<< . .

( 36)

. . >>

4. Enable “Permit Multi-Room Viewing” for all TiVo that will trade content.
5. Click Save Preferences on the Web site.
6. Now on the TiVo DVR screen, go to Phone and Network Setup and select “Connect to
the TiVo service now” to download the latest settings.

Once the Home Media Option is enabled on both TiVo units, an update to the TiVo service
has taken place, and 5 minutes have elapsed, the TiVo units will now be able to view content on
each other through the “Now Playing on TiVo” screen.
Scroll to the bottom of the “Now Playing on TiVo” screen to see other TiVos in your home
network. Click the TiVo icon to view its contents.
Make sure the TiVo units are all on the same TCP/IP subnet due to possible limitations with
Home Media Option discovery and routing. Theoretically, two TiVo units can share content
even though they are miles apart. Try it out!

TiVo Hacks
To really dig in to the possibilities of using TiVo to the extreme, open up the world of hardware
and software hacks. There are myriad sites available with endless helpful hints on hacking TiVo.
There are also books available that spell out step-by-step instructions for performing a number
of tweaks and upgrades. If you have a TiVo, it is money well spent to pick up one or two of
these TiVo hacking books.
Table 12-3 shows a list of TiVo hacking destinations.
And for a wealth of information, search for “TiVo Hacks” or “TiVo Hacking.”

Table 12-3 Some TiVo Hacking Resources
Name Description From

Hacking TiVo Book by Jeff Keegan John Wiley & Sons
TiVo Community Online discussion forums
DealDatabase Online discussion forums

302 Part IV ” Just for Fun

Table 12-3 (continued)
Name Description From

PTVupgrade Online discussion forums
TiVo Hack FAQ Web site devoted to TiVo hacks
TiVo Steve-o Web site by Steve Jenkins
9th Tee Online TiVo Upgrade Retailer

TiVo has some great features. If you own one, you already use it to time-shift your viewing like
crazy. Adding wireless makes it even better. Now program updates occur in a flash. All of the
media options break the barrier between TV and PC. Listening to MP3s on an entertainment
center surround sound system is tremendous!
TiVo has changed the way we watch TV. With a wireless connection, you have unlocked addi-
tional features that may change the way you share digital media.
Next, get into a long-distance relationship by creating a wireless network capable of communi-
cating over 15 miles or more. Learn what you need to know to set up a link and keep it run-
ning. Beam a signal from your house to a friend in the next county with a couple of add-on
antennas and a little planning.
Create a Long-
Distance Wi-Fi Link
here will come a time when you are ready to span a great divide
using wireless. It™s obvious that free space radio signals can travel
great distances. Previous chapters spoke at length about high-gain
antennas, picking up signals while wardriving, and even broadcasting a
signal to the neighbors.
in this chapter
But what about beaming a signal 5, 10, 20 miles, or more? Wireless is a
natural replacement for land lines, T1s, DSL, and other high-speed data
Selecting a site
when needed in a remote location. Or even a location that™s not so remote,
but where DSL or cable Internet may not be available. Figure 13-1 shows
Choosing an
a prime example. A long-distance Wi-Fi link creates a high-bandwidth
connection to the mountain operating at the speed of light.
Creating a long-distance link gathers many of the essentials of wireless
Determining the
and adds a healthy dose of physics to overcome the obstacles of a long-
Fresnel zone
distance, free space link.
This chapter is a compilation of practical guidelines designed to enable
Planning the link
you to establish a long-distance Wi-Fi link of your own . For your conve-
nience, we™ve condensed the most essential aspects of strategy, design,
Testing your
and experimental deployment for you here. We™ll start with site selection,
then take on design considerations including antenna location considera-
tions, and work our way through important hindrances”such as Fresnel
zones, path loss, and the Earth™s curvature”many of which can be mathe-
matically determined. We™ll move on with a discussion on link planning
and actual deployment strategies, and conclude the chapter with tips and
recommendations for creating a successful link.
A typical long-distance Wi-Fi link will require:
¤ Two wireless access points or wireless bridges
¤ Two high-gain, directional antennas
¤ Two people
¤ Spotting scope or binoculars (optional)
¤ Topographic software (optional)
¤ Handheld radio system/cellular phone
304 Part IV ” Just for Fun

FIGURE 13-1: Experimental link to a mountaintop eight miles away.

Selecting a Site
One of the most important fundamental aspects of setting up a long-distance wireless link lies
in the matter of site selection. Choosing the proper location of your links can mean the differ-
ence between a quick and easy setup and a long day of problems when it finally comes time to
establish the link.
The time you spend on initial site selection can be drastically reduced by using topographical
mapping software. This easy-to-use software can show the terrain profile of a line drawn
between two or more points (see Figure 13-2). From that line, you can quickly gauge whether
or not line-of-sight is possible given the terrain.
In the case of large obstructions blocking your path, you™ll need to seek an alternative. One
alternative is to employ a repeater, as was described in Chapter 9 (see Figure 13-3). Other solu-
tions are to shift the site requirements slightly. You can run Ethernet cabling up to 100 meters
from network equipment, and fiber cable can be run for several kilometers. The possibility of
stretching the wired portion of the link horizontally or vertically to a suitable transmission
point is apparent.
Chapter 13 ” Create a Long-Distance Wi-Fi Link

FIGURE 13-2: Forgiving terrain for a mountain-to-valley link.

Software is only the first step. You™ll need to make an actual site visit to determine if foliage,
buildings, or other obstructions will interfere with the link path. One of the best tools for this
is a spotting scope (see Figure 13-4). Binoculars will also help, but the magnification level is
not as high as a spotting scope.

As magnification increases, things like field of view, image brightness, image steadiness, and
even sharpness decrease. Also, higher magnifications are much more sensitive to atmospheric
turbulence and pollution. Be sure to use the magnification to just cover the distance between
your antennas.

Design Considerations
When you set up your long-distance Wi-Fi link, there are several factors to consider, includ-
ing background research and testing. Through the course of this section, we™ll work our way
through the most commonly used types of antennas, followed by antenna location, and
finally review potential obstacles and impedance problems and how to deal with them
306 Part IV ” Just for Fun

FIGURE 13-3: A mountaintop repeater may be necessary to establish this link.

FIGURE 13-4: Spotting scope used to determine direction and angle.
Chapter 13 ” Create a Long-Distance Wi-Fi Link

Antenna Types
There are several types of antennas and characteristics to consider for deployment in a dis-
tanced WLAN. This section is a synopsis of the most common types you should be aware of.
First, let™s review some important general definitions.

Isotropic antenna. A hypothetical, loss-less antenna that has an equal radiation intensity
in all directions. Used as a zero dB gain reference in directivity calculation (gain).
Antenna gain. Basically a measure of directivity, it is defined as the ratio of the radiation
intensity in a given direction that would be obtained if the power accepted by the
antenna was radiated equally in all directions. (Antenna gain is expressed in dBi.)
Radiation pattern. A graphical representation in either polar or rectangular coordinates
of the spatial energy distribution of an antenna.
Side lobes. The radiation lobes in any direction other than the main lobe.
Omnidirectional antenna. Radiates and receives equally in all directions in azimuth.
Directional antenna. Radiates and receives most of the signal power in one direction.
Antenna bandwidth. The directiveness of a directional antenna is defined as the angle
between two half-power ( 3 dB) points on either side of the main lobe of radiation.

We™ll only focus on three types of antennas you could deploy in your outdoor WLAN as a link
between two points or point-to-multipoint: the dipole antenna, coaxial antenna, and the dish
antenna. Albeit interrelated, each type has its own design strengths.
A dipole antenna is a straight electrical conductor measuring half a wavelength from end-to-
end and connected at the center to a radio frequency. This antenna, also called a doublet, is one
of the simplest types of antennas, and constitutes the main RF radiating and receiving element
in various sophisticated types of antennas. The dipole is inherently a balanced antenna, because
it is bilaterally symmetrical. For best performance, a dipole antenna should be more than half a
wavelength above the ground, the surface of a body of water, or other horizontal, conducting
medium such as sheet metal roofing. The element should also be at least several wavelengths
away from electrically conducting obstructions such as supporting towers, utility wires, guy
wires, and other antennas.
A coaxial antenna is a variant of the dipole antenna, designed for use with an unbalanced feed
line. One side of the antenna element consists of a hollow conducting tube through which a
coaxial cable passes. The shield of the cable is connected to the end of the tube at the center of
the radiating element. The center conductor of the cable is connected to the other half of the
radiating element. The element can be oriented in any fashion, although it is usually vertical.
A dish antenna (also known simply as a dish, see Figure 13-5) is common in microwave sys-
tems. This type of antenna consists of an active, or driven, element and a passive parabolic or
spherical reflector. The driven element can be a dipole antenna or a horn antenna. The reflector
has a diameter of at least several wavelengths. As the wavelength increases (and the frequency
decreases), the minimum required dish diameter becomes larger. When the dipole or horn is
properly positioned and aimed, incoming electromagnetic fields bounce off the reflector, and
308 Part IV ” Just for Fun

FIGURE 13-5: A parabolic dish antenna.

the energy converges on the driven element. If the horn or dipole is connected to a transmitter,
the element emits electromagnetic waves that bounce off the reflector and propagate outward
in a narrow beam.

Antenna Location
A long-distance Wi-Fi link is not an easy accomplishment. There are many factors working
against successful communications such as distance, open space, interference, obstructions, and
inherent equipment limitations. To start building a strategy, you should consider location very
carefully. Your radio signal path must have a clear, line-of-sight path”end-to-end”and a clear
Fresnel zone (covered in more detail later on). Be sure to use GPS and a spotting scope to visu-
ally map and sight your path over long distances. Incidentally, Fresnel zone losses of up to 6 dB
can be avoided by ensuring that there are no objects large enough to act as diffracting edges
within the first 0.6 Fresnel zone. If a large, rounded object is in your path, losses may exceed 20
dB through several Fresnel zones. This will force you to mount your antennas on towers or
buildings at a significant height. Unfortunately, microwave frequencies can also be affected by
too much antenna height, and the signal can be degraded due to ground reflections canceling
out the signal. Signals will propagate through a few obstructions such as trees or small build-
ings, and the radio signal will slightly extend over the line-of-sight horizon, but you shouldn™t
always count on it.
Chapter 13 ” Create a Long-Distance Wi-Fi Link

Table 13-1 Microwave Attenuation
Frequency (MHz) Approximate Attenuation (dB/meter)

432 0.10“0.30
1296 0.15“0.40
2304 0.25“0.50
3300 0.40“0.60
5600 0.50“1.50
10000 1.00“2.00

For all practical purposes, it™s safe to assume that if light can™t penetrate a stand of trees,
microwave losses will be unacceptable. Consider Table 13-1.

Potential Obstacles and Impedance
Although typically microwaves are not affected by the ionized layers in our atmosphere because
these layers are higher than the normal line-of-sight transmission of the signals, temperature
inversions can still prove to be a problem. This is because as the hot air rises, moisture rising
within the air causes attenuation of the signal. One might assume that lower microwave fre-
quencies are affected by water vapor and oxygen, but this is not the case.
Also consider the temperature effects on paths such as: reflections, refractions, diffractions,
transmission “ducts” and even tropospheric reflections and scattering. These atmospheric con-
ditions can cause a link to fail even though you have visual line-of-sight. A basic understanding
of these conditions may help you when troubleshooting a long-distance link.
Other sources of performance degradation in frequency hopping systems are spectrum back-
ground noise, received signal fading, interference from other services in that frequency range, ran-
dom FM components in the signal, “click” noise resulting from the phase discontinuities between
frequency hops, errors in receiver synchronization, or even the wind moving your antennas.

The antennas will also have to have the same RF signal polarization. The polarization of the
signal will depend on the direction the actual antenna is positioned. If it™s up/down, the polar-
ization is vertical; if the antenna is left/right, the polarization is horizontal. If the antenna is
diagonal (45 degrees usually), you™ll have diagonal polarization. By not having the same polar-
ization on your network™s antennas, you can receive a 20 dB loss of signal strength. This is an
enormous loss, but can also be very useful. By changing antenna polarization, you can help
eliminate certain types of radio interference, or allow many antennas in one location.
Horizontal antenna polarization at microwave frequencies will generally provide less multipath
and may also provide lower path loss in non line-of-sight situations, but you should always
experiment with different polarizations.
310 Part IV ” Just for Fun

Try to avoid installing your antenna in areas that are located near multipoint microwave distribu-
tion system (MMDS) or instructional television fixed service (ITFS) transmitter sites. You can query
FCC or PerCon frequency databases for the coordinates to transmitter locations in your area. You
can then look up the sites via these coordinates at the Tiger map server here: http://tiger.cen- You should also note that MMDS uses horizontal antenna
polarization, so if you need to locate your antenna site near one, use vertical polarization. Other
things to look out for at your antenna site are high-power PCS wireless cell phone transmissions in
the 1.8“1.9 GHz band, broadband noise from high-power colocated transmitters, harmonics from
mobile radio and paging transmitters, and other nearby microwave links.

The proper Earth grounding of your antenna tower is essential for lightning protection and
static discharge. Many towers are inadequately grounded by using only a few grounding rounds
and large gauge round copper cables. This is not correct. The small number of grounding rods
are inadequate, and round copper cable has a relatively high impedance to an instantaneous rise
in electric current (lightning hit). Extremely high voltages will develop across these cables and
instead of going to ground, these charges will go directly into your building equipment. A min-
imum of four ground rods per tower leg with some sort of chemical grounding material should
be used. The chemical grounding material will help to lower the ground rod resistance. Copper
straps should be used to connect the ground rods to the tower due to their low inductance. In
areas with sandy soil or excessive wind-generated static, it™s advisable to use a more elaborate
grounding method. Most likely a radial grounding system like that found in AM radio.
You should also try to have all your transmission line runs inside the tower column. This will
help shield them from lightning if it hits the tower. You should also securely bond the lines to
the tower every 15 meters or so. Use the recommended bonding kits that your tower manufac-
ture approves of.

Beam Tilt
Antennas mounted on very high towers may need to take into account beam tilt. Beam tilt is
needed when a radiating signal™s vertical beam width is narrowed (by using high-gain anten-
nas), and the areas near the tower location lose service because most of the signal is wasted by
broadcasting into open air. The beam must be tilted either mechanically or electrically to steer
the signal back into its proper location.
Mechanical beam tilting is relatively easy. The antenna can be mounted slightly less than 90
degrees from the horizontal plane so the tilted beam illuminates the desired service area.
However, in the opposite direction, the signal will be pointed toward the sky, reducing the
effective service area in that direction of the antenna.
If the signal needs to be “bent” downward in all directions around the antenna site, an electrical
tilting method must be used. This is commonly referred to as “null fill”. Electrical tilting is pro-
duced by controlling the current phase in the antenna itself. Thus, it must be done during the
antenna™s design stages by an engineer with expensive equipment.

Finally, consider potential weather problems. Ice buildup on antenna elements will result in an
increased SWR (impedance mismatch, standing wave ratio) that will de-tune a transmitter sys-
Chapter 13 ” Create a Long-Distance Wi-Fi Link

tem, significantly reducing its output power. Ice can also cause severe transmission line damage,
and falling icicles can kill. The easiest way to prevent ice buildup is with special antenna heaters
or by covering the antenna system with a fiberglass radome. Radomes will increase the wind
load on the tower and antenna heaters can be expensive. For more information, visit the Web at:

There™s a great guidebook to building your own custom WLAN antenna on the Web at

Determining the Fresnel Zone
In radio communications”especially given a point-to-point signal between two antennas”the
Fresnel zone is part of the concentric ellipsoids of revolution of a circular aperture. In other
words Fresnel zones are caused by diffraction by a circular aperture; it™s an elliptical region sur-
rounding the line-of-sight path between the transmitting and receiving antennas. To further
explain, imagine line-of-sight between two antennas with a signal that spreads out in an ellipti-
cal path between the ends (shown in Figure 13-6). The path is divided into different zones that
accommodate radio waves that are traveling at different velocities. The Fresnel zone™s radius at
the point where the ends of the ellipse peak out (known as the midpoint) should be free and
clear to provide adequate signal strength. In a good long-distance Wi-Fi design, you should
calculate the elliptical shape to determine height and placement of your antennas. In this sec-
tion we will examine Fresnel zone calculations in detail, but first we™ll review some of the
obstacles to take into consideration.

Path Loss and Earth Curvature
Path loss between two antennas in a long-distance Wi-Fi link can be caused by a number
of objects, including buildings, trees, and landscape features such as protruding hills
(see Figure 13-7), and even open air (which we™ll talk more about later). However, although
many times it is insignificant, one entity to also consider is the curvature of the Earth
given by the distance between the two endpoints. Typically this can be an issue when the
distance between endpoints is over 10 miles.

FIGURE 13-6: Fresnel zone. Notice the elliptical path between
312 Part IV ” Just for Fun

FIGURE 13-7: Path loss. The elliptical path between endpoints should be clear from obstruction.

For all practical purposes, let™s assume that the earth is a sphere with a radius of 3,958 miles. If
you are at some point on the Earth and move tangent to the surface for a distance of 1 mile,
then you can form a right-angled triangle as shown in the diagram in Figure 13-8. Next, using
the theorem of Pythagoras, a2 39582 12 15665765. Therefore, a 3958.000126 miles.
As a result of this calculation, your position is 3958.000126 3958 0.000126 miles above the
Earth™s surface. Furthermore, 0.000126 miles 12 5280 0.000126 7.98 inches. For this
reason, we can speculate that the Earth™s surface curves approximately 8 inches given the particu-
lars in the scenario. Eight inches isn™t very much, and a mile isn™t all that far either. But this
“dropoff ” of the horizon adds up over distance. If your long-distance link goes much farther than
a few miles, you will need to raise antenna elevation to compensate for the curvature of the Earth.
As shown in Figure 13-8, we™re assuming the Earth has a radius of 3,958 miles and you are at
some point on the surface moving in tangent for 1 mile.

<< . .

( 36)

. . >>

Copyright Design by: Sunlight webdesign