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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

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.

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:

How far is the bridge link?

Is there clear line of sight?

What is the minimum acceptable data rate at which the link will run?

Is this point to point or point to multipoint?

Are the proper antennas available for testing?

Has a path-loss analysis been performed (or some calculation utility used to check figures)?

Is there physical access to both of the bridge locations?

Have the proper permits, if any, been obtained?

Will there be two engineers available for this survey? (Never attempt to survey or perform work on a roof or tower alone.)

Have the products been configured prior to any on-site visit?

Are the proper tools and equipment available to complete the survey?