PoE Challenges and How to Troubleshoot Them (by Chris Greer)

Now more than ever, IT Organizations are making use of Power over Ethernet (PoE) to power end devices such as VoIP Phones, Wireless access points, and even NetBooks. These are often critical business systems that drive important communication services. As the demand for PoE goes up, so does the demand for the amount of power provided. Video phones, security cameras, alarm systems, and 802.11n AP’s have driven the need for high power standards, delivering up to 25.5W to these end devices.

Since PoE is driving such critical systems, it is very important that during the installation and deployment stage it is load tested and verified before powered devices are connected. In most environments, testing PoE during deployment is limited to connecting an AP to the switch and seeing if it powers up. Often this is done only for a port or two, and does not take the cable infrastructure, powered device type, nor power standard into consideration. For example, I can walk around with an 802.3af AP and see if a PoE switch will light it up on different connections, but there are several questions that this test does not answer. Is the switch providing standards-based power? How much power is really provided? Is this whole circuit going to support PoE Plus, the next generation of power? Will it support 25.5W?

We see that turning on an LED on an AP doesn’t necessarily properly test PoE. Nor does it check the common challenges experienced with PoE, including:

1.    Incompatible Technologies – Vendor Proprietary vs. Standards Compliant.
2.    PoE Power Budget Limitations
3.    Cabling is too long, or incorrect for PoE Type. 802.3at requires at least 5e
4.    Bad cabling – Opens, shorts, length issues.
5.    Misconfiguration on the PSE – PoE is not always on by default. Not exactly Plug and Play.

The interesting part about all of these installation headaches is that they can all be detected and resolved during roll out. All we need is the proper tool.

Fluke Networks has a great feature built into their new OneTouchTM AT and LinkRunnerTM AT called TruePowerTM­. This test puts a selected, standards based load on the switch port, requesting and measuring supplied power. If the switch does not provide the requested power, the test will fail. If the test does fail, both of these tools are armed with the Ethernet and cable testing features necessary to troubleshoot whether the root cause is a cable problem, port problem, or incompatibility in the PoE Type.

With a technology as important as PoE, which powers business critical network services, don’t leave testing to chance! Validate and verify PoE before powered devices are connected. Resolve issues such as technology incompatibility, cabling problems, and poor power budgeting before they affect the business.

Author Profile - Chris Greer is a Network Analyst for Packet Pioneer. Chris has many years of experience in analyzing and troubleshooting networks. He regularly assists companies in tracking down the source of network and application performance problems using a variety of protocol analysis and monitoring tools including Wireshark. When he isn’t hunting down problems at the packet level, he can be found teaching various analysis workshops at Interop and other industry trade shows. Chris also delivers training and develops technical content for several analysis vendors. He can be contacted at chris (at) packetpioneer (dot) com.