Structural Considerations

 
Physical considerations are classified into two major groups of issues that can
affect wireless connectivity and performance—path fading and propagation
losses. Below these two groups are specific causes that produce either propagation
loss or path fading.The following sections explain propagation losses and path
fading and then explore common causes for each, keeping in mind their affect
on wireless design. As discussed in Chapter 2, radio waves are propagated through space at the
speed of light.This speed is attained assuming that there are no obstructions for
the electromagnetic wave to pass through.The reason for this is because electromagnetic
waves pass through different substances at different speeds.The greater
the density of the substance, the slower the wave propagates through it. For
example, a radio wave will travel faster through the air than it will water and
faster through water than a concrete building.
Under normal circumstances, as the signal radiates out from an antenna and
encounters objects within the environment, it will exhibit one or more of the
following reactions: the signal may penetrate the object, reflect off the object, or
be absorbed by the object.
In most cases, all of these reactions will occur to varying degrees, depending
on the density and type of object encountered.This is the propagation of the
signal.The strength of the signal decreases as it propagates. Penetration, reflection,
and absorption all factor into the signal as it travels, each taking with it some
amount of signal strength.These actions not only weaken the signal, but they
may affect the direction in which the electromagnetic wave travels and the speed
at which it travels.
As the radio wave propagates through the Earth’s atmosphere and encounters
objects within the environment, the strength of the signal will decrease.Any distortion
of a wave’s amplitude, phase, or direction can affect the strength of the
received signal.This is known as path fading.The strength of the received signal is
equal to the strength of the transmitted signal minus path fading.
As you can see, propagation loss and path fading are very similar.The difference
is really a matter of perspective. Receivers can suffer from path fading and
transmitters can suffer from propagation losses. Ideally, because most WLAN
radios both send and receive, elimination or minimization of propagation losses
and path fading are extremely desirable.
Differing environments can have substantial structural considerations to work
around or overcome to successfully implement a WLAN solution.The following
list takes a look at some of the common problems encountered in various environments
and the solutions or alternatives available for each.
 Hospitals The most obvious issue that comes to mind in any medical
environment is compatibility of wireless networks with existing medical
equipment and, more importantly, medical diagnostic devices.
Another consideration is the need for many healthcare providers to
meet federal regulations in terms of their information systems.The Health Insurance Portability and Accountability Act of 1996 (HIPAA) is
causing a major reassessment regarding privacy and related issues in
healthcare information systems. Because of the lack of security in previously
implemented WLANs used in these organizations, data encryption
is a must for compliance in these facilities.
Structurally, hospitals offer a variety of radio frequency obstacles.You
should be aware of X-ray areas in particular, because most hospitals have
lead-lined or extremely thick walls surrounding these areas to prevent
X-ray bleed-through. Consider these areas “dead zones” to radio frequency.
If coverage is necessary in these areas, you will have to install
your antenna directly in each room requiring coverage. Full site surveys
are recommended for all areas of a hospital, and all equipment normally
used in the hospital should be on during the survey.
 Warehouses Warehouses generally contain stock and rows of shelving.
This presents coverage problems due to the density of the stock items
and the metal construction of the shelving. Be sure to find out what the
current stock levels are when performing your site survey. If the stock
levels are high, you will get a much more accurate picture of coverage in
your wireless implementation.The type of product that is stored in the
warehouse also makes a difference . For example, a fully stocked warehouse
that contains only cases of empty plastic water jugs will have
better coverage with fewer APs than the same warehouse containing
cases of full plastic water jugs.
 Metal Construction In general, WLAN radio devices do not penetrate
metal construction very well, if at all. Keep this in mind when
designing your WLAN. In most cases, you will need to place antennas in
each area that is contained by metal construction.Your site survey will
aid you in verifying this requirement.
 Other Construction The materials used in the construction of walls,
pillars, and supports can also cause radio frequency impairments and
impair coverage in a given area. Exterior walls tend to be thicker and
contain more reinforcement materials than interior walls and partitions.
Rebar (metal rods used to increase the strength of concrete construction)
reinforced cinder block or concrete walls and pillars can present a
design issue in most facilities that use them. In general, the denser the
material, the more difficult it will be for radio waves to penetrate.  RF-producing Devices In addition to the construction and application
considerations in your site survey and wireless design, you should
constantly be on the lookout for potential interference from other electronic
devices.There are many devices that can potentially cause interference
and require you to change your AP channel assignments.The
most common culprits are 2.4 GHz cordless phones and microwave
ovens. Be sure to have someone use these devices if they are inside a
coverage area in your design.This will help you to determine the best
channel to use on your AP.
Other potential interfering devices can be arc welding and telemetry
equipment, 2.4 GHz lighting systems, and Spectralink phone systems.
Spectralink phone systems are used to provide cellular phone coverage
within a company and are based on the IEEE 802.11b standard—the
same standard used for WLANs.