Bach Seat

Gary Audin at No Jitter warns that Power over Ethernet (PoE) is not always a plug-and-play environment and PoE should be monitored, managed, and efficient. In this article, Mr. Audin observes that PoE has evolved into an electrical power device utility platform. POE started out as a centralized power source for IP phones, backed up with an Uninterruptible Power Supply (UPS). (rb- Click here and here for my overview of PoE) Since those early Cisco dominated days. The article says PoE now is called upon to support wireless access points; environmental controls; point, tilt, and zoom cameras; lighting control; clocks; door controls; Bluetooth devices; RFID; now laptops, and still more to come.

The LAN switch is the PoE source, but the article warns it can be overwhelmed with the power drain, which produces headaches for IT. Unless properly managed, the PoE function can experience:

• A blown-out power supply. Smoke is an indicator of this condition.
• Reduced power to all devices with degraded service from all the attached devices.
• An added PoE device does not work.
• The more power is drawn by PoE, the shorter the UPS battery life. The original UPS design could last 20 minutes. Added PoE devices could shorten this to 3 minutes.

PoE IP phones and other devices can signal to the PoE network what class of device it belongs to and how much power it may need. Class 0 devices, usually older devices, do not indicate their PoE power requirements. These devices may draw any power level from none to maximum. The other standard classes, 1-3, range from very low power to mid-level power consumption.

Class 4 is a newer class of device requiring PoE+ (802.1at) and needs to draw more than the 12.95 Watt maximum provided by the original standard PoE. Class 4 devices must be powered by PoE+ ports and may not function correctly on an 802.3af PoE port. Most IP phones are in class 2. IP phones with color screens and other advanced features may be categorized as class 3 devices.

PoE Access Points Wireless LAN access points are also common PoE devices, many of which started out as class 2 and 3 devices. As the wireless speeds increased, so did the power requirements. The 802.11ac standard means that the access points (AP) will have a 1 Gbps connection back to the switches and routers.

At issue is the PoE required. It is likely that each AP could need 20 to 30 watts, the limit that the 802.1at PoE+ standard delivers. Many installed switches cannot support PoE+. So the enterprise has to buy new switches or power supplies or power injectors. (rb- add this to your site-survey when you plan to implement 802.11ac)

Mr. Audin spoke to Tim Titus, CTO, and founder of PathSolutions, (they happen to sell a network management tool) about what he considers a good approach to monitoring and managing POE. He told No Jitter,

“Regardless of whether there are any PoE or PoE+ devices on a network, it can be very helpful to monitor the health of our network equipment’s power supplies. The best monitoring system watches the status and power consumption of each power supply, what percentage of utilization it is running, and which interfaces are drawing power, so power policing can be achieved.”

He provided this example of missing power management.

“Keeping an eye on power supplies avoids unpleasant discoveries. One unlucky network administrator had two power supplies installed in a network chassis (one primary and one backup). Unfortunately, when the primary power supply stopped working, nobody knew, since the backup power supply was doing its job of keeping everything running. The problem wasn’t noticed for over six months. Nobody was in the empty remote wiring closet to notice the lack of lights on the power supply. The users remained blissfully unaware of impending doom until the wee hours of a weekend when the second power supply was shut off by a circuit-breaker trip!” 

Mr. Titus pointed out to Mr. Audin, that monitoring should happen at the port level,

“Not only will a monitoring system show you what mode a PoE port is operating in, but it should also provide a view of relevant error counters.

• MPS Absent and Invalid Signature errors frequently point to broken or defective powered devices.
• Overload conditions and short-circuits typically point to wiring problems (or somebody re-wiring devices in use).
• Denied errors can point to devices asking for more power than the switch has available, and may indicate that it is time to consider adding another power supply to a large Ethernet chassis.”

How did that happen?

Finally, many network engineers try to buy limited PoE due to the cost premium of POE ports, only to find that half of their PoE ports are used by non-PoE devices such as PCs. With a monitoring tool, the engineers could have avoided buying expensive PoE ports or purchased less expensive “ordinary” Ethernet ports.  The engineers should have an up-to-date PoE port inventory and use it to avoid over-buying the PoE by playing safe in their design. (rb- Been there done that, I’ve been in many customer’s closets and found POE switches full of PC and printer access ports.)

rb-

The author warns not to assume that PoE is always a plug-and-play environment. PoE should be handled like a utility–monitored, managed, and efficient.

I have tried to build custom fields by working with reports in SolarWind’s Orion by working with MIBs, it’s not the funnest thing in the world. I wonder if this product does a better job.

Related articles

• Have you ever stopped to think…that PoE is good for the environment? (lukefrostblog.wordpress.com)

Ralph Bach has been in IT long enough to know better and has blogged from his Bach Seat about IT, careers, and anything else that catches his attention since 2005. You can follow him on LinkedIn, Facebook, and Twitter. Email the Bach Seat here.