• Hi, I have a question related to Power injectors.

    All we know that las longest line from the poe switch and the powered device must be no more than 100m. What if you place a power injector, let´s say, 50 m away from the switch. Would it extend the cable to 150 m?

    Thank in advanced!!


  • By Howard - edited: January 24, 2019

    A PoE injector will not, by itself, extend the maximum range.

    But there are extenders that can be used.  For example, some fiber-optic models can extend the range for kilometers, but you still need power at the other end. 

    The maximum range of Ethernet is limited by several factors, only one of which is power and/or voltage. There are timing issues, and the signal's quality is as important  as its power level.  Cable shielding, copper quality, twist rates, and poor line terminations can all affect the attainable range.  

    Remember, it's termed the Maximum range, not the guaranteed range.

    The cable category (CAT-3, 5, 5e, 6, etc.) can affect the range, but know that counterfeit cable is a large problem in the industry.   

     We could go on for days about cables and range.....

    I once worked at a location with hundreds of PC's.  These machines were all updated from a central server and one update often included hundreds of files.  One day, a machine started failing when a particular data file was sent to it.  The other hundred files transferred just fine, and there was nothing special about the file type or length.  Even PC's located just a few few away form the problem PC worked fine.  Dozens of updates were attempted, all ending in failure. 

    The problem was finally resolved when someone lifted a ceiling tile and saw that its Ethernet cable had been shifted years before,  from its original location and now ran over the top of a light fixture.   After the cable was restored to its original location, the problem disappeared. 

    It was the combination of that cable, EMI, and the specific bit bit pattern in THAT file that caused the failure.

  • I agree with Howard.

    The 100 meter limit is for Ethernet, not PoE. The main reasons are signal degration and timing problems. Some Ethernet categories only run for 40 meters.

    Very seldom does PoE provide sufficient power over the full 100 meters. The wires are thin and the voltage is relatively low. I don't trust PoE for 50 meters. Note also that that PoE is only defined for Cat5e cabling. In Cat6 and above the wires are even thinner to provide for more distance between the wires for less crosstalk. And that holds for real copper wires, CCA is a disaster.

  • I should mention that sometimes, communication gear can reach beyond its theoretical maximum, but it probably is due to the quality of every electrical component AND phase of the moon.

    Some  IBM-mainframe-compatible channel equipment (FEP's) that I worked on, had a theoretical range of only 200 feet.   Our test setups worked reliably at over 1,000 feet.

    Of course, we only guaranteed to the IBM limit. 

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