A wireless local area network links two or more devices using some wireless distribution
method, and usually providing a connection through an access point to the wider Internet.
This gives users the ability to move around within a local coverage area and still be
connected to the network. Most modern WLANs are based on IEEE 802.11 standards, marketed
under the Wi-Fi brand name. Wireless LANs have become popular in the home
due to ease of installation, and in commercial complexes offering wireless access to their
customers; often for free. New York City, for instance, has begun a pilot program to
provide city workers in all five boroughs of the city with wireless Internet access.
History Norman Abramson, a professor at the University
of Hawaii, developed the world’s first wireless computer communication network, ALOHAnet,
using low-cost ham-like radios. The system included seven computers deployed over four
islands to communicate with the central computer on the Oahu Island without using phone lines.
WLAN hardware initially cost so much that it was only used as an alternative to cabled
LAN in places where cabling was difficult or impossible. Early development included
industry-specific solutions and proprietary protocols, but at the end of the 1990s these
were replaced by standards, primarily the various versions of IEEE 802.11. An alternative
ATM-like 5 GHz standardized technology, HiperLAN/2, has so far not succeeded in the market, and
with the release of the faster 54 Mbit/s 802.11a and 802.11g standards, it is even more unlikely
that it will ever succeed. In 2009 802.11n was added to 802.11. It operates
in both the 2.4 GHz and 5 GHz bands at a maximum data transfer rate of 600 Mbit/s.
Most newer routers are able to utilise both wireless bands, known as dualband. This allows
data communications to avoid the crowded 2.4 GHz band, which is also shared with Bluetooth
devices and microwave ovens. The 5 GHz band is also wider than the 2.4 GHz band, with
more channels, which permits a greater number of devices to share the space. Not all channels
are available in all regions. A HomeRF group formed in 1997 to promote a
technology aimed for residential use, but it disbanded at the end of 2002.
Architecture 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 controllers. Wireless stations fall
into one of two categories: wireless access points, and clients. Access points, normally
wireless routers, are base stations for the wireless network. They transmit and receive
radio frequencies for wireless enabled devices to communicate with. Wireless clients can
be mobile devices such as laptops, personal digital assistants, IP phones and other smartphones,
or fixed devices such as desktops and workstations that are equipped with a wireless network
interface. Basic service set
The basic service set is a set of all stations that can communicate with each other. Every
BSS has an identification called the BSSID, which is the MAC address of the access point
servicing the BSS. There are two types of BSS: Independent BSS,
and infrastructure BSS. 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.
Extended service set An extended service set is a set of connected
BSSs. Access points in an ESS are connected by a distribution system. Each ESS has an
ID called the SSID which is a 32-byte character string.
Distribution system A distribution system connects access points
in an extended service set. The concept of a DS can be used to increase network coverage
through roaming between cells. DS can be wired or wireless. Current wireless
distribution systems are mostly based on WDS or MESH protocols, though other systems are
in use. Types of wireless LANs
The IEEE 802.11 has two basic modes of operation: ad hoc mode and infrastructure mode. In ad
hoc mode, mobile units transmit directly peer-to-peer. In infrastructure mode, mobile units communicate
through an access point that serves as a bridge to other networks.
Since wireless communication uses a more open medium for communication in comparison to
wired LANs, the 802.11 designers also included encryption mechanisms: Wired Equivalent Privacy,
Wi-Fi Protected Access, to secure wireless computer networks. Many access points will
also offer Wi-Fi Protected Setup, a quick method of joining a new device to an encrypted
network. Peer-to-peer
An ad hoc network is a network where stations communicate only peer to peer. There is no
base and no one gives permission to talk. This is accomplished using the Independent
Basic Service Set. A WiFi Direct network is another type of network
where stations communicate peer to peer. In a Wi-Fi P2P group, the group owner operates
as an access point and all other devices are clients. There are two main methods to establish
a group owner in the Wi-Fi Direct group. In one approach, the user sets up a P2P group
owner manually. This method is also known as Autonomous Group Owner. In the second method,
also called negotiation-based group creation, two devices compete based on the group owner
intent value. The device with higher intent value becomes a group owner and the second
device becomes a client. Group owner intent value can depend on whether the wireless device
performs a cross-connection between an infrastructure WLAN service and a P2P group, remaining power
in the wireless device, whether the wireless device is already a group owner in another
group and/or a received signal strength of the first wireless device.
A peer-to-peer network 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. This can basically occur in devices within a closed
range. 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.
IEEE 802.11 defines the physical layer and MAC layers based on CSMA/CA. 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 out of range of each other.
Bridge A bridge can be used to connect networks,
typically of different types. A wireless Ethernet bridge allows the connection of devices on
a wired Ethernet network to a wireless network. The bridge acts as the connection point to
the Wireless LAN. Wireless distribution system
A Wireless Distribution System enables the wireless interconnection of access points
in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access
points without the need for a wired backbone to link them, as is traditionally required.
The notable advantage of WDS over other solutions is that it preserves the MAC addresses of
client packets across links between access points.
An access point can be either a main, relay or remote base station. A main base station
is typically connected to the wired Ethernet. A relay base station relays data between remote
base stations, wireless clients or other relay stations to either a main or another relay
base station. A remote base station accepts connections from wireless clients and passes
them to relay or main stations. Connections between "clients" are made using MAC addresses
rather than by specifying IP assignments. All base stations in a Wireless Distribution
System must be configured to use the same radio channel, and share WEP keys or WPA keys
if they are used. They can be configured to different service set identifiers. WDS also
requires that every base station be configured to forward to others in the system as mentioned
above. WDS may also be referred to as repeater mode
because it appears to bridge and accept wireless clients at the same time. It should be noted,
however, that throughput in this method is halved for all clients connected wirelessly.
When it is difficult to connect all of the access points in a network by wires, it is
also possible to put up access points as repeaters. Roaming
There are two definitions for wireless LAN roaming:
Internal Roaming: The Mobile Station moves from one access point to another AP within
a home network because the signal strength is too weak. An authentication server performs
the re-authentication of MS via 802.1x. The billing of QoS is in the home network. A Mobile
Station roaming from one access point to another often interrupts the flow of data among the
Mobile Station and an application connected to the network. The Mobile Station, for instance,
periodically monitors the presence of alternative access points. At some point, based on proprietary
mechanisms, the Mobile Station decides to re-associate with an access point having a
stronger wireless signal. The Mobile Station, however, may lose a connection with an access
point before associating with another access point. In order to provide reliable connections
with applications, the Mobile Station must generally include software that provides session
persistence. External Roaming: The MS moves into a WLAN
of another Wireless Internet Service Provider and takes their services. The user can independently
of his home network use another foreign network, if this is open for visitors. There must be
special authentication and billing systems for mobile services in a foreign network.
Applications Wireless LANs have a great deal of applications.
Modern implementations of WLANs range from small in-home networks to large, campus-sized
ones to completely mobile networks on airplanes and trains. Users can access the Internet
from WLAN hotspots in restaurants, hotels, and now with portable devices that connect
to 3G or 4G networks. Oftentimes these types of public access points require no registration
or password to join the network. Others can be accessed once registration has occurred
and/or a fee is paid. References