Tip

Network energy efficiency starts in the LAN closet

When it comes to measuring and managing enterprise IT power use, many focus on data center servers and storage, but energy efficiency can start in the network closet where LAN switches and routers live alongside a host of other power-hungry equipment.

Network closet components and their costs

The first step to network energy efficiency in the LAN is to assess all of the devices attached to the network and then measure both their power use and what it takes to cool them.

Understanding network closet components

The network closet has LAN switches and sometimes routers, as well as other forgotten devices, such as fans,

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UPS with batteries, power supplies, testing equipment, lighting and security access devices.

A 48-port LAN switch alone can cost between $98 and $554 per year to power and cool, while a 192-port router can cost between $1,082 and $2,217.

Like all electrical devices, switches and routers are inefficient and cause heat, which is wasted energy. Network infrastructure represents between 12% and 15% of data center power consumption and the equivalent in HVAC.


Measuring the cost of cooling equipment in the network closet


The cost of cooling equipment can be a major energy expense, and it's important to measure how much cooling will cost per watts used. It's disturbing to note that inadequate cooling directly affects network performance. Every 18 degree increment over 70 degrees reduces reliability by 50%. In data center cooling studies, up to 72% of the cooling capacity bypassed IT equipment and was wasted.        

The following table illustrates a method for determining the cooling cost in watts:

Add up the watts for IT equipment (switches,

routers, test equipment, etc.)                                               _______

LAN end span (internal power supply)

PoE input power x.6                                                              _______

Mid-span (external power supply)

PoE input power x .4                                                             _______

Lighting at full watts                                                            _______

UPS power rating x .09                                                         _______

Add all together for total watts                                            _______

To calculate the total wattage expended in the closet, take the total from the table and multiply by .86. This means that for every 1,000 watts of energy used in the closet, another 860 watts is required to cool the closet contents.

Prior to 2004, the operating temperature for data centers and closets was 72 degrees Fahrenheit. In 2008, it was 81 degrees Fahrenheit according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers. New equipment can operate reliably at higher temperatures. However, the older the equipment is, the lower the required operating temperature will be. What's more, each vendor has its own temperature operating limits.

How to measure power for network energy efficiency


There are passive electrical power measurement devices on the market that can measure power use without disrupting devices. They cost between $200 and $500. Here are recommendations to follow when determining energy consumption:

  • Read the specification sheets to get an approximation of the power consumption.
  • Specification sheets do not always reflect reality. The rate of power usage can be either higher or lower than reality.
  • Consider actual measurement in the closets with portable measuring equipment.
  • Investigate the present energy usage in detail by location, room and closet. Develop a baseline of the present usage and costs for comparison with changes and improvements that will occur in the future.
  • Procure test equipment to meter the power use in the closets and rooms, if they are not separately metered.
  • Determine the distribution of usage across lighting, IT equipment HVAC, UPS losses and other consumers of power.
  • Measure the usage over time to develop energy usage trends for the enterprise.
  • Compare the energy efficiency of any new core or edge switches before purchasing them.

Measuring Power Usage Effectiveness (PUE)

It's also important to understand the Power Usage Effectiveness (PUE) in data centers. The PUE, which can be found in the “Recommendations for Measuring and Reporting Overall Data Center Efficiency,” is a metric for reporting the overall data center infrastructure efficiency. The closer the PUE is to 1.00, the more effective the IT operation.  A number greater than 1.00 means lower effectiveness. A way to define the total power consumption is to measure the power entering the closet at the connection to the main power source in the building.

The PUE is calculated using the following formula:

PUE = Total energy consumption (includes IT and non-IT energy like cooling, lighting and support infrastructure) / IT energy consumption

The efficiency of the data center is the reciprocal of the PUE, where: Efficiency = IT energy consumption / Total data center energy consumption

In this case, an efficiency of 1.00 would be perfect, but not really possible. Therefore the closet efficiency will be less than 1.00, probably about .80 to .90.

Another recommendation is to measure annual power consumption in kilowatt hours (kWh). The enterprise should attempt to calculate an initial PUE even if the measurements are not all inclusive. This will produce a benchmark for future PUE calculations for comparison.

The next tip, “Creating the Energy Efficient Network Closet,” will explore methods for managing energy consumption in the network closet.

This was first published in July 2011

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