• Unbounded media have network signals that are not bound by
any type of fiber or cable; hence, they are also called wireless
technologies
• Wireless LAN media are becoming extremely popular in
modular office spaces.
• Today’s networks aren’t composed of a single technology or
wiring scheme—they are heterogeneous networks.
• Wireless technologies are just one way of solving a particular
networking need in a heterogeneous cabling system. Although
cabled networks are generally less expensive, more robust
transmission-wise, and faster (especially in the horizontal
environment), in certain situations, wireless networks can carry
data where traditional cabled networks cannot. This is
particularly the case in backbone or WAN implementations.
Everyone who has a
television with a remote
control has performed an
infrared transmission.
• It has a wavelength
• shorter than visible light
(actually, it’s shorter than the
red wavelength in the visible
spectrum)
• with more energy. Infrared is a
very popular method of
wireless networking. The
sections that
• follow examine some of the
details of infrared
transmissions.
• Infrared (IR) transmissions are signal transmissions that
use infrared radiation as their transmission method.
Infrared radiation is part of the electromagnetic
spectrum. It has a wavelength shorter than visible light
(actually, it’s shorter than the red wavelength in the
visible spectrum) with more energy. Infrared is a very
popular method of wireless networking.
• Infrared transmissions are very simple. All infrared connections
work similarly to LAN transmissions, except that no cable
contains the signal. The infrared transmissions travel through the
air and consist of infrared radiation that is modulated in order
to encode the LAN data.
• A laser diode, a small electronic device that can produce single
wavelengths or frequencies of light or radiation, usually
produces the infrared radiation. A laser diode differs from a
regular laser in that it is much simpler, smaller, and lower
powered; thus, the signals can only travel over shorter distances
(usually less than 500 feet).
• Besides needing an infrared transmitter, all devices that
communicate via infrared need an infrared receiver. The
receiver is often a photodiode, or a device that is sensitive to a
particular wavelength of light or radiation and converts the
infrared signals back into the digital signals that a computer
will understand.
•Cost
•Installation
•Capacity
•Attenuation
•
•
•
•
•
•
Relatively inexpensive
High bandwidths
No FCC license required
Ease of installation
High security on point-to-point connections
Portability
• Line of sight needed for focused transmissions.
• Weather attenuation.
• Radio-frequency
(RF)
transmission systems are those
network transmission systems
that use radio waves to
transmit data.
• 1999
• 2003
Radio waves have frequencies
from 10 kilohertz (kHz) to 1
gigahertz (GHz), and RF
systems use radio waves in this
frequency band. The range of
the electromagnetic spectrum
from 10kHz to 1GHz is called
radio frequency (RF).
ADVANTAGES OF RF
• No line of sight needed Radio waves can penetrate walls
and other solid obstacles, so a direct line of sight is not
required between sender and receiver.
• Low cost Radio transmitters have been around since the
early twentieth century. After 100 years, high-quality radio
transmitters have become extremely cheap to manufacture.
• Flexible Some RF LAN systems allow laptop computers with
wireless PC NICs to roam around the room while remaining
connected to the host LAN.
DISADVANTAGES OF RF
• Susceptible to jamming and eavesdropping Because RF signals are
broadcast in all directions, it is very easy for someone to intercept
and interpret a LAN transmission without the permission of the sender
or receiver. Those RF systems that use spread-spectrum encoding are
less susceptible to this problem.
• Susceptible to RF interference All mechanical devices with electric
motors produce stray RF signals, known as RF noise. The larger the
motor, the stronger the RF noise. These stray RF signals can interfere
with the proper operation of an RF-transmission LAN.
• Limited range RF systems don’t have the range of satellite
networks (although they can travel longer distances than infrared
networks). Because of their limited range, RF systems are normally
used for short-range network applications (e.g., from a PC to a
bridge, or shortdistance building-to-building applications).
• Microwave communication makes use of the lower
gigahertz frequencies of the electromagnetic spectrum.
These frequencies, which are higher than radio
frequencies, produce better throughput and
performance than other types of wireless
communications.
• Terrestrial microwave systems typically use directional
parabolic antennas to send and receive signals in the
lower gigahertz frequency range. The signals are
highly focused and must travel along a line-of-sight
path. Relay towers extend signals. Terrestrial
microwave systems are typically used when the cost of
cabling is cost-prohibitive.
•
•
•
•
•
Frequency range
Cost
Capacity
Attenuation
EMI
• Satellite microwave systems transmit signals between
directional parabolic antennas. Like terrestrial
microwave systems, they use low gigahertz frequencies
and must be in line of sight. The main difference with
satellite systems is that one antenna is on a satellite in
geosynchronous orbit about 50,000 kilometers
(22,300 miles) above the earth. Therefore, satellite
microwave systems can reach the most remote places
on earth and communicate with mobile devices.
•
•
•
•
•
Frequency range
Cost
Capacity
Attenuation
EMI
• Very high bandwidth
• Transmissions travel over long distances
• Signals can be point-to-point or broadcast
•
•
•
•
•
Equipment is expensive
Line of sight required
Atmospheric attenuation
Propagation delay
Safety
• Microwave equipment differs from infrared and RF
equipment because it is more specialized and is
usually only used for WAN connections.