Wireless Local Area Networks
- Transmission Technology
- Microwave Transmission
- Spread Spectrum Transmission
- Infrared Transmission
- Technical Standards
- Types of Wireless LAN
- Ad-hoc Networks
- Infrastructure Networks
Wireless local area networks (WLANs) are the same as the traditional LAN but
they have a wireless interface. With the introduction of small portable devices
such as PDAs (personal digital assistants), the WLAN technology is becoming very
popular. WLANs provide high speed data communication in small areas such as a
building or an office. It allows users to move around in a confined area while
they are still connected to the network. Examples of wireless LAN that are
available today are NCR's waveLAN and Motorola's ALTAIR.
In this article, the transmission technology used in WLANs is considered. We
will also discuss some of the technical standards for WLANs developed by the IEEE
Figure 1 : The Motorola Envoy (PDA) 
There are three main ways by which WLANs transmit information : microwave,
spread spectrum and infrared.
Motorola's WLAN product (ALTAIR) transmits data by using low powered microwave
radio signals. It operates at the 18GHz frequency band.
Spread Spectrum Transmission
With this transmission technology, there are two methods used by wireless LAN
products : frequency hopping and direct sequence modulation.
- Frequency Hopping
The signal jumps from one frequency to another within a given frequency range.
The transmitter device "listens" to a channel, if it detects an idle time (i.e.
no signal is transmitted), it transmits the data using the full channel
bandwidth. If the channel is full, it "hops" to another channel and repeats the
process. The transmitter and the receiver "jump" in the same manner.
- Direct Sequence Modulation
This method uses a wide frequency band together with Code Division Multiple Access
(CDMA). Signals from different units are transmitted at a given frequency range.
The power levels of these signals are very low (just above background noise).
A code is transmitted with each signal so that the receiver can identify the
appropriate signal transmitted by the sender unit.
The frequency at which such signals are transmitted is called the ISM
(industrial, scientific and medical) band. This frequency band is reserved for
ISM devices. The ISM band has three frequency ranges : 902-928, 2400-2483.5 and
5725-5850 MHz. An exception to this is Motorola's ALTAIR which operates at
Spread spectrum transmission technology is used by many wireless LAN manufacturers
such as NCR for waveLAN product and SpectraLink for the 2000 PCS.
This method uses infrared light to carry information. There are three types of
infrared transmission : diffused, directed and directed point-to-point.
The light source used in infrared transmission depends on the environmemt. Light
emitting diode (LED) is used in indoor areas, while lasers are used in outdoor
The infrared light transmitted by the sender unit fills the area (e.g. office).
Therefore the receiver unit located anywhere in that area can receive the signal.
The infrared light is focused before transmitting the signal. This method
increases the transmission speed.
- Directed point-to-point
Directed point-to-point infrared transmission provides the highest
transmission speed. Here the receiver is aligned with the sender unit.
The infrared light is then transmitted directly to the receiver.
Infrared radiation (IR) has major biological effects. It greatly affects the
eyes and skin. Microwave signals are also dangerous to health. But with proper
design of systems, these effects are reduced considerably.
Technical standards are one of the main concerns of users of wireless LAN products.
Users would like to be able to buy wireless products from different manufacturers
and be able to use them on one network. The IEEE Project 802.11 has set up
universal standards for wireless LAN. In this section we will consider some of
In March 1992 the IEEE Project 802.11 established a set of requirements for
wireless LAN. The minimum bandwidth needed for operations such as file transfer
and program loading is 1Mbps. Operations which need real-time data transmission
such as digital voice and process control, need support from time bounded services.
Types of Wireless LAN
The Project 802.11 committee distinguished between two types of wireless LAN :
"ad-hoc" and "infrastructred" networks.
Figure 2 : (a) Infrastructred Wireless LAN; (b) Ad-hoc Wireless LAN. 
Figure 2b shows an ad-hoc network. This network can be set up by a number mobile
users meeting in a small room. It does not need any support from a wired/wireless
backbone. There are two ways to implement this network.
Suppose that a mobile user A wants to send data to another user B in the same area.
When the packets containing the data are ready, user A broadcasts the packets. On
receiving the packets, the receiver checks the identification on the packet. If
that receiver was not the correct destination, then it rebroadcasts the packets.
This process is repeated until user B gets the data.
- Temporary Infrastructure
In this method, the mobile users set up a temporary infrastructure. But this
method is complicated and it introduces overheads. It is useful only when there
is a small number of mobile users.
Figure 2a shows an infrastructure-based network. This type of network allows users
to move in a building while they are connected to computer resources.
The IEEE Project 802.11 specified the components in a wireless LAN architecture.
In an infrastructure network, a cell is also known as a Basic Service Area (BSA).
It contains a number of wireless stations. The size of a BSA depends on the power
of the transmitter and receiver units, it also depends on the environment. A
number of BSAs are connected to each other and to a distribution system by Access
Points (APs). A group of stations belonging to an AP is called a Basic Service Set
(BSS). Figure 3 shows the basic architecture for wireless LANs.
Figure 3 : Architecture for Wireless LANs 
Wireless LAN provide high speed data communication. The minimum data rate specified by
the IEEE Project 802.11 is 1Mbps. NCR's waveLAN operates at 2Mbps, while Motorola's
ALTAIR operates at 15Mbps.
Because of their limited mobility and short transmission range, wireless LANs can be
used in confined areas such as a conference room. In the U.S, almost all WLANs products
use spread spectrum transmission. Therefore they transmit information on the ISM band.
But with this frequency band, users can experience interference from other sources
using this band.
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for an article on Intelligent Agents (by M. K. Hauge).