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So I have a 3650 poe switch from cisco and 2 7960 ip phones that use poe, I have one phone on a short patch lead and this phone powers on fine

Now with the second phone, it has a 50m patch cable and for some odd reason only powers up when on port 10 of the switch, I have tried all the other phone ports that are configured and only port 10 works.

The same config has been applied to all other ports but the phone persists to only power on on port of

What is the problem here?

Thanks in advance

  • Does the second phone work on all ports with a short patch cord? – Ron Trunk Nov 7 '17 at 17:44
  • Yes the other phone works ok on all ports – NickCarlt Nov 7 '17 at 17:46
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    You know that by the standard, you are only allowed a total of 10 meters of stranded patch cable, divided between both ends. The total channel is limited to 100 meters, with up to 90 meters of solid-core horizontal cable (performs better), and up to 10 meters of stranded patch cable (performs poorly). – Ron Maupin Nov 7 '17 at 17:46
  • Oh so I need 50m of solid core cable? – NickCarlt Nov 7 '17 at 17:48
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    Available:370.0(w) Used:12.6(w) Remaining:357.4(w) Interface Admin Oper Power Device Class Max (Watts) --------- ------ ---------- ------- ------------------- ----- ---- Fa0/1 auto off 0.0 n/a n/a 15.4 Fa0/2 auto off 0.0 n/a n/a 15.4 Fa0/3 auto off 0.0 n/a n/a 15.4 Fa0/4 auto off 0.0 n/a n/a 15.4 Fa0/5 auto off 0.0 n/a n/a 15.4 Fa0/6 auto off 0.0 n/a n/a 15.4 Fa0/7 auto off 0.0 n/a n/a 15.4 Fa0/8 auto off 0.0 n/a n/a 15.4 Fa0/9 auto off 0.0 n/a n/a 15.4 Fa0/10 auto on 6.3 IP Phone 7960 n/a 15.4 – NickCarlt Nov 7 '17 at 17:57
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From the fact that Phone 2 works with long cable into port 10 but not 9, and that the switch doesn't enable power when plugged into port 9 ...

It does indeed look like the cable.

POE sensing is a fiddly business which to simplify a little is as follows: the switch puts a voltage down the cable and senses the resistance. If it measures as a characteristic resistance then it enables power (ie on port 10), otherwise it doesn't (ie port 9).

If the cable is too long or too thin it will have too high a resistance. Fatter or shorter cables address this. (There are many other ways for a cable to be problematic: this is just the POE story.)

The best guess I can have for port 10 working and not others: just a little variation in the internal resistance or resistance-measuring circuit on that port. There's a threshold, it looks like you found it!

  • So will using port 10 be reliable or am I better getting a power supply for the phone? – NickCarlt Nov 7 '17 at 19:09
  • I would get a better cable: solid if possible, or 22 AWG stranded. Then I expect all the ports will work, with both phones. I'd leave the power cube as last resort. – jonathanjo Nov 7 '17 at 19:26
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    Or a marginal connection. I've seen crimp connectors make poor contact many times. – Ricky Beam Nov 7 '17 at 20:20
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Patch cable has a smaller effective diameter (usually AWG 26-28) and a larger serial resistance than horizontal (rigid) cable (AWG 22-24). This causes a higher voltage drop from end to end. With 30+ m it is likely to exceed the tolerance of PoE. 50 m patch cable will also very likely exceed the specifications for 100BASE-TX and 1000BASE-T (maximum attenuation) - the standard reach of 100 m assumes 90 m of rigid cable and 10 m of patch cable.

You either need to find a patch cable with a greater effective diameter (should be AWG 23 or better) or cover a large part of the distance with horizontal cable.

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