IP Phone may have power, but it doesn’t mean it’s connected to the network.

Last week I had one of the phone techs come to my desk and ask me to check the configuration on a switch port. He was setting up an IP Phone for a user at a new desk and the phone powered on but would not register with Call Manager. I checked the switch port, the configuration was correct, but the port was down. To double check the port, I went to the closet with the tech to make sure we were looking at the correct port. After confirming we were looking at the correct port, we headed to the desk.

When I arrived at the desk, the IP Phone had lights illuminated with an error of “Network Not Available”. I suggested that the tech replace the CAT5 jumper at the desk to see if that would resolve the issue, it did. With a new patch cable, the IP Phone powered up and registered with Call Manager.

So why did a new CAT5 patch cable fix the problem?

CAT5 has 4 pair of wires inside of it. 10/100 Ethernet uses pins 1,2,3,6 Ethernet (Data). Power Over Ethernet (PoE) uses pins 4,5,7,8 to power the end device.

In this case, the IP Phone used 100BASE-T, so one or more of pins 1,2,3 or 6 were damaged while 4,5,7 and 8 worked fine.

The troubleshooting steps that the tech went through were correct.
– Phone powered up, but wouldn’t register.
– Switch port was enabled, but didn’t look configured
– Switch port was configured correctly.
– Had to be a cable issue, or a phone issue

By understanding how the basics of PoE worked, I was able to easily identify the best action to troubleshoot the issue. After confirming the configuration was correct, I was able to easily troubleshoot the issue.

Understanding the basic foundations of the technology that you use can really reduce the time it takes to troubleshoot. There are many resources on how PoE works, here is the resource I used to identify what pins are used in the CAT5 cable for PoE. There is a nice chart towards the bottom of the page.

Have you run into this PoE issue on a PoE device?
Do you have any good stories to share about a co-worker not understanding the basics of a technology they are using?

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One thought on “IP Phone may have power, but it doesn’t mean it’s connected to the network.

  1. This is not entirely accurate. 10/100 does, in fact, use 1,2,3 and 6, but according to 802.3af, the switch MAY put power on either the data pairs, or the unused pairs, or just simply prefer the data pairs for power as well. Some switches are smart enough to try one set and then the other. One of the side effects of this is that in older networks or where running new cables isn’t available, you can use 1,2,3, and 6 for one PoE device, and 4,5,7, and 8 for another PoE device on the same wire in the wall.

    Also, a different, but related problem arises when using non-gig aware PoE injectors on gig-capable endpoints. Enough information is exchanged on 1,2,3 and 6 to get the link to come up in gig, but gig-e uses all 4 pairs for data. In this case, you’ll not only get power, but even link, and it still won’t work unless you force one end into 100M.

    Source: Senior engineer for ITSP.

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