Wireless Local
Area Networks
Contents
- Introduction
- Transmission Technology
- Microwave Transmission
- Spread Spectrum Transmission
- Infrared Transmission
- Technical Standards
- Requirements
- Types of Wireless LAN
- Ad-hoc Networks
- Infrastructure Networks
- Conclusion
- References
Introduction
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 Project 802.11.
Figure 1 : The Motorola Envoy (PDA) [2]
Transmission Technology
There are three main ways by which WLANs transmit
information : microwave, spread spectrum and infrared.
Microwave Transmission
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
18GHz.
Spread spectrum transmission technology is used by many wireless LAN
manufacturers such as NCR for waveLAN product and SpectraLink for the 2000
PCS.
Infrared Transmission
This method uses infrared light to carry information.
There are three types of infrared transmission : diffused, directed and directed
point-to-point.
- Diffused
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.
- Directed
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.
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 areas.
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
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 these standards.
Requirements
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.
[3]
Ad-hoc Networks
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.
- Broadcasting/Flooding
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.
Infrastructure Networks
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 [2]
Conclusion
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.
References
- Anderson D. and Molyneaux J. Three
Technical Hurdles on the Road to a Wirless office-integration, integration,
integration. 1992 IEEE International Conference on Selected Topics in
Wireless Communications. pp 387-390.
- Links C. , Diepstraten W. and Hayes
V. Universal Wireless LANs. Byte. (May 1994) Vol. 19 No. 5 pp 99-108.
- Chen K. Medium Access Control of
Wireless LANs for Mobile Computing. IEEE Network. (Sept/Oct 1994) pp
50-63.
- Cox D. Wireless Personal
Communications : What is it? IEEE Personal Communications. (1994)
Vol. 1 No. 1 pp 20-31.
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