One of the most revolutionary technologies to come out of Ethernet is the ability to provide power to a device. The IEEE 802.3af -2003 standard known as power over Ethernet (PoE) delivers power to an Ethernet device, and the right PoE solution can maximize investment returns.

Val Oliva is the director of product management for Foundry Networks‘ enterprise business unit, Santa Clara, Calif. Oliva is also a board member and secretary of The Ethernet Alliance, and chair of the marketing committee for power over Ethernet/power over Ethernet plus.

In addition, a new IEEE standard is being developed, called 802.3at or PoE Plus, that intends to deliver ~30 watts for medium power (two-pair) and ~59 watts for high power (four-pair). PoE Plus is backward compatible with PoE, specifically for those that use Category 5 connections for their powered devices (PD), such as voice-over-IP (VoIP) phones. Although the upcoming standard supports high power, most PDs available today, including pan-tilt-zoom cameras, can be powered with 802.3af and 802.3at medium power.

PoE supports four separate power classifications, each negotiated by the power-sourcing equipment (PSE) or a PoE switch and PDs. If a PSE cannot deliver adequate power because power is being consumed by other PDs, then the PD may not operate properly.

Common to 802.3af and 802.3at, and often neglected by installers, is the power consumed by the CAT 5 connection–a 100-meter CAT 5 cable can consume as much as 2.45 watts. Installing the right PSE helps maximize the capital return from PoE investments.

To make PoE deployments simple, the right PSE should support full Class 3 power for the same available ports. For example, a 48-port fixed PSE should support 48 full Class 3 (15.4 watts). Without this capability, network managers will need to carefully document their PSE deployments to include PD power usage to eliminate power starvation.

Chassis-based PSE should follow the same rule because most chassis-based PSE systems deliver power to the PD and the chassis system. A chassis-based PSE will not perform when power to the chassis system and the modules is limited, and because of PoE density, an outage from a chassis-based PSE is severe. In addition, PoE deployments that use a chassis-based PSE should support enough wattage for future additions of 802.3at modules.

PoE power redundancy is required for PSEs to ensure that high power availability is on hand for a PD, such as an IP phone.

One of the main challenges in delivering PoE power redundancy is installing the right power supply within a PSE. A power requirement of 739.2 watts is needed to deliver 48 full Class 3 (15.4 watts), and 1,478.4 watts is required for PoE power redundancy.

To support high-wattage requirements and PoE power redundancy in a fixed PSE, a load-sharing and removable power supply system is needed, allowing a PSE to get power from more than one power supply. This maximizes the amount of power available to support more PDs.

To support the higher wattage required for PoE power redundancy, a chassis-based PSE should be designed to have two power distribution sources: one for the system and one for the PoE output. This design delivers more power for PDs that are PoE or PoE Plus compliant, and ensures redundant power for the system.

Intelligent PSEs that give detailed information of the traffic pattern are needed because traffic patterns of applications that use PoE behave differently. Having complete network visibility sheds light on detailed information required for managing real-time applications such as VoIP.

Complete visibility of the network traffic within a PSE is critical and sFlow (RFC 3176) gives network managers complete network visibility of their VoIP traffic. Enabling sFlow at the PSE delivers real-time VoIP usage and appropriate quality of service settings, allowing network managers to monitor VoIP traffic.

The next wave of devices to be powered by PoE will be video cameras, specifically for video surveillance. In addition, 802.3at or PoE Plus will fuel the need for PSE switches that deliver more power per port. Enterprise customers will begin to use 802.3at ports to power pan-tilt-zoom video cameras and, at the same time, use it for 802.3af PDs

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