Wireless features operate at the physical layer; therefore, your wireless-interface feature
evaluation covers a broad range of features. The following sections cover these feature
categories:
NLOS — Non line-of-sight and near line-of-sight equipment capabilities
Wireless frequency bands — Propagation characteristics and equipment availability
for each of the license-free bands
Modulation types — Various types of modulation used by license-free equipment
Bandwidth and throughput — Tradeoffs between data rate, data throughput, and link
distance
Noise and interference-reduction features — Receiver and antenna features that
improve signal reception abilities
Security — Physical layer wireless security features
Miscellaneous wireless features — Transmit and receive features that can play a
significant role in the performance of your wireless network operation
Non Line-of-Sight Features
In the broadband wireless industry, there is no agreement about the exact meaning of the
term NLOS. Here are two common ways that NLOS is used:
Near line-of-sight — When equipment vendors state that their equipment has near-LOS
capabilities, they are claiming that it operates satisfactorily even when there is a
partially obstructed line-of-sight path, as long as there are not too many obstacles to the
line-of-sight path. For example, perhaps a few trees are intruding into the Fresnel zone.
Non line-of sight — When equipment vendors state that their equipment has non-LOS
capabilities, they are claiming that it operates satisfactorily even when there is an
obstructed line-of-sight path. For example, perhaps buildings, trees, and hills are
completely blocking the path.
Because there is no standard definition of NLOS, the process of evaluating NLOS performance
claims is a challenging one. Almost all vendors of NLOS equipment (either accidentally
or intentionally) exclude information about the range of their NLOS equipment. The
impression is left with the customer (you) that the NLOS equipment has the same communications
range as LOS wireless equipment. This is never the case; the range of NLOS
equipment is always substantially less than the range of equipment that is operating over a
true, unobstructed LOS path.
Now, you will learn about features that actually improve performance in NLOS environments.
Two significant challenges that an NLOS environment presents for wireless
equipment are as follows:
Multipath — Any equipment feature that improves performance in a multipath
environment also improves performance in an NLOS environment. These features are
as follows:
Diversity antennas
Circularly polarized antennas
Smart antennas that constantly adjust their beamwidth to receive and
transmit energy directly to and from each individual end user antenna
Adaptive equalization
Multicarrier modulation, such as OFDM
Whenever possible, always try to design your wireless WANs to use LOS paths. You
will achieve more reliable coverage at longer distances.
Attenuation — Attenuation losses in a non-LOS environment are the reason that the
communications range in an NLOS environment is less than in a LOS environment.
The following equipment and network features reduce attention and improve NLOS
performance:
Receiver sensitivity
900 MHz frequency band
Mesh networks
Wireless Frequency Bands
The following sections describe the license-free frequency bands, including the propagation
characteristics and the power levels of each band.
900 MHz
900 MHz is the lowest-frequency industrial, scientific, and medical (ISM) band. The total
width of the band is 26 MHz. Signals in this band have a wavelength of approximately
12 inches (30 cm). These signals have the capability to pass through some obstructions
without being completely lost. For example, they can pass through light trees and diffract
over one low hill and still be strong enough to be received several miles away. 900 MHz is
the best band to use when there are just a few obstacles to the LOS path. Table 6-1 shows
900-MHz power levels.
2.4 GHz
2.4 GHz is the middle ISM band. The total width of the band is 83 MHz. Signals in this
band have a wavelength of approximately 4.8 inches (12 cm). These signals have little
capability to pass through obstructions without being lost. Passing through one wall can
result in 10 to 12 dB of attenuation. Attenuation from trees varies depending on the
presence of leaves and whether the leaves are wet or dry but, on average, the attenuation
from trees is approximately .5 dB per meter. One 30-ft (10-meter) diameter tree (the tree
canopy/leaves are 30 feet across, not the tree trunk) results in about 5 dB of attenuation;
6 dB of attenuation reduces the length of a wireless link to 1/2 of its previous length. You
can see that passing a 2.4-GHz signal through a few trees can easily reduce the usable
length of the wireless path to a few hundred feet. Table 6-2 shows 2.4-GHz power levels.
3.5 GHz
The 3.5-GHz band is not available for use in the United States; however, some frequency
subbands between 3.3 and 4.0 GHz are available for use (usually on a licensed basis) in a
number of other countries. This band is mentioned here because equipment for this band is,
in many cases, similar to equipment for the 2.4-GHz band. Signals in this band have a
wavelength of approximately 9 cm (3.4 in). The propagation characteristics are somewhat
similar to the 2.4-GHz band, although attenuation from trees and other obstructions is
higher.
5 GHz
There are four license-free subbands at 5 GHz, although two of these bands overlap each
other. There is one ISM band from 5725 to 5850 MHz (5.725 to 5.850 GHz), and there are
three Unlicensed National Information Infrastructure (U-NII) bands: 5150 to 5250 MHz,
5250 to 5350 MHz, and 5725 to 5825 MHz. The ISM band is 125 MHz wide, and each
U-NII band is 100 MHz wide. Signals in the 5-GHz subbands have a wavelength of approximately
2 inches (5 cm). Each 5 GHz subband is wider than the entire 2.4-GHz band;
therefore, it is possible to build 5-GHz wireless equipment that provides more bandwidth
and more throughput than equipment for any other license-free band. The attenuation from
trees at 5 GHz is about 1.2 dB per meter; therefore, each 30-ft (10-meter) diameter tree
(crown) that blocks an LOS path reduces the length of a wireless link by approximately 75
percent. Table 6-3 shows 5-GHz power levels.
60 GHz
The 59 to 64-GHz ISM band was approved for use in the United States in 1999. The total
width of this band is almost 5 GHz. Signals in this band have a wavelength of about 2/10
of an inch (1/2 cm). Signals at this frequency are attenuated by the presence of oxygen in
the air; therefore, the maximum wireless link distance is approximately half a mile (800 m),
assuming that a LOS path is available. Obstructions completely block the signal. The
advantage of this band is that equipment is available that provides point-to-point raw data
rates up to 622 Mbps. In addition, the oxygen absorption means that the likelihood of interference
from other networks is low.
Modulation Types
This section covers the following information:
A quick review of the modulation process
A direct sequence spread spectrum (DSSS) description
A frequency hopping spread spectrum (FHSS) description
An orthogonal frequency division multiplexing (OFDM) description
A brief mention of other spread spectrum and non-spread types of modulation
Understanding the Modulation Process
Chapter 1, "An Introduction to Broadband License-Free Wireless Wide-Area Networking,"
defined modulation as the process of adding intelligence to the signal. The modulation
process creates a change in some combination of the amplitude, the frequency, or the phase
of a signal. Many types of modulation exist, including amplitude modulation used by
commercial AM broadcast stations and frequency modulation (FM) used by police departments
and fire departments, for example.
Spread spectrum modulation was originally designed for use by the military to camou.age
the existence of and the content of military communications. Descriptions of the two types
of spread spectrum modulation follow.
Direct Sequence Spread Spectrum (DSSS) Modulation
DSSS modulation simultaneously widens (spreads) a data signal out and reduces the
amplitude (technically, it reduces the power density) of the signal. The resulting modulated
signal resembles a low-level noise signal that is widely dispersed around a single frequency.
The modulated DSSS signal is wider than the bandwidth of the original data. For example,
an 11-Mbps raw data rate signal becomes a 22-MHz-wide DSSS signal. In the 2.4-GHz
frequency band, there is enough room for three nonoverlapping 11 Mbps-wide signalreless Equipment
channels. Each time a DSSS signal is transmitted, the wireless energy is centered around
only one frequency; therefore, DSSS modulation is a single-carrier modulation scheme.
Frequency Hopping Spread Spectrum Modulation
Frequency Hopping Spread Spectrum (FHSS) modulation does not spread its signal energy
out, but it rapidly shifts the energy from frequency to frequency. A narrowband FHSS signal
is transmitted first on one narrow (1-MHz) channel and then quickly shifted to another
channel, then another, and another, and so on. The rapid frequency hopping gives this type
of modulation its name. The two following types of FHSS are now allowed in the 2.4-GHz
band:
Narrowband frequency hopping — Narrowband FHSS signals are 1 MHz wide.
They hop using a total of 79 different frequencies. The signal can hop between these
frequencies in 78 unique hopping patterns or hopping sequences. 802.11 standards
define narrowband frequency hopping.
NOTE: Hopping sequences are sometimes different in different countries. Check with your
national telecommunications authority for the regulations in your country.
Wideband frequency hopping — Wideband FHSS signals can be up to 5 MHz wide
and can hop using a total of less than 75 different frequencies. Typical wideband
FHSS systems use far less than 75 frequencies; one such system uses 43 frequencies
with a signal that is 1.7 MHz wide. Wideband frequency hopping systems are
relatively new and are just beginning to be deployed in outdoor wireless WANs.
FHSS equipment changes its center frequency each time it hops; however, each time it
transmits, the wireless energy is still centered on only one frequency. FHSS is, therefore,
a single-carrier modulation scheme.
Orthogonal Frequency Division Multiplexing Modulation
Orthogonal Frequency Division Multiplexing (OFDM) modulation transmits bursts that
use more than one carrier frequency simultaneously. Compared to a DSSS signal, an
OFDM signal has the following characteristics:
Occupies the same amount of bandwidth
Uses 52 carriers instead of one carrier
Carries more information with each transmitted burst
Is more resistant to multipath fading
802.11a equipment uses OFDM modulation and operates on the 5-GHz band; 802.11g uses
OFDM on the 2.4-GHz band. OFDM is a multicarrier modulation scheme because it
transmits using more than one carrier simultaneously.
Other Spread Spectrum Modulation Types
Other types of spread spectrum modulation are now legal to use. These versions are proprietary
to particular manufacturers and do not interoperate with 802.11b, 802.11a, or 802.11g
systems.
One example of a proprietary spread spectrum modulation type is multicarrier DSSS.
Rather than using just one DSSS carrier, multicarrier DSSS uses several simultaneous
carrier frequencies to transport data. This multicarrier approach is a hybrid combination
of single-carrier and multicarrier modulation.
Other Nonspread Spectrum Modulation Types
In the ISM bands, the FCC originally required that spread spectrum modulation be used.
Recent rule changes now allow additional modulation types. In the U-NII bands, nonspread
spectrum modulation types are permitted, and equipment manufacturers use proprietary
digital modulation schemes to offer a variety of high-bandwidth point-to-point and pointto-
multipoint systems.
Bandwidth and Throughput
It is important for you to understand wireless throughput so that you can meet (or exceed)
the expectations of your wireless end users. This section describes the following:
The difference between the wireless data rate and the wireless throughput
The tradeoff between throughput and distance
Examples of low, medium, and high throughput equipment
Comparison Between Data Rate and Throughput (Including Simplex Versus Duplex
Throughput)
There is a common misunderstanding regarding the bandwidth, the data rate, and the
throughput of a wireless device:
Bandwidth refers to the raw data rate of the device.
Throughput refers to the actual amount of end user data that the device can transfer in
a given time interval.
The result of this misunderstanding is that wireless network users are frequently disappointed
in the wireless throughput (data transfer speeds) that they experience.
Understandably, wireless equipment manufacturers want their equipment to look as
attractive as possible to potential buyers. For this reason, they usually use the raw data rate
in their sales and advertising material. An 802.11b AP, for example, provides a raw data rate
of 11 Mbps.
Wireless users have a different expectation; they are interested in how fast a web page or a
file downloads. They are interested in the capability of the wireless device to deliver their
data. When the wireless users’ 802.11b AP delivers just 5.5 Mbps of data throughput, they
feel that there must be a problem with the equipment.
Most frequently, the real data throughput potential of a half-duplex wireless network is
approximately 50 percent of the raw data rate. An 802.11b AP operating at the maximum
11-Mbps raw data rate has a maximum throughput potential of about 5.5 Mbps. This
difference between raw data rate and actual throughput has several causes, including these:
The framing and signaling overhead
The half-duplex turnaround time between transmit and receive
The lower efficiency inherent in the transmission of small packets
Collisions between wireless users and interference from other networks can reduce the
throughput below 50 percent. Chapter 8, "Solving Noise and Interference Problems,"
discusses this issue in more detail.
Remember that your end users rely on you to set their throughput expectations realistically.
When they measure their throughput and discover that it meets or slightly exceeds the
throughput that you told them to expect, they will judge your wireless network performance
to be good.
Tradeoff Between Data Rate and Distance
As you evaluate wireless equipment, you will invariably compare different equipment
brands based on how long of a link they can support. Link distance is important; however,
during your comparison, it is important that you compare apples to apples. When you
compare two brands of wireless equipment side by side, you must compare their link
distances at the same data rate. Other factors being equal, the higher the throughput (or the
higher the raw data rate), the shorter the communications range. Table 6-4 lists the typical
outdoor link distances from an 802.11b AP (using a standard low-gain omnidirectional
antenna).
As the data rate increases, the maximum AP link distance decreases. AP data rates automatically
fall back to the next lower level when the AP detects the signal quality decreasing as
the link distance increases.
Sub-1 Mbps Data Rates
Two types of wireless systems operate at sub-1 Mbps data rates:
Low-speed (such as 4800 bps to 128 kbps) wireless modems that provide a point-topoint
wireless extension for an RS-232 serial data system.
128 kbps to 1 Mbps point-to-point or point-to-multipoint wireless Ethernet bridges or
AP systems. These systems are useful for Internet access.
-Mbps to 11-Mbps Data Rates 1
Most point-to-multipoint wireless WAN systems are in this category. This category also
includes both 802.11 (2 Mbps) and 802.11b (11 Mbps) systems.
12-Mbps to 60-Mbps Data Rates
This category includes both high-bandwidth point-to-point backbone equipment and pointto-
multipoint equipment. Here are some examples:
Point-to-point equipment is available with bandwidths of 12 Mbps, 20 Mbps,
24 Mbps, and 45 Mbps.
Point-to-multipoint equipment is available with shared, aggregate bandwidths of
20 Mbps, 40 Mbps, 54 Mbps, and 60 Mbps.
802.11a WLAN equipment is becoming available. However, as this book was being
written, this equipment was not yet appropriate for use in outdoor wireless networks.
The power level was too low, and there is no connector to attach an outdoor antenna.
Over 60-Mbps Data Rates
The higher in frequency wireless equipment operates, the more bandwidth that is available.
Products that provide more than 60 Mbps of bandwidth operate almost exclusively in the
5.3- and 5.8-GHz U-NII bands, although a few short-range products operate in the 60-GHz
band. Products that operate in these bands have aggregate bandwidths of 90 Mbps, 100 Mbps,
155 Mbps (OC-3), 200 Mbps, 480 Mbps, 622 Mbps (OC-12), and 872 Mbps. All these are
full-duplex products.
Noise and Interference Reduction Features Noise is defined as anything and everything other than the desired signal. Interference
reduces the throughput of a wireless network. Interference has many sources, so it is
important that you consider and utilize all possible noise-reduction features.
NOTE: Chapter 8 is devoted completely to the topic of understanding and minimizing the effects
of noise and interference. Refer to Chapter 8 for additional information as you read about
the following interference-reduction features.
In the outdoor wireless environment, many potential interference sources exist. You can use
a few equipment characteristics to provide some help in minimizing the effects of interference.
The following interference-reduction features operate at the physical layer to help reduce
the effects that interference can have on both AP and CPE throughput.
Receiver Selectivity Selectivity is the capability of a receiver to reject signals that are not exactly on the desired
receiving frequency. No receiver is perfectly selective; no receiver has the capability to
completely reject all off-frequency signals; therefore, all receivers are susceptible to being
overloaded by nearby, strong off-frequency signals. These off-frequency signals can be
within the license-free band (in-band interference), or they can be outside the band (out-ofband
interference).
Overloading causes a receiver to become desensitized (to experience a reduced sensitivity)
to the desired signals. The symptom of a desensitized receiver is a reduction in the receiving
distance. Some receivers allow you to configure a higher receive threshold level. This
feature enables you to intentionally reduce the sensitivity and therefore reduce the intensity
of the overloading. This feature is similar to a "squelch" control on an FM two-way radio.
It can be difficult for you to compare receiver selectivity and to predict overload resistance
because most manufacturers do not publish overload specifications. Keep the following
general guidelines in mind as you evaluate wireless equipment:
Wireless equipment that is designed for outdoor WAN use should be less susceptible
to being overloaded when compared to indoor wireless LAN equipment.
Wireless equipment that is designed for indoor LAN use is likely to be more
susceptible to being overloaded when used outdoors.
All wireless equipment might need to have an external bandpass filter added when
there are one or more strong, nearby transmitters such as FM, AM, or television
broadcast transmitters.
Multipath Resistance
Multipath fading is a fact of life at microwave frequencies. Multipath is caused when signal
reflections cause several signals (echoes) to be received almost simultaneously. Equipment
features that minimize the effects of multipath include the following:
Antenna diversity — Antenna diversity helps minimize multipath by using two
separate antennas. The antennas are separated from each other, and when the signal
fades, one antenna receives a stronger signal than the other antenna. The receiver
automatically selects the strongest antenna signal on each incoming packet, so fading
is reduced.
Circular antenna polarization — Circularly polarized antennas discriminate against
multipath interference. Equipment that offers the option of using a circularly polarized
antenna provides more protection against multipath compared to equipment without
a circularly polarized antenna.
OFDM — Equipment that uses orthogonal frequency division multiplexing
modulation provides more immunity to multipath interference compared to non-OFDM equipment.
Multipath interference is worse in a physical environment where you find many obstacles
that reflect wireless signals. The center of a city with many tall, flat, reflective metal
building surfaces is a high multipath environment. If you plan to deploy wireless service in
a high-multipath environment, use as many multipath-reduction features and techniques as
possible.
Miscellaneous Interference Reduction Techniques
There are many sources of noise and interference besides multipath. The following miscellaneous
features help reduce the distance-robbing effects of noise and interference:
Selectable antenna polarization — Interference from other wireless systems is
usually either vertically polarized or horizontally polarized. Equipment that allows
you to select either vertical or horizontal polarization allows you to minimize
interference from other systems by selecting polarization opposite to other, interfering
networks.
Smart antenna technology — Smart antennas enable the antenna pattern beamwidth
to be automatically adjusted under software control. In this way, the antenna pattern
can be automatically steered to minimize or avoid interference. Smart antenna
technology is a relatively new technology that is just beginning to appear in licensefree
wireless WAN equipment.
Smart radio features — Smart radio features include the radio’s capability to
automatically scan the available frequencies and to choose the frequency with the
least amount of interference. Automatic power adjustment is another smart radio
feature. The wireless equipment measures the strength and the quality of the received
signal and adjusts the transmit power level up or down to maintain the desired link
quality. Using only the amount of power needed minimizes the interference to other
wireless systems.
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