Most wireless LANs consist of a sea of access points installed within a building
that provide overlapping radio cells. One issue with this approach, however,
is that it involves careful access point positioning and assignment of radio channels.
A site survey also becomes an essential step.
Furthermore, the installation of access points requires a distribution system,
such as Ethernet, to interface the access points to the corporate network. This
necessitates an initial investment and additional funding as the building undergoes
remodeling and consequential re-cabling.
A new, practical approach
Recently, I spent time evaluating the Vivato wireless LAN switch solution in various
indoor and outdoor scenarios. One rather interesting test that I conducted under
this evaluation was using Vivato’s "outdoor-to-indoor" configuration.
The Vivato outdoor switch is an antenna panel measuring approximately forty
inches square and seven inches thick. The panel enclosure includes the equivalent
of thirteen interlinked and coordinated 802.11b radios,
with each one operating on a separate beam of a phased array and beam-former
system.
Standard 802.11b wireless clients associate with the beam having the strongest
signal. As users move through the beams, they roam by re-associating with the
next radio.
The antenna array and beam-former system create beams of radio frequencies
(RF) with a peak gain of 25dBi. Each beam varies from six to nine degrees horizontally
and is twelve degrees in height. The thirteen beams together create a field
of view that exceeds one hundred degrees of horizontal coverage. The switch
packs a pretty good punch in the direction it’s pointing, to say the least.
This makes it feasible to mount the switch outside a building at a specific
distance and orientation that illuminates the entire building with radio waves.
With this solution, you can avoid the time and expenses related to installing
multiple access points, with a lower cost of ownership for a number of wireless
applications.
Some real results
As part of the evaluation, I setup the Vivato switch at a hotel located in
the Dayton, Ohio area. I installed the antenna panel in the hotel parking lot
120 feet from the hotel. The antenna was located four feet off the ground and
aimed at the center of a large six story wing of the hotel.
The wing contains standard sleeping rooms, with rooms on both sides of a center
hallway that runs the entire length of the wing. Half of the rooms face the
front of the hotel, on the side where the antenna panel was installed. The other
rooms are opposite the hallway on the backside of the wing and away from the
panel.
The construction of the exterior walls of the hotel includes red brick and
center block interior wall with steel reinforcement. The inside walls consist
primarily of drywall and a mix of metal and wood frame construction. The windows
are clear with no metallic coatings.
The hotel manager was nice enough to escort me through many of the rooms to
record signal strengths. We only took measurements within empty rooms, but we
timed the testing to occur during the week around 1 p.m., which is when most
the rooms at this particular hotel are empty.
Through the use of an Airmagnet handheld,
I found the signal-to-noise ratio (SNR) in all rooms on the side facing the
panel to be 25dB to 35dB, which is strong enough to easily support 11Mbps associations
throughout the rooms. 15dB to 20dB SNR was found in rooms opposite the center
hallway on the side of the wing away from the Vivato panel. This was still sufficient
to allow good performance with varying data rates, depending on the position
in the room.
One antenna panel easily covered the entire six story hotel wing, including
rooms opposite the hallway away from the antenna panel. As a result, the outdoor-to-indoor
solution is certainly feasible. In fact, this is a less expensive and more practical
approach as compared to installing multiple access points inside the building.
A RF site survey conducted at the hotel using conventional indoor access points
resulted in the need for twelve access points to cover the same area illuminated
by the outdoor antenna panel. The cost of deploying the solution with traditional
access points is $22,000 for hardware and installation, which includes access
points and Ethernet distribution system.
The cost of deploying the Vivato antenna panel solution would be $15,500, which
includes the outdoor antenna panel and installation. In addition, the outdoor-to-indoor
solution will not entail any costs when the hotel undergoes major renovations,
which the hotel manager told me will occur within the next couple years. As
a result, the outdoor-to-indoor solution is certainly a good alternative for
the hotel.
Antenna placement matters
In order to fully cover larger buildings, plan on placing the antenna panel
approximately 75 to 100 feet away from the building. You’ll need to position
the panel far enough away to ensure full illumination of the building. Minimize
the distance, however, in order to maximize signal strength inside the building.
This is a tradeoff that you’ll need to play with a bit in order to obtain best
results.
In some cases, you’ll need to locate the panel near the perimeter of the facility
to allow enough range between the panel and the building. If mostly large parking
lots surround the building, then you might need to seize some parking lot spaces
for the panel installation. Be sure to think about obstacles such as trees,
though, that may get in the way and cause significant attenuation.
As part of the installation, plan on running network cabling to the installation
location in order to provide connectivity between the panel and the Internet
or corporate network. This might limit installation locations due to cabling
difficulties. Alternatively, you can install a wireless backhaul link to the
panel to avoid any cabling issues.
Even though the antenna panel will require some installation planning and related
work efforts, the lower costs and flexibility of this approach is certainly
worth considering before going forward with the traditional indoor access point
approach.
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 training
focusing on wireless LANs.
