Category:HowTo:PoE

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Introduction

Power over Ethernet has three main advantages:

  1. There is only one cable needed to connect to the phone, which clears up the desktop.
  2. It is relatively easy to have a separate backup power supply for the telephone system. This is important when emergency procedures require that the telephone system works even when the main power is gone.
  3. By centralized remote power management, single devices can be checked for power. This allows simple remote booting and hardware failure detection.

Power over Ethernet is sometimes also called Power over LAN. We use the term Ethernet here because the underlying technology is Ethernet for snom products.

Detecting Power over Ethernet

Different mechanisms are used to detect whether or not the attached Powered Device (PD) supports Power over Ethernet (PoE). This is important in order to avoid damaging a PD that does not support this feature. Power over Ethernet can either be provided over the spare twisted pairs (pins 7 & 8 and pins 4 & 5) or over the data pairs (pins 1 & 2 and pins 3 & 6) in an ordinary category 5 cable.

The Cisco Power over Ethernet uses a proprietary method. The IEEE committee approved a different standard called 802.3af. In IEEE 802.3af, a PSE uses a resistive signature to detect and measure the current required by the PD. Before IEEE decided to use the resistive signature, the capacitive signature was used (so-called „big capacitor method“). At the moment the following methods are available for PoE:

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