13 Feb What is the difference between PoE and Passive PoE?
The general meaning of Power over Ethernet (PoE) is the capability of passing electrical power along with a data signal over the same Ethernet cable. This can be a pretty useful arrangement, since it requires only a single cable to provide both the electrical power and the data to receptors like Access Points (APs) and devices like IP cameras. The power signal can be carried on an entirely different conductor within the Ethernet cable, or it can be carried on the very same conductor as the data signal.
Active PoE is the standard, and it is what is assumed when anyone mentions ‘Power over Ethernet’, or PoE. The big advantage of PoE is that it allows greater flexibility in locating your Access Points, since they don’t have to be situated near a power source – power is carried to them right in the Ethernet cable. That also frequently results in significantly lower installation costs, especially where many Access Points must be setup.
What makes Active PoE work is a Cat-5 injector, which injects a DC voltage into the Cat-5 cable, and is then typically accepted at the terminus by, for instance; a wireless Access Point through its RJ-45 jack. Devices like these which can directly accept the injected power in the line are considered to be ‘PoE compatible’, and those which are not PoE compatible must be converted to compatibility by a ‘picker’ or ‘tap’ (also known as a PoE splitter).
PoE+ was the original name of cabling standard 802.3at, which differs from active PoE, described in standard 802.3af. The main difference between these two standards is in how much power is provided over the cabling. With 802.3af (PoE), a maximum of 15.4 watts of power is provided, while (PoE+) could provide as much as 25.5 watts of power.
Passive PoE is, as you might think, very similar to active PoE, with one notable exception. In active PoE, the switch and the remote equipment actually communicate and negotiate the proper voltage and the pins to use for the transfer of electrical power. In passive PoE, no negotiation takes place between the two devices, but instead the known cable layout is used from Standard 802.3af, mode B.
The IEEE standard 802.3af preceded the 802.3at standard by several years, and allowed for considerably less power to be carried over a cable. But this proved inconvenient for devices which needed more than a few watts in order to run properly as a powered device, so 802.3at was implemented, and this currently calls for up to 50 watts of power to be injected into a Cat-5 cable for powering endpoint devices.
Devices for use with PoE
Industry-leading devices are available from T3 Innovation for use with PoE cabling applications and installations. Both the Net Prowler and Cable Prowler from T3 can detect the presence and specific class of PoE, per IEEE 802.3af/at with load test for voltage drop. The Cable Prowler combines the functions of a high-end cable tester with link status and PoE detection, while Net Prowler includes all this functionality, plus adds the capability to troubleshoot and monitor LANs.