TEAM FLY WIRELESS NETWORK DEPLOYMENTS phần 10 doc

Chapter 12

WIRELESS LANs NETWORK DEPLOYMENT IN

PRACTICE

ANAND R. PRASAD, ALBERT EIKELENBOOM, HENRI MOELARD, AD

KAMERMAN AND NEELI PRASAD

Wireless Communications and Networking Division, Lucent Technologies, Nieuwegein, The

Netherlands

Abstract: Wireless LANs most commonly use the Industrial, Scientific, and Medical

(ISM) frequency band, of 2.45 GHz. Although there have been a variety of

proprietary solutions, the IEEE approved a standard, 802.11, that organizes

this technology. Planning the network, which fulfills the requirements of the

user in such systems, is a major issue. In this chapter we will discuss some

critical issues faced during wireless LAN deployments from a practical point

of view.

236 Chapter 12

1. INTRODUCTION

Proliferation of computers and wireless communication together has

brought us to an era of wireless networking. Continual growth of wireless

networks is driven by, to name a few, ease to install, flexibility and mobility.

These benefits offer gains in efficiency, accuracy and lower business costs.

The growth in the market brought forward several proprietary standards for

Wireless Local Area Networks (WLANs), this chaos was resolved by

harmonizing effort of IEEE with an international standard on WLANs: IEEE

802.11 [1].

Wireless LANs in a Nutshell

Wireless LANs mostly operate using either radio technology or infrared

techniques. Each approach has it own attribute, which satisfies different

connectivity requirements. Majority of these devices are capable of

transmitting information up to several 100 meters in an open environment. In

figure 1 a concept of WLAN interfacing with a wired network is given. The

components of WLANs consist of a wireless network interface card, often

known as station, STA, and a wireless bridge referred to as access point, AP.

The AP interface the wireless network with the wired network (e.g. Ethernet

LAN) [1], [2], [3].

Wireless LAN Deployment in Practice 237

The most widely used WLANs use radio waves at the frequency band of

2.4 GHz known as ISM (industrial, scientific and medical) band. The release

of the ISM band meant the availability of unlicensed spectrum and prompted

significant interest in the design of WLANs. An advantage of radio waves is

that they can provide connectivity for non line of sight situations also. A

disadvantage of radio waves is the electromagnetic propagation, which

might cause interference with equipment working at the same frequency.

Because radio waves propagate through the walls security might also be a

problem.

WLANs based on radio waves usually use spread spectrum technology

[2], [4], [5]. Spread spectrum spreads the signal power over a wide band of

frequencies, which makes the data much less susceptible to electrical noise

than conventional radio modulation techniques. Spread spectrum modulators

use one of the two methods to spread the signal over a wider area: frequency

hopping spread spectrum, FHSS, or direct sequence spread spectrum, DSSS.

FHSS works very much as the name implies. It takes the data signal and

modulates it with a carrier signal that hops from frequency to frequency as a

function of time over a wide band of frequencies. On the other hand direct

sequence combines a data signal at a sending STA with a higher data rate bit

sequence, thus spreading the signal in the whole frequency band.

Infrared LANs working at 820 nm wavelength provide an alternative to

radio wave based WLANs. Although infrared has its benefits it is not

suitable for mobile applications due to its line of sight requirement. There

are two kinds of infrared LANs, diffused and point to point.

The first WLAN products appeared in the market around 1990, although

the concept of WLANs was known for some years. The worldwide release of

the ISM band at 2.4 GHz meant the availability of unlicensed spectrum and

prompted significant interest in the design of WLANs. The next generation

of these WLAN products is implemented on PCMCIA cards (also called PC

card) that are used in laptop computers and portable devices[2], [3], [6]. The

major technical issues for WLAN systems are size, power consumption, bit

rate, aggregate throughput, coverage range and interference robustness.

Considered Wireless LAN

In this chapter we consider WLANs based on DSSS technology as given

by IEEE 802.11. The IEEE 802.11 WLAN based on DSSS is initially aimed

for the 2.4 GHz band designated for ISM applications as provided by the

regulatory bodies world wide [1], [2], [3].

The DSSS system provides a WLAN with 1 Mbit/s, 2 Mbit/s, 5.5 Mbit/s

and 11 Mbit/s data payload communication capability. According to the FCC

regulations, the DSSS system shall provide a processing gain of at least 10