Power over Ethernet (PoE): An energy-efficient Ethernet solution

Overview

Power over Ethernet ( PoE ) technology is becoming an important tool for simplifying network deployment while improving overall energy efficiency. Recent technological advances have brought new opportunities to further reduce power consumption and related energy costs.

PoE enables data and power to be transmitted over a single Ethernet cable. On existing LAN facilities, this technology provides up to 60W of safe, uninterrupted power to power powered devices (PDs) with a voltage range of 50V to 57V, supporting the latest high-power IEEE802.3at-2009 standard (see Figure 1). The IEEE802.3at-2009 standard is backward compatible and replaces the previous IEEE802.3af-2003 specification.

Figure 1. Network power supply facilities with PoE function

PoE enables network administrators to deploy VoIP phones, WLAN access points (APs), IP security cameras, access control systems and other power-consuming devices anywhere throughout the facility without installing AC outlets or sending electricians to run wiring, changing building plans or obtaining safety approvals. PoE technology also eliminates the need for separate wires and power outlets, providing up to 50% cost savings compared to traditional network power supply facilities.

Choosing the right power supply equipment: Midspan advantages

PoE can be implemented using either type of power sourcing equipment (PSE): through a PoE switch or by installing a PoE midspan between an existing switch and the network power device. Midspans provide the easiest way to quickly deploy high-power PoE (see Figure 2).

Figure 2: Deploying PoE using midspan technology

Unlike PoE switches, midspans allow PoE ports to be added incrementally over time, rather than installed all at once during initial installation. They also typically offer improved mean time between failure (MTBF) rates compared to PoE switches, because PoE switches combine the high-power dissipation PoE portion and the highly sensitive data portion in a single box.

Midspans are also a more cost-effective, scalable solution than PoE switches, allowing network administrators to more easily upgrade their power facilities without being limited by their data facilities. Network administrators often cannot justify the cost of upgrading to new PoE switches when existing switches are only a few years old, and/or they may only need a few ports with PoE capabilities. In addition, midspans enable administrators to upgrade their switches as needed without having to change the power facilities at the same time. Finally, midspans enable network administrators to remotely manage PDs while centralizing all power management and backup systems, further improving energy efficiency and in-use facility reliability, reducing operating costs, and enhancing safety. Because midspans can detect and automatically disconnect non-PoE-compliant PD devices when events such as overload, short circuit, or underload conditions occur.

Additional energy saving opportunities

There are three main ways to use midspans to get the most power out of a network. The first is to take advantage of the midspan's ability to deliver power over all four pairs of structured cable wires. Today's midspans have two interfaces, each capable of delivering 25.5W into the same box (one interface uses wires 1, 2, 3, and 6 over two pairs, and the other interface uses wires 4, 5, 7, and 8 over two pairs). Connecting the two interfaces doubles the standard delivered power to 51W while still being fully compliant with the 802.3at-2009 standard (see Figure 3).

Figure 3: Four-pair power supply

Another approach is to use four-pair powering to reduce power dissipation. This same four-pair structure can be used to power two pairs of devices with 30W of power, instead of delivering 51W over the CAT5 cable, which dissipates up to half the power, while reducing power consumption by almost 15% over the traditional two-pair solution, saving more than 2.5W per port. Using this approach, with a 12-port midspan, power dissipation can be reduced by nearly 30W, saving approximately 263 kilowatt-hours (kWh) per year (30W x 24 hours x 365 days). At $0.10 per kWh, this saves $26 per 12 WLAN APs per year.

To improve energy efficiency, the next strategic step is to use the midspan's remote PD monitoring and configuration capabilities. Network administrators can monitor the power consumption of each PoE port and the total power consumption, and set the PD to achieve instant and preset port ON/OFF functions, as well as UPS status port ON/OFF functions. For example, an organization with 12 WLAN APs can reduce the operating hours from 24 hours a day to 10 hours a day (from 8 am to 6 pm Monday to Friday), reducing power consumption from 2,933KWh (12 APs x 27.9W x 24 hours x 365 days) to 837KWh (12 APs x 27.9W x 10 hours x 250 days), saving 2,096KWh per year, and saving $210 per year at $0.10 per kWh of electricity.

The third way to improve power efficiency is to minimize the impact of idle power consumption. Many PoE midspans and switches use switching power supplies (SPS), which can achieve 90% efficiency at full load, which means that for a 200W PoE source, up to 220W of AC power is consumed, or for a 400W PoE source, up to 440W is consumed. The problem is that SPS devices have high switching power losses when idle—for a 200W-rated device, the power consumption is as much as 20W to 40W at 0W load, and for a 400W-rated device, the power consumption is as much as 40W to 80W at 0W load.

The solution is to develop a distributed power architecture using midspans, where the PSE uses an internal or default power supply, and can then use additional power supplies to increase power as needed, depending on customer demand. Using this approach, the midspan can be configured with an internal power supply to power the real demand, and each port can be boosted to full power with an external power supply only when and where it is needed. With this efficiency improvement, it is possible to use a small (450W) internal power supply to handle all real-time needs, and then use an external 450W to 1kW power supply to increase power when needed. This can save 394kWh per year (45W x 24 hours x 365 days), which is equivalent to $39 per year for 12 WLAN APs at $0.10 per kWh of power.

By taking these approaches and stacking the savings opportunities together, organizations can use the latest energy-efficient PoE midspan technology to significantly reduce costs. Savings opportunities include:

. Four-pair power delivery: Save 2W per port or $53 per year

. Power Management: Save $210 per year by shutting down ports when not in use.

Smaller internal power supply (with external power supply to augment power when needed): Saves $39 per year by reducing idle power consumption

PoE technology is easy to deploy and expand the network. Organizations can use PoE technology to further improve network availability and energy efficiency, while reducing equipment and operating expenses by selecting the latest high-power PoE midspans. These systems implement four-pair power supply and built-in power management capabilities, while minimizing idle power consumption by using a smaller internal power supply.