Information Technology Reference
In-Depth Information
Cisco Catalyst Switch PoE
If you were to create an Ethernet cable (Category 5 or 6), you would find that there are
eight wires (four pairs of wires) to crimp into an RJ-45 connector on each end of the con-
nection. Further study reveals that only four of the wires are used to transmit data. The
other four remain unused and idle...until now.
The terms inline power and PoE describe two methods you can use to send electricity
over the unused Ethernet wires to power a connected device. There is now a variety of de-
vices that can attach solely to an Ethernet cable and receive all the power they need to op-
erate. In addition to Cisco IP Phones, other common PoE devices include wireless access
points and video surveillance equipment.
Powering devices through an Ethernet cable offers many advantages over using a local
power supply. First, you have a centralized point of power distribution. Many users ex-
pect the phone system to continue to work even if the power is out in the company of-
fices. By using PoE, you can connect the switch powering the IP phones to an
uninterruptible power supply (UPS) instead of placing a UPS at the location of each IP
phone. PoE also enables you to power devices that are not conveniently located next to a
power outlet. For example, it is a common practice to mount wireless access points in the
ceiling, where power is not easily accessible. Finally, PoE eliminates much of the “cord
clutter” at employees' desks.
PoE became an official standard (802.3af) in 2003. However, the IP telephony industry was
quickly developing long before this. To power the IP phones without an official PoE stan-
dard, some proprietary methods were created, one such method being Cisco Inline Power.
Note: The IEEE standards body has recently created the 802.3at PoE standard (also
called PoE Plus), the goal of which is to increase the current maximum PoE wattage from
15.4W to 25.5W. In addition, some proprietary implementations of PoE have reached 51W
of power by using all four pairs of wire in the Ethernet cable.
Powering the IP Phone Using a Power Patch Panel or Coupler
Many companies already have a significant investment in their switched network. To up-
grade all switches to support PoE would be a significant expense. These organizations
may choose to install intermediary devices, such as a patch panel, that are able to inject
PoE on the line. The physical layout for this design is demonstrated in Figure 3-3.
By using the power patch panel, you still gain the advantage of centralized power and
backup without requiring switch upgrades.
Note: Keep in mind that Cisco switches must also provide quality of service (QoS) and
voice VLAN support capabilities, which may require switch hardware upgrades. Be sure
your switch supports these features before you consider a power patch panel solution.
Inline PoE injectors provide a low-cost PoE solution for single devices (one device per
coupler). These are typically used to support wireless access points or other “single spot”
PoE solutions. Using inline PoE couplers for a large IP Phone network would make a mess
 
 
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