Bring on the Power

Understanding that PoE impacts network design

• By Steven Olen
• Mar 01, 2015

When selecting a PoE switch for an IP surveillance network, system integrators should be aware that not all switches are created equal. PoE switch manufacturers offer a variety of different features that can impact functionality and pricing. For example, port speeds may support 10/100, or Gigabit, or perhaps a combination of both. Form factors can vary from compact/desktop to rack-mountable. Power supplies may be external power bricks, or internal components. More importantly, PoE power budgets will vary from switch to switch.

Understanding the power budget of a PoE switch, and how it impacts network design, will help system integrators make better buying decisions and ensure the right product gets used at the right price.

Background: PoE Standards

Standards-based PoE switches typically support either IEEE 802.3af or IEEE 802.3at. IEEE 802.3af—the original PoE standard ratified in 2003—provides up to 15.4W of power at each PoE switch port. Accounting for voltage drop along the cable, it ensures at least 12.95W of power is available at the IP camera, IP phone or other PoE device.

IEEE 802.3 at—the newer standard ratified in 2009—is backwards compatible with 802.3af, and supports higher power levels. At the PoE switch port, 802.3at supports up to 30W of power, of which 25.5W are ensured to be available to the PoE device.

When shopping for a PoE switch for an IP surveillance network, it’s important to document the power requirements for each camera in the network, and plan accordingly. These specs are typically available on the camera datasheets. Most common indoor cameras will draw under 6 watts. For such cameras, a PoE switch supporting 802.3af is normally sufficient. Some outdoor cameras may contain heaters/blowers for temperature control. Other cameras may have PTZ motors. Cameras in these categories may require additional power beyond what 802.3af can support, and would therefore need a PoE switch that supports 802.3at.

PoE Power Budget

The power budget of a PoE switch is the total amount of PoE power that the switch can provide to all the connected devices at the same time. The power can be allocated across all PoE ports as needed. Port 1 may require 2W, port 2 may require 10W, and port 3 may require 12W. As long as the switch has a budget of at least 24W, all devices will function as intended.

Some PoE switches are designed with a PoE power budget sufficient to provide “full” 802.3af power (15.4W), or full 802.3at power (30W), concurrently on all ports. An 8-port switch supporting full 802.3af power, for example, would have a power budget of 8 x 15.4W = 123W (approximately).

However it is important to remember that powered devices like IP cameras will only draw as much power as they need. Why pay for excess power capacity in a switch if it’s never going to be needed? A full power PoE switch will always be more expensive than its counterpart with a smaller PoE power budget. If a full power 8-port PoE switch was connected to a network of eight IP cameras, with each camera drawing only 2W, for example, a switch with a power budget of 16W would suffice. The 123W PoE switch would be over-designed and overly expensive.

Not only do larger PoE power budgets drive up costs, they also may present thermal challenges. An 8-port PoE switch with 78W PoE budget will run cooler than a similar 8-port switch with 123W budget. The 78W switch will likely not need fans, whereas the 123W switch may.

Sometimes, a case can be made for deploying a full power PoE switch, in spite of the additional costs. In surveillance networks for example, conditions and requirements often change after initial installation.

Additional cameras may get added, or existing cameras may get swapped out. Daytime-only cameras may get replaced with cameras that have integrated IR illumination. Fixed dome cameras may get replaced with PTZ dome cameras. So installing full power PoE switches on “day-one” gives the system integrator peace of mind that his network is future-proof. Although he doesn’t need all that power today, he may need it tomorrow.

Unmanaged versus Smart PoE Switches

A final consideration for a system integrator would be whether to deploy unmanaged or smart PoE switches. An unmanaged PoE switch has no user configurable options and no user interface, whereas a smart PoE switch will have an intuitive, easy to navigate graphical user interface (GUI) that can be used for configuration and monitoring of the network.

A smart switch will have a distinct advantage over an unmanaged switch when it comes to monitoring the PoE power budget. With an unmanaged switch, the network administrator has no visibility into how much PoE power is actually being drawn from the switch. As new cameras are turned up, power demands on the switch may eventually exceed its budget and the network administrator may not be aware.

With a smart switch, the network administrator can easily monitor the real time PoE power draw using the switch’s GUI.

System integrators have many important things to consider when setting up a surveillance network, and PoE power budget should be one of them. Clearly understanding the power requirements of all devices in the network will help with high-quality network planning, and will ensure appropriate switches are used that meet the project’s budget.

Smart PoE switches have an advantage over unmanaged PoE switches with their ability to configure and monitor various switching functions, including budgets.

This article originally appeared in the March 2015 issue of Security Today.