An Introduction to Wireless Technology, Second Edition

DSSS system that is minimally impacted by strong sources of radio

frequency interference; it is generally agreed that by its very nature SFH is

more immune and secure from this phenomenon.

5.3 Building a Radio LAN System

A radio LAN will consist of a number of stations which can be configured into a

LAN in various ways. These configurations or “topologies” are described below:

5.3.1 Topologies

There are many possible radio LAN topologies the main three type are as

follows:

•

Peer-to-peer

•

Base-to-remote

•

Base-to-remote with wired LAN

Peer-to-Peer

This is where there is no base station and traffic is directly transmitted

from user to user.

This mode of operation is considered essential because many users will

want to set up ad hoc connections between small numbers of stations on

an as-needed basis. This is shown in Figure 45.

Figure 45. Peer-to-Peer Wireless LAN

LANs such as these might be set up, used, and dispersed again within

the space of an hour.

Such a system could use multiple channels (FDM or CDMA) between

pairs of users or a single channel with TDM or CSMA operation to share

the available capacity between users.

106 An Introduction to Wireless Technology

Use of a Base Station

When a base station is used, all transmissions from workstations are

routed to or from the base station. This is shown in Figure 46 on

page 107.

Figure 46. Base-to-Remote Wireless LAN

The base station performs the function of a bridge (optionally) connecting

the radio LAN segment to a wired LAN.

Connection to a Wired LAN

Most often a radio LAN will require a connection to a wired LAN. In this

case the base station should perform the function of a bridge connecting

the radio LAN segment to the wired LAN segment. This is shown in

Figure 47 on page 108.

Chapter 5. Radio Communication in LANs 107

Figure 47. Base-to-Remote with Wired LAN

It is now possible to have multiple base stations connected together by a

wired LAN with a mobile user moving around and being passed from one

base station to another much like a cellular telephone user. There are

many applications in large plant environments where this is very useful.

The IBM Entry Level WLAN system is capable of supporting this function.

5.3.1.1 Topologies Compared

In comparing a peer-to-peer (ad hoc) configuration to a base-station

configuration the following points should be considered:

1. In a peer-to-peer configuration the maximum size of a LAN is the area which

encompasses the radii of the maximum transmission ranges of all the

terminals. (This assumes that each terminal is within the transmission range

of

all

the other terminals.) Figure 48 on page 109 explains this.

108 An Introduction to Wireless Technology

Figure 48. Peer-to-Peer LAN Size

In the base-station approach the maximum size of the LAN is the area of a

circle whose

diameter

is equal to the maximum range of the transmission.

See Figure 49 to help with understanding this.

Figure 49. Base-Station LAN Size

Thus, the base-station approach allows a single radio LAN to be

geographically much larger than the peer-to-peer approach (all other

considerations being equal).

Chapter 5. Radio Communication in LANs 109