It’s difficult for those without radio frequency
(RF)
points. Without paying close attention to the factors that contribute to RF
propagation, you’ll end up with spotty coverage, inadequate performance, and
frustrated users. As we’ve discussed in a previous tutorial,
you should perform an RF site survey to identify the number and location of
access points. Let’s take a closer look now at what you should consider when
making decisions regarding access point installation locations.
Requirements First
As the basis for making decisions about these locations,
take into account application requirements,
which are generally attributes of the solution that you don’t want to (or can’t)
change. Have a clear understanding of the areas where users will require access
to the wireless LAN. This helps you scope out the coverage needed from the access
points and where you should place them. If users don’t need access from the
parking garage, then you don’t need to install any access points there.
However, whether or not users need access from
the lunch room will very likely impact the placement of access points. You should
know precisely where users will operate to make decisions on access point locations
during the site survey. In order to cover the lunch space, you may need to position
an access point closer or within the applicable area, which could then require
the addition of another access point to cover an adjacent portion of the facility.
Performance requirements provide some insight into
placement. If users need 11Mbps throughout the facility, access points will
need to be close together (e.g., 100 feet). When performing the RF site survey,
your goal will be to ensure that access points are placed in a manner where
the edge of the access point’s propagation overlaps the propagation of the adjacent
access point. On the other hand, if users must have connectivity regardless
of data rate you could allow the placement of access points much farther apart
(e.g., 500 feet) because the propagation overlap can be held at a lower data
rate.
Facility construction plays a role in positioning of access points as well.
In general, choose a location that enables the access point antenna to have
maximum line-of-sight propagation with the users. For example, don’t hide the
access point in a broom closet in a far-reaching corner of the building. Instead,
make sure that there’s a clear path between the access point antenna
and the users, free from obstructions such as overheat vents, office partitions,
walls, etc., that offer attenuation
pattern of the antenna over areas of the facility where users will reside (not
outside or in non-user areas).
Some companies may have policies related to the
dicor of the facility. If this is the case, fully understand where and how you
can mount the access points and antennas. Ideally, you should mount the access
points (more importantly the antennas) as high as possible, unless there are
obstructions. The higher vantage point increases the horizontal range of the
RF signal.
If policies dictate that WLAN hardware must be
completely out of sight, then you’ll probably need to mount the access points
above the ceiling. This is doable in most offices because of the use of false
ceilings to hide electrical wires and ventilation ducts. In warehouses and manufacturing
facilities, you’ll probably be free to expose the hardware, but you may be limited
to installing the access points on pillars or overhead beams. Keep in mind too
that your selected mounting location will need to accommodate data cabling and
possibly electrical connections, assuming you’re not using Power-over-Ethernet
Consider Configurations
When negotiating access point locations, think about configuration attributes,
such as signaling method (i.e., 802.11a or 802.11b), transmit power and antenna
type. These configurations affect range; therefore, use them to your advantage
when tweaking the positioning of the access points.
802.11b access points generally offer greater range than 802.11a,
mainly because 802.11b operates using lower frequencies (2.4GHz instead of 5GHz
band). As a result, the use of an 802.11a network requires access points to
be closer together (e.g., 100 feet) as compared to 802.11b (e.g., 500 feet).
Keep this in mind when positioning the access points.
Enterprise class access points generally offer
several transmit power settings, which affect range (i.e., coverage) of the
RF signals. The maximum transmit power for 802.11b is 100mW, which provides
the greatest range. In most cases, base the selection of access point location
with the highest transmit power.
Some applications, especially those requiring support
for a large density of users and high throughput, need a large number of access
points in a relatively small area. In order to accomplish this and avoid inter-access
interference, try using a lower transmit power. Of course, this will require
the access points to be closer together.
Similar to transmit power, the antenna
type affects the positioning of access points. The antenna affects range,
and it also affects the pattern of the propagated signal. For example, an omni-directional
antenna broadcasts horizontally in all directions. The use of omni-directional
antennas provides widespread coverage, which are best for applications in office
complexes, warehouses, homes, etc. The access points in these cases need to
have fairly equal spacing, assuming the construction and layout of the facility
is fairly uniform.
A directional antenna focuses the RF signal more in one direction than others
(thus increasing the range more in that direction). To cover relatively long,
narrow areas, such as airport concourse or convention center hallways, a directional
antenna may make most sense to minimize the number of access points. In these
environments, the access point should be located near one end of the long corridor
to focus most of the signal in the right direction. This technique can make
use of only one access point instead of several access points having omni-directional
antennas.
As you begin to deploy more and more wireless LANs, you’ll quickly find that
locating access points is more of an art rather than a cut-and-dry procedure.
In fact, you eventually develop a knack for predicting the propagation of radio
waves in various scenarios. Don’t let that, however, rule out the need for an
RF site survey
to verify your conclusions!
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001) and offers computer-based
training (CBT) courses on wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
