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Wireless LAN
A wireless LAN or WLAN is a
wireless local area network, which is the linking of two or more computers
without using wires. WLAN utilizes spread-spectrum technology based on radio
waves to enable communication between devices in a limited area, also known as
the basic service set. This gives users the mobility to move around within a
broad coverage area and still be connected to the network.
For the home user, wireless has become popular
due to ease of installation, and location freedom with the gaining popularity of
laptops. For the business, public businesses such as coffee shops or malls have
begun to offer wireless access to their customers; some are even provided as a
free service.
Benefits
The popularity of wireless LANs is a testament
primarily to their convenience, cost efficiency, and ease of integration with
other networks and network components. The majority of computers sold to
consumers today come pre-equipped with all necessary wireless LAN technology.
The benefits of wireless LANs include:
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Mobility:
With the emergence of public wireless networks, users can access the internet
even outside their normal work environment. Most chain coffee shops, for
example, offer their customers a wireless connection to the internet at little
or no cost.
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Productivity:
Users connected to a wireless network
can maintain a nearly constant affiliation with their desired network as they
move from place to place. For a business, this implies that an employee can
potentially be more productive as his or her work can be accomplished from any
convenient location.
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Deployment:
Initial setup of an
infrastructure-based wireless network requires little more than a single
access point. Wired networks, on the other hand, have the additional cost and
complexity of actual physical cables being run to numerous locations (which
can even be impossible for hard-to-reach locations within a building).
Disadvantages
Wireless LAN technology, while replete with the
conveniences and advantages described above, has its share of downfalls. For a
given networking situation, wireless LANs may not be desirable for a number of
reasons. Most of these have to do with the inherent limitations of the
technology.
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Security:
Wireless LAN transceivers are designed to serve computers throughout a
structure with uninterrupted service using radio frequencies. Because of space
and cost, the antennas typically present on wireless networking cards in the
end computers are generally relatively poor. In order to properly receive
signals using such limited antennas throughout even a modest area, the
wireless LAN transceiver utilizes a fairly considerable amount of power. What
this means is that not only can the wireless packets be intercepted by a
nearby adversary's poorly-equipped computer, but more importantly, a user
willing to spend a small amount of money on a good quality antenna can pick up
packets at a remarkable distance; perhaps hundreds of times the radius as the
typical user. In fact, there are even computer users dedicated to locating and
sometimes even hacking into wireless networks, known as wardrivers.
On a wired network, any adversary would first have to overcome the physical
limitation of tapping into the actual wires, but this is not an issue with
wireless packets. To combat this consideration, wireless networks users
usually choose to utilize various encryption technologies available such as
Wi-Fi protected access.
Some of the more older encryption methods, such as WEP are known to have
weaknesses that a dedicated adversary can compromise.
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Range:
The
typical range of a common 802.11g network with standard equipment is on the
order of tens of meters. While sufficient for a typical home, it will be
insufficient in a larger structure. To obtain additional range,
repeaters or
additional access points will have to be purchased. Costs for these items can
add up quickly. Other technologies are in the development phase, however,
which feature increased range, hoping to render this disadvantage irrelevant.
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Reliability:
Like any radio frequency transmission,
wireless networking signals are subject to a wide variety of interference, as
well as complex propagation effects (such as multipath, or especially in this
case Rician fading) that are beyond the control of the network administrator.
In the case of typical networks, modulation is achieved by complicated forms
of phase-shift keying (PSK) or quadrature amplitude modulation (QAM), making
interference and propagation effects all the more disturbing. As a result,
important network resources such as servers are rarely connected wirelessly.
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Speed:
The
speed on most wireless networks (typically 1-108 Mbps) is far slower than even
the slowest common wired networks (100Mbps up to several Gbps). There are also
performance issues caused by
TCP and its
built-in congestion avoidance.
For most users, however, this observation is irrelevant since the speed
bottleneck is not in the wireless routing but rather in the outside network
connectivity itself. For example, the maximum ADSL
throughput (usually 8Mbps or less) offered by telecommunications companies to
general-purpose customers is already far slower than the slowest wireless
network to which it is typically connected. That is to say, in most
environments, a wireless network running at its slowest speed is still faster
than the internet connection serving it in the first place. However, in
specialized environments, the throughput of a wired network might be
necessary. Newer standards such as 802.11n are addressing this limitation and
will support peak throughputs in the range of 100-200 mbps.
Architecture
Wireless LAN architecture using an
infrastructure BSS
You can connect through a cell phone card via
GSM network, satellite hardware from your satellite company, or the most common
a 802.11 router and either a network card for PCI/PCI express slot on a desktop,
or PCMCIA card for a laptop. Some laptops already come prepared with built-in
wireless.
802.11a
“The 802.11a amendment to the original
standard was ratified in 1999. The 802.11a standard uses the same core protocol
as the original standard, operates in 5 GHz band, with a maximum raw data rate
of 54 Mbit/s, which yields realistic net achievable throughput in the mid-20
Mbit/s. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6 Mbit/s if
required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4
to point to point. It is not interoperable with 802.11b, except if using
equipment that implements both standards. 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.”
802.11b
“The 802.11b amendment to the original
standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s
and uses the same CSMA/CA media access method defined in the original standard.
Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput
that an application can achieve is about 5.9 Mbit/s using TCP and 7.1 Mbit/s
using UDP. 802.11b products appeared on the market very quickly, since 802.11b
is a direct extension of the DSSS (Direct-sequence spread spectrum) modulation
technique defined in the original standard. The dramatic increase in throughput
of 802.11b (compared to the original standard) along with substantial price
reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN
technology. 802.11b is usually used in a point-to-multipoint configuration,
wherein an access point communicates via an Omni-directional antenna with one or
more clients that are located in a coverage area around the access point.
Typical indoor range is 30 m (100 ft) at 11 Mbit/s and 90 m (300 ft) at 1 Mbit/s.
With high-gain external antennas, the protocol can also be used in fixed
point-to-point arrangements, typically at ranges up to 8 kilometers (5 miles)
although some report success at ranges up to 80–120 km (50–75 miles) where line
of sight can be established. This is usually done in place of costly leased
lines or very cumbersome microwave communications equipment. 802.11b cards can
operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1 Mbit/s if
signal quality becomes an issue. Since the lower data rates use less complex and
more redundant methods of encoding the data, they are less susceptible to
corruption due to interference.”
802.11g
"In June 2003, a third modulation
standard was ratified: 802.11g. This works in the 2.4 GHz band (like 802.11b)
but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net
throughputs (like 802.11a). 802.11g hardware is compatible with 802.11b
hardware. Details of making b and g work well together occupied much of the
lingering technical process. In older networks, however, the presence of an
802.11b participant significantly reduces the speed of an 802.11g network. Even
though 802.11g operates in the same frequency band as 802.11b, it can achieve
higher data rates because of its similarities to 802.11a. The maximum range of
802.11g devices is slightly greater than that of 802.11b devices, but the range
in which a client can achieve the full 54 Mbit/s data rate is much shorter than
that of 802.11b. Despite its major acceptance, 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."
802.11n
In January 2004, IEEE announced that
it had formed a new 802.11 amendment to the 802.11 standard for wireless
local-area networks. The real data throughput is 100Mbit/s (which require an
even higher raw data rate at the physical layer), and is up to 50 times faster
than 802.11b, and up to 10 times faster than 802.11a or 802.11g. 802.11n builds
upon previous 802.11 standards by adding MIMO (multiple-input multiple-output).
MIMO uses multiple transmitter and receiver antennas to allow for increased data
throughput through spatial multiplexing and increased range by exploiting the
spatial diversity, through coding.
An 802.11n-only network may be impractical
for many users because the existing computer stock is predominantly 802.11b/g
only. Replacement of incompatible WiFi cards or of entire laptop stock is
necessary for older computers to operate on the network. Consequently, it may be
more practical in the short term to operate a mixed 802.11b/g/n network until
802.11n hardware becomes more prevalent.
The 802.11n technical draft is expected to be finalized in March 2009 with
publication in December 2009 but major manufacturers are already releasing
'pre-N', 'draft n' or 'MIMO-based' products based on early specs.
Depending on the manufacturer, a firmware update may
eventually be able to make current "Draft-N" hardware compatible with the final
version.
Current information suggests a maximum data throughput of approximately 300 mbit/s
based on both the wireless send and receive units having 4 antennae send arrays.
Lower antennae numbers will supply proportionately lower data transmission rates
(e.g. 2 antennae both ends ..... max possible throughput 150 mbit/s approx.)
Stations
All components that can connect into a
wireless medium in a network are referred to as stations. All stations are
equipped with wireless network interface cards (WNICs). Wireless stations fall
into one of two categories: access points and clients.
Access points
Access points (APs) are base stations
for the wireless network. They transmit and receive radio frequencies for
wireless enabled devices to communicate with.
Clients
Wireless clients can be mobile devices
such as laptops, personal digital assistants, IP phones, or fixed devices such
as desktops and workstations that are equipped with a wireless network interface
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Basic service set
The basic service set (BSS) is a set
of all stations that can communicate with each other. There are two types of
BSS: independent BSS and infrastructure BSS. Every BSS has an identification
(ID) called the BSSID, which is the MAC address of the access point servicing
the BSS.
Independent basic service set
An independent BSS is an ad-hoc
network that contains no access points, which means they can not connect to any
other basic service set.
Infrastructure basic service set
An infrastructure BSS can communicate
with other stations not in the same basic service set by communicating through
access points.
Extended service set
An extended service set (ESS) is a set
of connected BSSes. Access points in an ESS are connected by a distribution
system. Each ESS has an ID called the SSID which is a 32-byte (maximum)
character string. For example, "linksys" is the default SSID for Linksys
routers.
Distribution system
A distribution system connects access
points in an extended service set. A distribution system is usually a wired LAN
but can be a wireless LAN.
Types of wireless LANs
Peer-to-peer
Peer-to-Peer or
ad-hoc wireless LAN
A peer-to-peer (P2P)
allows wireless devices to directly communicate with each other. Wireless
devices within range of each other can discover and communicate directly without
involving central access points. This method is typically used by two computers
so that they can connect to each other to form a network.
If a signal strength
meter is used in this situation, it may not read the strength accurately and can
be misleading, because it registers the strength of the strongest signal, which
may be the closest computer.
802.11 specs define
the physical layer (PHY) and MAC (Media Access Control) layers. However, unlike
most other IEEE specs, 802.11 includes three alternative PHY standards: diffuse
infrared operating at 1 Mbps in; frequency-hopping spread spectrum operating at
1 Mbps or 2 Mbps; and direct-sequence spread spectrum operating at 1 Mbps or 2
Mbps. A single 802.11 MAC standard is based on CSMA/CA (Carrier Sense Multiple
Access with Collision Avoidance). The 802.11 specification includes provisions
designed to minimize collisions. Because two mobile units may both be in range
of a common access point, but not in range of each other. The 802.11 has two
basic modes of operation: Ad hoc mode enables peer-to-peer transmission between
mobile units. Infrastructure mode in which mobile units communicate via an
access point that serves as a bridge to a wired network infrastructure is the
more common wireless LAN application the one being covered. Since wireless
communication uses a more open medium for communication in comparison to wired
LANs, the 802.11 designers also included a shared-key encryption mechanism,
called wired equivalent privacy (WEP), or Wi-Fi Protected Access, (WPA, WPA2) to
secure wireless computer networks.
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