·
Infrared
Infrared (IR) radiation is electromagnetic radiation of a
wavelength longer than that of visible light, but shorter than that of
microwave radiation. The name means "below red" (from the Latin
infra, "below"), red being the color of visible light of longest
wavelength.
Bluetooth
Is an industrial specification for wireless personal area
networks (PANs). Bluetooth provides a way to connect and exchange information
between devices like personal digital assistants (PDAs), mobile phones,
laptops, PCs, printers and digital cameras via a secure, low-cost, globally
available short range radio frequency.
FHSS
Frequency-hopping spread spectrum is a spread-spectrum method of
transmitting radio signals by rapidly switching a carrier among many frequency
channels, using a pseudorandom sequence known to both transmitter and receiver.
Spread-spectrum transmission offers these advantages over a fixed-frequency
transmission:
·
Highly resistant to
noise and interference.
·
Signals are difficult
to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary
noise burst or simply an increase in the background noise for short
Frequency-Hop codes on any narrowband receiver except a Frequency-Hop
spread-spectrum receiver using the exact same channel sequence as was used by
the transmitter.
·
Transmissions can
share a frequency band with many types of conventional transmissions with
minimal interference. As a result, bandwidth can be utilized more efficiently.
DSSS
direct-sequence spread spectrum is a modulation technique where
the transmitted signal takes up more bandwidth than the information signal that
is being modulated, which is the reason that it is called spread spectrum. Direct
Sequence Spread Spectrum (DSSS) uses one channel to send data across all
frequencies within that channel. Complementary Code Keying (CCK) is a method
for encoding transmissions for higher data rates, such as 5.5 and 11 Mbps, but
it still allows backward compatibility with the original 802.11 standard, which
supports only 1 and 2 Mbps speeds. 802.11b and 802.11g support this
transmission method.
Comparison of DSSS and
Frequency Hopped SS
|
DSSS
·
Flexible support of
variable data rates
·
High capacity is
possible with enhancements (interference cancellation, adaptive antenna,
etc.)
·
Suffers from
near-far effect
|
FHSS
·
Suitable for ad hoc
networks (no near-far problem)
·
Robust to
interference
·
Limited data rate
|
OFDM
Orthogonal frequency-division multiplexing, also called discrete
multitone modulation (DMT), is a transmission technique based upon the idea of
frequency-division multiplexing (FDM). OFDM (Orthogonal Frequency
Division Multiplexing) increases data rates by using a spread spectrum: modulation.
802.11a and 802.11g support this transmission method.
·
Used in some wireless
LAN applications, including WiMAX and IEEE 802.11a/g
·
Used in many
communications systems such as: ADSL, Wireless LAN, Digital audio broadcasting.
MIMO (Multiple Input
Multiple Output)
MIMO (Multiple Input Multiple Output) transmission, which uses
DSSS and/or OFDM by spreading its signal across 14 overlapping channels at 5
MHz intervals. 802.11n uses it. Use of 802.11n requires multiple antennas.
|
802.11a
|
802.11b
|
802.11g
|
802.11n
|
|
|
Data Rate
|
54 Mbps
|
11 Mbps
|
54 Mbps
|
248 Mbps (with 2×2
antennas)
|
|
Throughput
|
23 Mbps
|
4.3 Mbps
|
19 Mbps
|
74 Mbps
|
|
Frequency
|
5 GHz
|
2.4 GHz
|
2.4 GHz
|
2.4 and/or 5 GHz
|
|
Compatibility
|
None
|
With 802.11g and the
original 802.11
|
With 802.11b
|
802.11a, b, and g
|
|
Range (meters)
|
35–120
|
38–140
|
38–140
|
70–250
|
|
Number of Channels
|
3
|
Up to 23
|
3
|
14
|
|
Transmission
|
OFDM
|
DSSS
|
DSSS/OFDM
|
MIMO
|
Radio Frequency
Transmission Factors
Radio frequencies (RF) are generated by antennas that propagate
the waves into the air. Antennas fall under two different categories:
·
Directional
·
Omni-directional
Directional Directional antennas are commonly used in point-to-point
configurations (connecting two distant buildings), and sometimes
point-to-multipoint (connecting two WLANs). An example of a directional antenna
is a Yagi antenna: this antenna allows you to adjust the direction and focus of
the signal to intensify your range/reach.
Omni-directional Omni-directional antennas are used in
point-to-multipoint configurations, where they distribute the wireless signal
to other computers or devices in your WLAN. An access point would use an
omni-directional antenna. These antennas can also be used for point-to-point
connections, but they lack the distance that directional antennas supply
Three main factors influence signal distortion:
·
Absorption Objects that absorb the RF waves, such as walls, ceilings, and floors
·
Scattering Objects that disperse the RF waves, such as rough plaster on a wall,
carpet on the floor, or drop-down ceiling tiles
·
Reflection Objects that reflect the RF waves, such as metal and glass
Responsible body
The International Telecommunication Union-Radio Communication
Sector (ITU-R) is responsible for managing the radio frequency (RF) spectrum
and satellite orbits for wireless communications: its main purpose is to
provide for cooperation and coexistence of standards and implementations across
country boundaries.
Two standards bodies are primarily responsible for implementing WLANs:
Two standards bodies are primarily responsible for implementing WLANs:
·
The Institute of Electrical
and Electronic Engineers (IEEE)
·
The Wi-Fi Alliance.
IEEE Defines the mechanical process of how WLANs are implemented in
the 802.11 standards so that vendors can create compatible products.
The Wi-Fi Alliance Basically certifies companies by ensuring that
their products follow the 802.11 standards, thus allowing customers to buy WLAN
products from different vendors without having to be concerned about any
compatibility issues.
Frequencies bands:
WLANs use three unlicensed bands:
·
900 MHz Used by older
cordless phones
·
2.4 GHz Used by newer
cordless phones, WLANs, Bluetooth, microwaves, and other devices
·
5 GHz Used by the
newest models of cordless phones and WLAN devices
900 MHz and 2.4 GHz frequencies are referred to as the
Industrial, Scientific, and Medical (ISM) bands.
5 GHz frequency the Unlicensed National Information
Infrastructure (UNII) band.
Unlicensed bands are still regulated by governments, which might
define restrictions in their usage.
A hertz (Hz) is a unit of frequency
that measures the change in a state or cycle in a wave (sound or radio) or
alternating current (electricity) during 1 second.
802.11g
Suffers from the same interference as 802.11b in the already
crowded 2.4 GHz range. Devices operating in this range include microwave ovens,
Bluetooth devices, and cordless telephones. Since the 2.4 GHz band is heavily
used, using the 5 GHz band gives 802.11a the advantage of less interference.
However, this high carrier frequency also brings disadvantages. It restricts
the use of 802.11a to almost line of sight, necessitating the use of more
access points; it also means that 802.11a cannot penetrate as far as 802.11b
since it is absorbed more readily, other things (such as power) being equal.
802.11a
Transmits radio signals in the frequency range above 5 GHz. This
range is "regulated," meaning that 802.11a gear utilizes frequencies
not used by other commercial wireless products like cordless phones. In
contrast, 802.11b utilizes frequencies in the unregulated 2.4 GHz range and encounters
much more radio interference from other devices.
IEEE 802.11a / IEEE
802.11h
This is also a physical layer enhancement. IEEE 802.11a provides
significantly higher performance than 802.11b, at 54 Mbps. Unlike 802.11b, the
802.11a standard operates within the frequency range of 5.47 to 5.725 GHz and
is not subject to the same interference from other commercial electronic
products. This higher frequency band allows significantly higher speeds of
communication over the 2.4 GHz range.
802.11g APs are backward compatible with 802.11b APs. This
backward compatibility with 802.11b is handled through the MAC layer, not the
physical layer. On the negative side, because 802.11g operates at the same
frequency as 802.11b, it is subject to the same interferences from electronic
devices such as cordless phones. Since the standard’s approval in June 2003,
802.11g products are gaining momentum and will most likely become as widespread
as 802.11b products. Table II-1 displays basic 802.11b/a/g characteristics.
The common range of operation for 802.11b is 150 feet for a
floor divided into individual offices by concrete or sheet-rock, about 300 feet
in semi-open indoor spaces such as offices partitioned into individual
workspaces, and about 1000 feet in large open indoor areas. Disadvantages of
802.11b include interference from electronic products such as cordless phones
and microwave ovens.
Range
The layout of your building can reduce the range.
·
A lot of concrete
walls can reduce your range.
·
The size of the
antenna and the placement greatly affect the range of their signals
·
The weather and amount
of water vapor in the air can affect your signals strength
Speed
·
The layout of your
building can reduce the speed
·
The size of the
antenna and its signal can affect your speed
·
The weather and amount
of water vapor can weaken the signal and affect your speed
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