classes detailed within IEEE 802.3af.

Class Usage Maximum Power

0 Default 15.4W

1 Optional 4.0W

2 Optional 7.0W

3 Optional 15.4W

4 Reserved for future use Treat as CLASS 0

All Silver Telecom PD modules have the option of setting the power class, if required.

What is Power over LAN?

Basically the Power over LAN or PoL is another term used for Power over Ethernet or PoE.

Do Silver Telecom's PoE modules comply with EMC regulations?

FCC Part 15 is the EMC spec. This is not directly relevant to our PD modules, but is relevant to the finished system. So, for example, a Wireless Access Point unit produced using our Ag9000 would have to meet FCC Part 15. In order to do this it may be necessary to add a common mode inductor at the input and/or output to filter out noise generated by the Ag9000 and prevent it radiating via the cabling. The Ag9000 module already has filters fitted internally at the input and output. These may not be sufficient for a particular system as there are limits to how much filtering can be done on such a small module. Also the customer's system may generate noise of its own, which can be conducted through our modules and thus require extra filtering to remove.

It would not be helpful for us to get an FCC Part 15 certificate for our modules because this would be specific to the one test configuration that we measured. As soon as it is put into a customer's product then the configuration is changed and the tests would have to be repeated.

FCC Part 68 is the telecoms specification. Since an Ethernet network is not classed as a "Telecommunications Network" by FCC Part 68 then this specification is not relevant.

POE Frequently Asked Questions: PoE-Frequently-Asked-Questions.pdf

Power Over Ethernet (PoE) Application Notes