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Introduction

This tutorial discusses the various components of a Wireless LAN system which are common to all forms of Wireless LAN deployment.

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Wireless Client Receiver

Wireless LAN client receivers are needed to connect a computing device, e.g. laptop, PDA, desktop, to the wired network via an access point.  The receiver takes care of data processing in the physical and MAC layer of the OSI framework.  Depending on the interfaces of the computing devices, there are different forms of receivers, however most of them come in PCMCIA and Compact Flash cards, PCI/ISA adapters, or USB adapters.

Access Point

Access points (APs) are needed only in the Infrastructure Mode of WLANs.  They provide the wireless clients with a point of access into a network, as its name already suggests.  They are comparable to a sophisticated Ethernet switch and operate in half-duplex mode, i.e. they either receive or transmit at a given time.  APs can be used in different constellations, and therefore support three modes of operation known as Root Mode, Bridge Mode, and Repeater Mode:

• In the Root Mode, the AP is connected to a wired network and the wireless client directly accesses this network via the AP.  Also, several clients can talk to each other by means of using APs that are connected to the same backbone network.  Root Mode is the default configuration of most APs.
• The Bridge Mode of APs makes is possible to wirelessly connect two separate wired network segments with each other.  No clients use the AP to enter a network.  The bridging functionality of APs is only found in a few commercially available APs, which are significantly more expensive than their non-bridging counterparts. 

• An AP in Repeater Mode provides a wireless upstream link into a network instead of being hard-wired to the network and using its Ethernet port.  It functions as intermediary between the clients and an AP in root mode for entry into the network, and is thought to either extend the perceived range of a WLAN or to make WLAN deployment possible in difficult environments.  Since an AP in repeater mode connects to another AP (root mode) and the wireless clients, its throughput is reduced dramatically.  The repeater mode should only be used if absolutely necessary.

WLAN Antennas

The wireless transmission of data and voice necessarily needs antennas – at sender and receiver – for proper operation.  The selection of appropriate antennas can impact the functionality of a system dramatically.  However, none of the WLAN standards regulates the use of antennas and one is free to choose.  Please note that adding an antenna does not increase the power you originally started with, but focuses it in a particular direction as to increase reception, but also limits the beam radius.  Proper installation of the antenna is crucial.  There are a wide range of antennas available on the market and the following description of antennas and parameters may help in selecting the correct one for an application. 

• Onmidirectional Antennas: radiate the signal outward equally in all directions.  They are shaped like a tall pole and have their highest sensitivity in horizontal direction.  They are used to cover large areas where the exact location of the receiver is unknown.  One disadvantage is that an omnidirectional antenna picks up a lot of noise surrounding it and distributes the transmit power over all directions making for a weaker signal.  

• Sectorized Antennas: have a similar shape of omnidirectional, but have reflectors behind the pole that direct the transmitted energy in a certain direction.  Mostly these sectors are 60° to 120° and form the radiation pattern as needed.  This kind of antenna is used when multiple clients need to access an AP from the same direction.  They increase range and decrease interference.

• Yagi Antennas: consist of multiple elements that are all aligned to guide incoming radio waves from a particular direction to the receiving dipole of the antenna.  They are similar to old TV aerial, but smaller in size.  They are very directional and the radiation pattern has an opening angle of anywhere from 15° to 60°.  Adding more elements means more gain, a longer antenna, but also more cost.  These antennas are used for point-to-point or point-to-multipoint application where long distances need to be spanned. 

• Parabolic (dish) Antennas: are the most directional antennas and deliver the highest gains.  They are tightly focused on a distinct direction, which makes them ideal for point-to-point operations.  The dish can either be a meshed wire grid or solid metal.  The diameter ranges from several inch to a few feet.  A correctly aligned pair of parabolic antennas can extend the range of wireless networks up to 20 miles. 

Polarization plays an important role in antennas.  This parameter refers to the direction the electro-magnetic waves travel through the air.  This can be horizontal or vertical, but also – in very rare cases – a circular polarization is used.  Transmitter and receiver must be used in the same polarization or no communication is possible.  Omni-directional and sectorized antennas are vertically polarized, whereas Yagi and parabolic antennas can be either horizontally or vertically polarized, depending on environment and application.