Gary Audin
Gary Audin is the President of Delphi, Inc. He has more than 40 years of computer, communications and security...
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Gary Audin | August 21, 2013 |


Overworked PoE

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

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

Power over Ethernet (PoE) has evolved into an electrical power device utility platform. We started with IP phones, with PoE supporting a central power source, usually with a UPS (Uninterruptible Power Supply). We have added 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 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 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
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. See the table below.

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. See my blog, "The IP Phone Power Bill Can Be High".

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 1 Gbps connections back to the switches and routers.

At issue is the PoE required. It is likely that each AP could require 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.

The wireless network staff will need to perform a new site survey before 802.11ac is implemented. See my blog "802.11ac: Another Wireless Standard to Adopt".

Managing PoE
I spoke to Tim Titus, CTO and founder of PathSolutions about what he considers a good approach to monitoring and managing POE. He said, "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!"

The above comments are at the switch level. Tim pointed out that on 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."

Too Many PoE Ports
PoE ports cost more than non-PoE Ethernet ports. In medium to large deployments, network engineers typically buy PoE ports, only to find that half of their PoE ports were taken by non-PoE devices such as PCs. The engineers could have avoided buying expensive PoE ports, or purchased less expensive "ordinary" Ethernet ports. They could keep "ordinary" Ethernet devices from using the expensive PoE enabled 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.

The warning here is to not assume that PoE is always a plug-and-play environment. PoE should be handled like a utility--monitored, managed, and efficient. Another blog of interest is of power management is "Business Processes to Increase Power Availability".


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