三种POE供电标准

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.

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.