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A
Quagmire of Wireless LAN Standards
By
Bhavish Sood
There
are many competing WLAN standards, each with its own advantages,
limitations and support groups. Get under the hood with
a through understanding of various WLAN standards
Wireless
LANs are the hot new enterprise technology that everyone
is talking about and considering implementing in varying
degrees for vertical markets like medical, education, and
manufacturing. Although the data communications market is
bringing out innovations at a break neck speed it is also
sending confusing signals to the consumers who are not sure
which standard will attract more products. Lets take a look
at some of the Wireless LAN standards and the technologies,
which power them.
Under
the hood
Before
delving straight into the details of Wireless LAN standards
lets take a look at technologies that makes it possible
for a wireless LAN to operate:
1.
Radio Frequency Systems: Radio Frequency is used for applications
where communications are over longer distances and terminals
are not seen by each other. The Radio Frequency signals
travel through walls and communicate where there is no direct
path between the terminals. In order to operate in the license
free portion of the spectrum called the ISM band (Industrial,
Scientific and Medical), the radio system must use a modulation
technique called Spread Spectrum (SS). In this mode a radio
is required to distribute the signal across the entire spectrum
and cannot remain stable on a single frequency. This is
done so that no single user can dominate the band and collectively
that all users look like noise. The fact that such signals
appear to be noise in the band means that they are difficult
to find and to jam.
2.
Infrared Systems: Infrared systems are rarely used in commercial
wireless LANs. Infrared systems use very high frequencies,
just below visible light in the electromagnetic spectrum,
to carry data. Like light, infrared cannot penetrate opaque
objects; it is either directed or diffuse technology. Inexpensive
directed systems provide very limited range (3 ft) and typically
are used for personal area networks but occasionally are
used in specific wireless LAN applications. High performance
directed infrared is impractical for mobile users and is
therefore used only to implement fixed sub-networks. The
application where infrared comes into use is as a docking
function and in applications where the power available is
extremely limited, such as a pager or PDA.
3.
Spread Spectrum Implementation: Most wireless LAN systems
use spread-spectrum technology; a wideband radio frequency
technique developed by the military for use in reliable,
secure, mission-critical communications systems. Spread-spectrum
is designed to trade off bandwidth efficiency for reliability,
integrity, and security. There are two methods of Spread
Spectrum modulation used to comply with the regulations
for use in the ISM band: Frequency Hopping Spread Spectrum
(FHSS) and Direct Sequence Spread Spectrum (DSSS).
-
Frequency Hopping Spread Spectrum (FHSS): Frequency-hopping
spread-spectrum (FHSS) uses a narrowband carrier that
changes frequency in a pattern known to both transmitter
and receiver. Properly synchronized, the net effect
is to maintain a single logical channel. To an unintended
receiver, FHSS appears to be short-duration impulse
noise.
-
Direct Sequence Spread Spectrum (DSSS): DSSS takes a
signal at a given frequency and spreads it across a
band of frequencies where the center frequency is the
original signal. The spreading algorithm, which is the
key to the relationship of the spread range of frequencies,
changes with time in a pseudo random sequence that appears
to make the spread signal a random noise source. The
strength of this system is that when the ratio between
the original signal bandwidth and the spread signal
bandwidth is very large, the system offers great immunity
to interference. For instance if a 1 Kbps signal is
spread across 1 GHz of spectrum, the spreading ratio
is one million times or 60 dB. This is the type of system
developed for strategic military communications systems
as it is very difficult to find and even more difficult
to jam.
Wireless
LAN Standards
Standards
are absolutely critical for both the consumer interest as
well as product manufacturers. The Wireless standards market
however is plagued by a plethora of standards promoted either
by Industry consortiums or by interest groups like IEEE,
which recently brought out IEEE 802.11a, which operates
at speeds of up to 54Mbps, allowing wireless LANs to accommodate
heavier, multimedia-rich traffic loads.
1.
IEEE 802.11: In the year 1990 the IEEE 802 standards groups
for networking setup a specific group to develop a wireless
LAN standard similar to the Ethernet standard. On June 26,
1997, the IEEE 802.11 Wireless LAN Standard Committee approved
the IEEE 802.11 specification; a major milestone in the
wireless arena for it now provides a solid specification
for the vendors to target, both for systems products and
components. This standard specifies a 2.4 GHz operating
frequency with data rates of 1 and 2 Mbps. The IEEE has
since published two supplements to this 802.11 standard:
802.11a (40 Mbps in the 5.8 GHz band), and 802.11b, (11
Mbps in the 2.4 GHz band).
2.
HiperLAN2: The HiperLAN2 technology takes advantage of the
latest developments in radio technology, MAC protocol for
high efficiency and quality of service (QoS), radio resource
management, power savings and plug-and-play. HiperLAN2 operates
in the dedicated spectrum in the 5 GHz band. It provides
connections, which can be assigned a specific QoS over the
air between the terminal and the base station. Whilst moving,
HiperLAN2 automatically performs handover to the nearest
base station (called access point). H2 also has strong security
support including both authentication and encryption and
has a built-in facility for automatic
frequency
allocation, removing the need for manual frequency planning.
The standard has tremendous benefits since it offers high-speed
wireless connectivity with up to 54 Mbps and seamless connectivity
with corporate LAN, 3G cellular systems, mobility and QoS
for future applications such as multimedia, voice over Internet
protocol (VoIP) and real-time video. By design HiperLAN2
supports time critical services as well as asynchronous
data. Thus packetized voice and video—and the QoS needed
to deliver them properly—have been included. This also
extends to the defined interfaces on the wired side; HiperLAN2
is compatible with ATM, 3G and 1394 networks as well as
IP networks. HiperLAN2 includes transmit power control and
dynamic frequency selection in the DLC/RLC layer, which
should provide greater spectrum efficiency and lower interference
probability with other systems operating on 5GHz. While
providing high-bandwidth connectivity ensures interoperability
with 3G cellular systems, HiperLAN2 is also complementary
to other wireless technologies such as Bluetooth.
3.
Bluetooth: Bluetooth is a short-range radio technology that
allows voice and data connections to be made up to 10 meters
(about 30 feet). The range can be extended to 100 meters
with an amplifier. The first generation of the technology
delivers performance up to 1Mbps. Subsequent versions may
deliver anywhere from 2 Mbps to 12 Mbps of throughput. Spanning
telecommunications, personal computing, and networking,
Bluetooth frees users from having to wrestle with numerous
cords and enables a host of new conveniences.
4.
HomeRF: IrDA: Infrared Data Association (IrDA) standard
provides a cordless, point-to-point cable replacement technology
operating from 0 to 1 meter at 9600 bps to 4 Mbps. Although
Bluetooth enables movement and greater distance between
devices than IrDA, IrDA will still be useful for some functions
such as quick e-business card exchange and faster data transfer.
Complexity
Keeps Growing
Signal
interference, cost and standards obstacles, and on top of
this, everyone has a competing technology, from Bluetooth,
Home RF, and phone line networking, to Sun's Jini, Universal
Plug and Play, and Apple's FireWire. This pace of innovation
and company promoted consortiums and standards are as usual
creating hindrance in both their adoption and use. HiperLAN
a standard currently being drafted by companies like Tenovis
(Bosch), Dell, Ericsson, Nokia, Telia, Texas Instruments
is fast gaining popularity and 802.11 a is already threatening
the Bluetooth popularity, its clear that no one clear standard
will see the light of the day and standards will be geographically
concentrated, at least for now.
Bhavish
Sood can be reached at bhavishsood@netscape.net
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