Feasibility Study
This section explains what is required to determine whether a
successful bridge link can be accomplished.
When determining the feasibility of a successful bridge link,
you need to define how far the bridge link is expected to transit, at what
frequency, and at what radio data rate. Very close bridge links (such as 1 mile
or less) are fairly easy to achieve assuming there are no obstructions. This is
referred to as a clear line of sight (LoS).
If both sites are very close, a link might be attained from a
window by using one of the upper floors of the building, avoiding the need to
install the bridge outdoors. This might work fine for a temporary event or in a
pinch to get a link up when time or weather conditions do not allow for a more
permanent solution. Keep in mind that some windows have metallic content for
tinting or conductive gas for insulation to prevent fogging, and such materials
might impede the radio signal, preventing a working link, even for short
distances. Therefore, links through glass are not a preferred method, but could
work for very short links.
In one real-world case, two bridges were used as a temporary
link between two buildings. Because it was temporary, the bridges were placed in
unused areas of the buildings, with the antennas located in the windows. The
bridges had no problem achieving a connection through windows, but the network
soon started to have troubles at a similar time each day. It turned out that the
areas in which the bridges were located at this time of day, and the office
inhabitants were closing the blinds (made of aluminum) each day to keep the
sun's glare out.
When preparing for a bridge system, you need to consider
several factors. LoS is a must for any outdoor bridge link of more than 100 feet
or so. You must also consider two distance parameters: the Fresnel zone and the
earth's curvature or bulge. These two factors impact you antenna height choices.
Environmental condition such as rain, fog, and snow do not have a big effect on
2.4-GHz or 5 GHz-links.
Determining Line of Sight
Because radio waves used by 2.4-GHz
and 5-GHz bridges are very high in frequency, the radio wavelength is relatively small.
As a result, the radio waves do not travel nearly as far (given the same amount
of power) as radio waves on lower frequencies. This fact also has an advantage:
It makes the bridge ideal for unlicensed use because the radio waves do not
travel far unless a high-gain antenna that can tightly focus the radio waves in
a given direction is used, reducing interference possibilities. Remember from Chapter 2 that high-gain antennas focus
radio waves, allowing them to go much farther, similar to adjusting the focus of
a flashlight from a flood type light into a tight beam. This not only provides
greater range, it provides a much smaller focus for both transmit and receive,
reducing also the possibility of interference to other systems as well as from
other systems. This in turn also means they are more critical to proper
alignment.
The higher the frequency used, the more dependent a system
becomes upon LoS. Therefore, longer distances (more than a couple hundred feet)
using 2.4- or 5-GHz products require LoS for successful operation. It is also
very difficult to acquire a good communication link when attempting to transmit
2.4- or 5-GHz Z radio waves through objects such as trees, foliage, hills, or
other buildings because these objects can absorb or reflect radio signals away
from the intended target. Distances greater than 6 miles (9.6 km) generally
require radio towers or high locations to overcome the LoS obstruction caused by
the curvature of the earth.
As frequency increases, so does signal loss through the
atmosphere. This is known as free-space loss or
just path loss. As the signal propagates from the
antenna, its power level decreases at a rate that is inversely proportional to
the distance and proportional to the wavelength of the signal. You can use this
variable to determine the maximum distance a bridge link can go. You can find
utilities available on the web that have been developed to assist in this
calculation. One such utility is the Cisco Outdoor Bridge Range Calculation
Utility available on the Cisco website.
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You can calculate the theoretical maximum distance for an RF
system in an outdoor environment before ever stepping outside the office by
using the following equations:
Distance = (300 / Freq) * (Conversion from metric to miles) *
EXP ((System gain First wavelength loss margin) / 6 * Natural log (2))
TO measure a wavelength in miles, the first part of the formula
is used:
(300 / Frequency) * (39 / 12) * (1 / 5280).
Then the overall system performance based on antennas, cables,
and radio capabilities is calculated:
System gain = Transmitter power + Antenna 1 gain Cable 1 loss +
Antenna 2 gain Cable 2 loss + Receiver sensitivity
Make sure to add in any losses for other devices such as
lightning arrestors or splitters.
The efficiency of an antenna to convert electrical energy to
radiated energy is 22 dB:
Distance = (300 / 2442) * (39 / 12) * (1 / 5280) * EXP ((Ant /
Radio parms 22 10) / 6 * LN(2))
The system gain determines how much overall path loss is
possible. It takes into account the gain of antennas at both ends of the RF
link, the transmitter power and minimum receiver sensitivity, and any associated
RF cables. Subtracting from this value, the efficiency of an antenna to convert
signals into radiated signals (the 22 in the formula) provides the signal
strength at a distance of one wavelength from the antenna. The 10 in the formula
provides an extra 10 dB of margin (fade margin) in the event of environmental
condition changes.
But the most useful item of this mathematical formula is that 6
* LN(2) provides the doubling of distance for every 6 dB. After a system is
designed and working, you can make "rule of thumb" estimates. Every increase of
6 dB (higher antenna gain, shorter cables) will double the distance. With every
decrease of 6 dB (loss such as cables or lower antenna gain), the range will be
cut in half.
Consider, for example, a system designed to operate at 18 miles
at a given data rate with given antennas. A change in length to the RF cables on
each end, adding 3 dB more cable loss per end, results in a total change of 6 dB
in the system gain parameter. This means that the overall range will drop to 9
miles (6 dB less).
If the antennas on each end of the link change from a 21-dBi
antenna to a 13.5-dB antenna (7.5 dB change on each end, for an overall change
of 15 dB), the range will drop to less than 4 miles. This is calculated by
reducing the distance in half for the first 6-dB drop (9 miles) and in half
again for the next 6-dB drop (4.5 miles). The remaining 3 dB will reduce the
range a bit farther, for an estimation of 3.5 to 4 miles.
The same can be done for increasing antenna gain. If a system
has a maximum range to 10 miles using two 13.5-dBi antennas, and one antenna is
replaced with a 21-dBi antenna (increase of 7.5 dB), the range will double for
the first 6 dB increase (20 miles) and a slight amount more for the next 1.5 dB.
You could make an estimate of 22 miles or so.
This 6-dB/range-doubling estimation is only for outdoor ranges.
Indoor ranges vary dramatically, but in many cases 9 dB can be substituted for a
similar estimation. |
Although you can use a Global
Positioning System (GPS) and topographical maps to determine whether any
hills or obstructions are in the way, it is always best to first visit the site
and physically assess the site to determine whether the sites to be linked can
be visually connected. An on-site assessment can answer many questions up front,
but accessing the rooftop of the building or climbing a tower might be necessary
to successfully perform this task.
When conducting a visual inspection, check whether the remote
site is behind trees or other obstructions. This is where some simple logic can
come into play. To determine whether you need a tower, you can just raise a
small weather balloon (or any other type of balloon that you can raise the
appropriate distance) at one site and look for the balloon from the other site.
Even the low-cost helium foil balloons available at any party shop might work if
the wind is minimal. You might need binoculars or a telescope to view the
balloon for longer-distance links. You could use strobe lights if doing this
task at night. Measuring the string used to float your "spotter balloon" would
give you an idea about how high a radio tower or other structure would need to
be to support your bridge antenna. Another method of spotting is to raise a
bucket truck or vehicle with a telescoping mast.
If all the sites have visual connection from the central site,
installing the links might be a simple matter of determining the distances and
data rates desired. If the buildings do not have LoS directly between them, you
might be able to install a radio tower or use a nearby radio tower or mast to
get above the obstruction. Another possible approach is to find a location that
both sites can see and install a bridge pair or repeater. As shown in Figure 14-5, you could use another building
or structure, such as a water tower, for this purpose.
One drawback with this type of solution is the reduction of
throughput (50 percent) that occurs when using a single radio device as a
repeater. An alternative design, as shown in Figure 14-6, uses two separate RF links on separate
channels to reduce this throughput degradation.
Environmental Issues
Now that you have learned about how free-space path loss and
LoS can affect the distance of a bridge link, you need to examine a few other
variables that can degrade a bridge link.
You might have heard that rain, snow, fog, and other
high-humidity weather conditions can obstruct or affect the LoS, introducing a
small loss (sometimes referred to as rain fade or
fade margin). Generally, these
weather conditions have minimal effect on RF links running at frequencies under
10 GHz. If you have established a good stable connection, such weather will
almost never be an issue; however, if the link was poor to begin with, bad
weather could degrade performance or cause loss of the link.
For this reason, most path-loss calculations should include
some type of fade margin error. Usually 10 dB is sufficient for data networks
running 2.4- or 5-GHz systems.
Fresnel Zone
A Fresnel zone is an imaginary ellipse around the visual LoS
between the transmitter and receiver (see Figure 14-7). If radio waves (or even light waves)
encounter an obstruction in the Fresnel area as the signal travels through free
space to their intended target, it can be attenuated, sometimes severely. The
best performance and range is attained when there is no obstruction of this
Fresnel area. Although this is not always completely unavoidable, engineers
should try to maintain a clear zone for 60 percent of the Fresnel area. Also
keep in mind that a Fresnel zone is not only vertical, but actually surrounds
the signal in a 360-degree zone. Fresnel zone clearance in all directions must
be maintained.
To improve a Fresnel zone impeded by an obstruction, it might
be necessary to get above (or away from, if the obstruction is on the side of
the LoS such as very tall building) the obstruction, which usually requires
mounting the antenna higher. This might be a simple matter of mounting the
antenna at another point on the building, such as an elevator room, or other
structure higher on the building's roof. However, it might also mean adding a
taller mounting structure.
It is possible to calculate the radius of the Fresnel zone (in
feet) at any particular distance along the path using the following
equation:
In this equation, F1 = the first Fresnel zone radius (ft.), D =
the total path length (mi.), and f = frequency (GHz).
Normally 60 percent of the first Fresnel zone clearance is all
that is required for a good, stable link. As such, you can modify the preceding
formula for 60-percent Fresnel zone clearance as follows:
Of course, it is much easier to forget the math and rely on
several of the tools available via the Internet. Try doing a Google search for
"Fresnel Zone calculation." Make sure you are using a Fresnel zone calculator
that provides 60 percent clearances (otherwise the Fresnel value will be much
larger). The Cisco Outdoor Bridge Range Calculation Utility mentioned previously
provides this calculation.
One thing to remember is that these theoretical range
calculations are based on the flat earth. As Christopher Columbus learned back
in the year 1492, the earth is not flat. So the earth curvature (also known as
the earth bulge) must be taken into account when planning for paths longer than
approximately 7 miles. To calculate the approximate earth bulge, you can use the
following formula:
Where D = distance in miles, and H = the earth bulge in
feet.
Looking at Figure 14-9,
you can see that at the midpoint, the LoS clearance needs to take into account
the maximum earth bulge and the maximum Fresnel zone clearance (or 60 percent of
it).
The required antenna height can add up quite quickly. As the
distance between antennas increases, the overall required height increases as
well. For example, two sites separated by 20 miles would have an earth bulge of
approximately 70 feet, and the 60-percent Fresnel zone value would be
approximately 63 feet. Adding these two values results in a required antenna
height of around 133 feet. Keep in mind that is the height above any obstructions in the center of the path!
Determining the Possible Coverage Distance
Determining the maximum distance in a strictly point-to-point
bridge link is fairly easy. As you can imagine, when linking only two sites your
antenna choices become easier because you need to concentrate your radio signal
only in one direction at the central bridge and vice versa.
When two or more remote sites are connected to the central
site, the central bridge might require an antenna with a much larger field of
view. Unlike a point-to-point link, the central site now has to transmit in more
than one direction to establish a radio path with the other remote bridges.
Directional antennas are not practical unless all remote sites are in the
coverage pattern of a directional antenna. (If this is the case, the rules
require the maximum EIRP to be less than 36-dBm EIRP. See Chapter 3 for point-to-multipoint system
regulations.)
A site survey can flush out problems such as interference,
Fresnel zone issues, or logistics problems that occur when installing a bridge
system. A proper site survey should involve temporarily setting up a bridge link
and taking some measurements to determine whether your antenna calculations
proved accurate and that you have picked the right location and antenna before
you spend a lot of time drilling holes, routing cables, and mounting
equipment.
Before attempting a site survey, you should have already
determined the following:
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How far is the bridge link?
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Is there clear line of sight?
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What is the minimum acceptable data rate at which the link will
run?
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Is this point to point or point
to multipoint?
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Are the proper antennas available for testing?
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Has a path-loss analysis been performed (or some calculation
utility used to check figures)?
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Is there physical access to both of the bridge locations?
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Have the proper permits, if any, been obtained?
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Will there be two engineers available for this survey? (Never
attempt to survey or perform work on a roof or tower alone.)
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Have the products been configured prior to any on-site
visit?
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Are the proper tools and equipment available to complete the
survey?
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