Wireless Local Area Networks

Contents


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.

Infrared Transmission

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 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.

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

  1. 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.
  2. Links C. , Diepstraten W. and Hayes V. Universal Wireless LANs. Byte. (May 1994) Vol. 19 No. 5 pp 99-108.
  3. Chen K. Medium Access Control of Wireless LANs for Mobile Computing. IEEE Network. (Sept/Oct 1994) pp 50-63.
  4. Cox D. Wireless Personal Communications : What is it? IEEE Personal Communications. (1994) Vol. 1 No. 1 pp 20-31.

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