Cisco Aironet Power over Ethernet
Application Note

Introduction

The goals of this document include the following:

•Identify Cisco inline-powered WLAN devices and their usage requirements with a Cisco inline-powered switch, mid-span device, or injector-powered solution.

•Help identify compatibility to enable the selection of the appropriate powered solution for the deployed device.

•Identify implementation requirements for all Cisco WLAN products that operate from inline power via Cisco pre-standard power, IEEE 802.3af, and the emerging 802.3at standard.

Types of Power over Ethernet

This section describes various types of Power over Ethernet (PoE) including Cisco pre-standard power, IEEE 802.3af power, and IEEE 802.3at power.

Cisco Pre-Standard Power

The first generation of Cisco pre-standard PoE was designed to power devices such as Cisco IP phones and access points. This pre-standard solution had relatively low power requirements (approximately 6 to 7 watts). Later generations of pre-standard power supported higher power modes (up to 15 Watts) and added power negotiation via Cisco Discovery Protocol (CDP). In July 2003, the IEEE ratified the 802.1af standard (up to 15.4 watts of power). With the ratification of IEEE 802.3af, Cisco supports both IEEE 802.3af and Cisco pre-standard PoE concurrently. Cisco has also extended pre-standard power management using CDP negotiation to Cisco IEEE 802.3af compliant devices to further optimize Power Source Equipment (PSE) power management.

When using a Cisco pre-standard source for power, it is important to check the power draw in watts (Table 1) and verify that the PSE can supply enough wattage to the powered device. Depending on the version of your PSE, enough power may not be available to supply some of the newer access points.

IEEE 802.3af Power

The IEEE 802.af-2003 PoE standard:

•Defines terminology to describe a port that acts as a power source to a powered device.

•Defines how a powered device is detected.

•Defines two methods of delivering PoE to the discovered device requiring power.

•End-Point PSE—PoE-capable Ethernet port. Power may be delivered by an end-point PSE to a powered device using the active data wires of an Ethernet port or the spare wires. An end-point PSE, such as a PoE-capable Ethernet switch may implement either scheme.

•Mid-span PSE—Can be used to deliver PoE if an existing non-PoE-capable Ethernet switch is used. If a mid-span PSE is used, it can only implement power delivery over the spare pairs of the copper cabling and cannot be used to deliver PoE over 1000BASE-T connections.

Note Only one mechanism may be used at a time to deliver power to a powered device because the design of the device.

Figure 1 End-Point PSE PoE Delivery

Power delivery, supported within Cisco Catalyst Ethernet switches, relies on the data pairs (pins 1-2 and 3-6) to transmit power (sometimes referred to as "phantom" power). The second mechanism relies on the unused data pairs (pins 4-5 and 7-8) to deliver power that is supported within mid-span power delivery.

IEEE 802.3at Power

In September 2005, the IEEE began work on a higher power standard for PoE. This standard has not yet been ratified. Unlike 802.3af (which has a limit of 15.4 watts and must work with the limitations of Category 3 cable), IEEE twice the PoE power support (approximately 30 watts using two pairs).

Although still in discussion, the standard may include PoE power support as high as 60 watts (using four pairs). The main purpose of 802.3at is to enable more power to operate over an Ethernet cable. Additionally, the task force has agreed to allow gigabit mid-span products and has granted them the ability to choose either active data pairs or spare pairs for the power delivery.

Note A 1000BASE-T mid-span device is a product in the middle of the circuit that can provide power. This product can be a powered Ethernet source, an injector, or other mechanism for inserting PoE.

PoE Requirements for Cisco Access Points

To determine if a particular switch or PSE can supply enough power to an access point, the installer needs to verify the amount of power the access point draws and confirm that there is enough power present to operate it. Depending on the make, model, and radio configuration, the access point can draw different amounts of power. Table 1 presents the power requirements (in watts) for Cisco Aironet access points.

The figures used in Table 1 were calculated as described below in Figure 2.

Note The power requested for the switch/box by the access point must take into account the resistance of 100 m of Category 5 Ethernet cable.

Figure 2 Power Calculation Formula for PSE

V = voltage, R = resistance of cable (Ohms), PowerPD = power Consumed by the access point (W)

The worst case voltage is 40 V. The resistance of the Category 5 cabling is in Ohms.

Note Always check the power specifications of the PSE (switch, mid-span power, or injector) to confirm that it can support the access point you are deploying. If the PSE cannot supply enough power, the access point radios may be disabled or the access point may repeatedly "reboot" as the device draws more power than the PSE can supply. This imbalance trips the breaker causing it to reset as the breaker cools down and causes the access point to appear to be cycling or rebooting.

Using a Cisco Power Injector

In environments with fewer installed access points, it may be more cost effective to use a Cisco Power Injector. Table 2 is a guide to assist with selecting the correct power injector.

Powering the Cisco Aironet 1250 Series Access Point in High Power (18.5-Watt Mode)

Powering access points with additional features such as radio modules may cause the access point to exceed 15.4 watts (the limits of 802.3af). There are several short-term ways to solve this problem.

Note Only use ONE source of power at any time.

1. Deploy using local power such as the Cisco Power Supply (AIR-PWR-SPLY1=)

2. Deploy using a mid-span device such as a Cisco Power Injector (AIR-PWRINJ4=)

3. Deploy on applicable Catalyst switches using Cisco Intelligent Power Management (See Figure 3).

Figure 3 Power from a Cisco Catalyst Switch Using Cisco Intelligent Power Management

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