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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