Regulators and other stakeholders are calling for data centres to use energy in the most efficient way. Power over Ethernet (PoE), which makes it possible to provide current to Ethernet devices using data cables already in place, can contribute to greater energy efficiency. It may even help structurally lower DC power consumption. However, several factors must be taken into consideration. Dr Thomas Wellinger, market manager data centres, R&M, explains
As the role of the data centre changes from data repository to vital economic and social enabler, the number and size of facilities is likely to keep growing exponentially. Every option for energy saving, however small, can help. First, let’s look at some of the main sources of energy consumption.
The energy efficiency of a Data Centre is typically measured in power usage effectiveness (PUE). If all the energy going into the Data Centre were to be used exclusively for IT, the PUE would be 1. A PUE of 2 indicates that for every watt of power used for computing, another is consumed for non-IT purposes such as cooling.
In 2014, the Uptime Institute published an average figure of 1.7 for respondents’ largest facilities. In other words: for every watt used for IT, three-quarters of a watt are used for power distribution, cooling and related processes such as lighting, heating and security systems. Power losses caused by idle circuitry are another major concern, especially as millions of new switches are added to the already substantial installed base each year.
Power loss through resistance
One key challenge related to PoE is power loss as a result of resistance. Four pair Power over Ethernet (4PPoE) has clear benefits when it comes to diminishing cable resistance, and thereby preventing power losses. Energising four available cable pairs, instead of the two that are normally utilised, can increase power efficiency. This not only reduces the environmental impact of PoE, but can also deliver significant savings to large data centres. The original PoE standard, introduced just over a decade ago, supported up to 12.95 watts. PoE+, introduced in in 2009 supported up to 25.5 watts. 4P PoE should enable a supply of up to 100 watts, which is four to five times greater than the current rate, allowing more powerful terminal equipment to be supplied with energy over network cables.
Resistance can also be lowered by opting for higher-category cables. Cat. 5e cables with a conductor cross section of AWG 24 (0.22 mm2) quickly reach their ‘natural limits.’ They grow hot quickly, especially when the heat is not being efficiently removed from large cable bundles. Cat. 6 or Cat 7A cables with cross sections of AWG 23 (0.26 mm2) or AWG 22 (0.33 mm2) offer significantly reduced resistance, and therefore less energy loss and heating.
Another way in which PoE can help cut energy usage is by replacing ‘wall warts’, the mains adapters which come with devices such as routers, printers, WiFi boosters and so on. These (often OEM) devices cannot be centrally monitored and managed, and are not always designed in the most efficient way possible. By replacing these with PoE connections, monitoring and standardisation become much easier. Unused devices can be shut down, for example, and devices attempting to draw a suspiciously high level of current could be locate and fixed easily. This example shows how PoE enhances efficiency across the power chain, including all required voltage conversions, from power conversion at the power sourcing equipment through to delivery at the powered device.
If used correctly, Power over Ethernet facilitated by current network convergence trends can help reduce power usage. Indirect energy savings are also realised through lower costs of installation, cooling and ongoing maintenance. What’s more, the benefits of being able to monitor data centre hardware with a DCIM system are extended to all peripheral hardware and systems. However, when developing a structured cabling solution that incorporates PoE, you do need to carefully consider the applications for which a cabling solution is being configured and make a realistic prediction of future bandwidth requirements.
Considering link lengths
As cables are heated by current passing through them attenuation increases. The result: reduced transmission range and higher energy consumption. Under realistic framework conditions with applications up to Power over Ethernet Plus (PoEP, 26 watt power), transmission length restrictions rarely need to be taken into account. However, if cabling must also to be suitable for future PoE generations, some rules of thumb may be applied.
Class D cabling for 1G Ethernet: Using Cat 5e cables (AWG 24), the permanent link length must be planned in accordance with the specific installation conditions and shortened if required. However, with Cat 6 cables (AWG 23), it is generally possible to reach the 90m standard link length.
Class EA cabling for 10G Ethernet: Using Cat 6A cables (AWG 23), the length of the permanent link must be planned in accordance with the specific installation conditions and shortened if necessary.