Choosing a Power-Over-Ethernet Approach

When planning the layout of a wireless LAN, you must take into consideration
power sources for access points. Some companies limit installation locations
to areas where a six foot cord can reach an electrical outlet. Other companies
opt to install new outlets, which require extra electrical wiring. Power-over-Ethernet
(PoE), however, can significantly decrease costs associated with supplying power
to access points.

PoE technology enables an access point to receive electrical power and data
over the same cable. A power-sourcing device, which connects directly to the
power source and is situated between the switch and access point, will detect
the access point and inject the right amount of electrical current over unused
pairs in the Cat5
Ethernet cable.

The access point can then operate without having a direct connection to an
electrical outlet. In addition to cost savings, this offers a variety of benefits,
such as flexible access point mounting locations, better reliability because
of less cables, and easier international deployment because the use of a common
power connection (i.e., no need for different power plugs).

Inline PoE

Many vendors incorporate inline PoE into their enterprise-class access point
solutions (you seldom find PoE in inexpensive home networking products). This
is the most basic type of PoE system, connecting only one access point at a
time to a power source. This is done through a PoE injector, which fits between
the source signal (modem, switch, etc.) and the access point via Cat5 cable.
The injector connects to a power outlet through a regular electrical cord. For
optimum benefits, you can plan the placement of the injector near a power outlet,
such as within the communications room near the switch.

The injector receives current from the power outlet and sends it through the
Cat5 cable to the access point. Think of this configuration as a "T,"
with the injector at the intersection, the source signal and access point at
either side of the top line, and the power outlet at the bottom point.

Inline PoE devices are best for small office applications. Because there are
very few access points in these applications (typically one), the costs for
the injectors are less than purchasing a more elaborate PoE hub. In some cases,
smaller installations won’t even benefit from PoE because you can generally
locate the access point close enough to an electrical outlet to use a traditional
(old fashioned) power cord.

Hub-based PoE

Another way to implement PoE is to install a "power hub" between
the source signal and the access points. This implementation follows the same
concept as the inline PoE system, except that the hub feasibly supports multiple
access points rather than just one. Multiple Ethernet cables run from the switch
into the power hub, which connects directly to a power outlet. Multiple power-infused
Ethernet cables run from the other side of the hub to various access points.

PoE hubs are a good approach to utilize when dealing with a large scale installation
that has existing switches, such as adding a wireless LAN to an existing wired
Ethernet network. PoE hubs give an installer the ability to run power to multiple
access points while using only one power outlet, saving time and preventing
the huge headache caused by dealing with too many wires (and electricians).
PoE hubs can each usually support up to twelve access points.

Integrated PoE

The most convenient way to implement PoE is to integrate it into the switch
itself. This eliminates the need to buy extra equipment, making it most cost
effective to deploy a wireless LAN. The switch itself connects to a power outlet,
and each port includes PoE capability integrated within the switch. The access
points connect directly to the PoE-based switch ports to receive both electrical
power and data over the Cat5 cable.

Integrated PoE is the optimum solution for providing electrical power to access
points. It is definitely the approach to use when installing a new, large-scale
network. Unfortunately most large companies already have an existing wired Ethernet
network, and replacing the existing switches is impractical and expensive. If
a company is just starting up or replacing their existing wired backbone, however,
go with integrated PoE.

Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs and
offers computer-based
training
focusing on wireless LANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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