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 cann
      ot 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.

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.