CISCO POWER OVER ETHERNET (POE)

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POWER OVER ETHERNET (POE)
Just walk around any building and you see electrical power outlets everywhere. When finishing the interior of a building, electricians run electrical cables and install electrical outlets to any and every location that might need power. They also run power cables so that devices such as light fixtures can be wired to power as well. And when network engineers thought about electrical power, they thought in terms of making sure the electricians had run enough power to the wiring closets and other locations to power the networking devices.
Power over Ethernet (PoE) changes that thinking so that the responsibility to provide electrical power to some devices can fall to the network engineering team.
Some classes of device types have been built to be able to receive their power over the Ethernet cable, rather than using a separate power cord. To make that work,
the LAN switch connected to the cable must supply that power over the cable. By using PoE, companies can gain several advantages, including reduced cost by requiring
fewer cable runs and better power management capabilities as compared with using a traditional electrical power cable run and power outlet. This final section of the chapter examines PoE.
PoE Basics
The family of standards that supply power goes by the general name Power over Ethernet (PoE). With PoE,some device, typically a LAN switch, acts as the Power
Sourcing Equipment (PSE)—that is, the device that supplies DC power over the Ethernet UTP cable (as shown in Figure 13-9). A device that has the capability to
be powered over the Ethernet cable, rather than by some other power connector on the device, is called the Powered Device (PD).

Figure 13-9 Power over Ethernet 􀀷erminology

PoE has a great advantage for devices installed to locations that often do not have a preinstalled power cable or power output. For instance, wireless design
places APs in a wide range across the ceiling of a floor(or story) in a building. Also, IP video cameras might be placed in the ceiling corners inside or at various outside
locations. Instead of running new power and new network cables to support each device, a single Ethernet cable run can supply power to the device while allowing
normal Ethernet communications over the same cable and same wire pairs.
PoE also helps in some less obvious practical ways because it supplies DC power over the Ethernet cable, so the device does not need an AC/DC converter. For
instance, devices like laptops and IP phones use a powercord that includes a power brick—an AC-to-DC converter—which converts the AC power from the power outlet to
the DC power needed by the device. PoE supplies DC current over the Ethernet cable. So, for an IP Phone, for instance, no more power cable and no more power brick
cluttering the desk or taking up a power outlet.
PoE Operation
PoE must have a means to avoid harming the devices on the end of the circuit. Every electrical device can be harmed by receiving too much current into the device,
which is why electricians install circuit breakers and why we use surge protectors. Applying power over anEthernet cable could have the same effect, harming the
device on the other end, if the device does not support PoE. So PoE must (and does) have processes in place to determine if PoE is needed, and for how much power,
before applying any potentially harmful power levels to the circuit.
PoE, standardized by the IEEE, extends the same IEEE autonegotiation mechanisms. In fact, the mechanisms need to work before the PD has booted, because the PD
needs power before it can boot and initialize. By using these IEEE autonegotiation messages and watching for the return signal levels, PoE can determine whether the
device on the end of the cable requires power (that is, it is a PD) and how much power to supply. This list details the major steps:

Step 1. Do not supply power on a PoE-capable port unless negotiation identifies that the device needs power.
Step 2. Use Ethernet autonegotiation techniques, sending low power signals and monitoring the return signal, to determine the PoE powerclass, which determines how much power to supply to the device.
Step 3. If the device is identified as a PD, supply the power per the power class, which allows the device to boot.
Step 4. Monitor for changes to the power class, both with autonegotiation and listening for CDP and LLDP messages from the PD.
Step 5. If a new power class is identified, adjust the power level per that class.

The negotiation processes result in the PDs signaling how many watts of power they would like to receive from the PSE. Depending on the specific PoE standard,
the PSE will then supply the power, either over two pairs or four pairs, as noted in Table 13-2.
Table 13-2 Power over Ethernet Standards


Cisco has been developing products to use some form of PoE since around 2000. Cisco has often developed prestandard power capabilities, like its original Cisco
Inline Power (ILP) feature. Over time, the IEEE has produced standards similar to Cisco’s power features, with Cisco supporting the standard version once
completed. However, for the most part, the Cisco literature refers to the more common names in the first column of the table.

PoE and LAN Design
Most of the LAN switch features discussed in this book exist as software features. Once you learn about a software feature, in some cases all you have to do is
configure the feature and start using it. (In some cases, you might need to research and license the feature first.) Regardless, adding the feature takes little or no
prior planning.
PoE does require some planning and engineering effort when designing a LAN, both when planning for the cable plant (both Ethernet and electrical), as well as when
planning for new networking hardware. Planning with PoE in mind prepares the network to supply power to network devices, rather than reacting and missing
opportunities to save money and time.

The following list includes some of the key points toconsider when planning a LAN design that includes PoE:
Powered Devices: Determine the types of devices and specific models, along with their power requirements.
Power Requirements: Plan the numbers of different types of PDs to connect into each wiring closet to build a power budget. That power budget can then be processed to determine the amount of PoE power to make available through each switch.
Switch Ports: Some switches support PoE standards on all ports, some on no ports, some on a subset of ports. Research the various switch models so that you purchase enough PoE-capable ports for the switches planned for each wiring closet.
Switch Power Supplies: Without PoE, when purchasing a switch, you choose a power supply so that it delivers enough power to power the switch itself. With PoE, the switch acts as a distributor of electrical power, so the switch power supply must deliver many more watts than it needs to run the switch itself. You will need to create a power budget per switch, based on the number of connected PDs, and purchase power supplies to match those requirements.
PoE Standards versus Actual: Consider the number of PoE switch ports needed, the standards they support, the standards supported by the PDs, and how much power they consume. For instance, a PD and a switch port may both support PoE+, which supports up to 30 watts supplied by the PSE. However, that powered device may need at most 9 watts to operate, so your power budget needs to reserve less power than the maximum for those devices.