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I have a set of 7912G IP Phones, which only accept Cisco pre-standard PoE. To save myself buying a Cisco switch, is it acceptable practice to change the wiring of my ethernet cables slightly and use a standard PoE injector from any manufacturer as detailed in some online tutorials?

CableWiring

Sources

  1. Wiring above: http://aikidokajeff.blogspot.co.uk/2017/10/raspberry-pi-freepbx-home-voip-network.html
  2. Standard patch cable wiring: http://www.bb-elec.com/Learning-Center/All-White-Papers/Ethernet/Cat5e-Cable-Wiring-Schemes.aspx
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    Never change the horizontal cable. Any custom cabling must be made outside the wall plate. It would seem that you could be putting your business in some legal liability by messing with power (even low-voltage). The simpler, safer solution is to use an AC adapter on your phones. – Ron Maupin Oct 27 '17 at 15:55
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 19:54
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It's your network, do whatever compromise is required by you, your management, and their budget.

I agree with comment above: don't mess with hidden wire, do it close to the phone or in the patch panel. [EDITED re safety] Be aware any legal consequence of non-professionally made cabling, especially as regards fire hazards. But because the power is allowed to be whichever way round (by 802.3ag), that itself isn't an issue: it's only the pre-standard phones which require it to be a particular way. From memory it was a fight between Cisco and another company about which way the polarity was, and the result was a late amendment to permit either polarity.

So 802.3af specifically says that "Alternative B" power (ie, 4+5/7+8) can go either polarity, and that it is up to the Power Sourcing Equipment (PSE) to decide whether to do A or B and which polarity.

So you had better check (empircally) what power your POE-supplying switch actually delivers, and whether it powers the phone -- 802.3af has pretty complex startup behaviour.

If that doesn't work, you could consider "Nasty Cisco Power Patch Panel", which you'll find available as mid-span injectors, or easy to do with a punch down patch panel and a suitable power supply.

My notebook tells me this about 802.3af

Details from standard, originally 802.3af 2003, now folded into 802.3 2012

  • Alternative A: power is on 1+2 and 3+6 data pairs
  • Alternative B: power is on 4+5 and 7+8 'spare' pairs

802.3 2012

  • section 33.2.3: "A PSE shall implement Alternative A, Alternative B, or both"
  • section 33.3.1: "The PD shall be capable of accepting power on either of two sets of PI conductors."
  • "The PD shall be implemented to be insensitive to the polarity of the power supply and shall be able to operate per the PD Mode A column and the PD Mode B column in Table 33–13. [polarity either way around]"
  • "PDs that implement only Mode A or Mode B are specifically not allowed by this standard."
  • "PDs that simultaneously require power from both Mode A and Mode B are specifically not allowed by this standard."

Let us know how you get on.

Jonathan.

  • My caution was not about the correct wiring of the alternative, but the possibility of an amateur making a mistake in changing the wiring, which can create a dangerous situation. UTP cabling is actually very flammable, even plenum-rated cables. – Ron Maupin Oct 27 '17 at 16:54
  • Completely agree about that. Also, 48V can be more sparky than you might think. Get your electrician to make it and ensure it's safe. Edited answer to include caveat. – jonathanjo Oct 27 '17 at 17:41
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is it acceptable practice to change the wiring of my ethernet cables slightly

Don't mess with standard cabling. In the longer term you (or whoever replaces you) will regret the decision. While your solution may seem simple enough now, but where do you do it and who will undo it when you no longer need it? What complexities may this add to operating or troubleshooting problems in the coming years?

To save myself buying a Cisco switch

Rather than messing with cabling or buying a Cisco switch, why don't you simply pick up some Cisco AIR-PWRINJ3 power injectors? These will provide you Cisco pre-standard PoE and are inexpensive on the used markets. Especially since you are already considering the use of power injectors.

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