Page 68 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes 1. Microwave Transmission: Motorola’s WLAN product (ALTAIR) transmits data by using
low powered microwave radio signals. It operates at the 18GHz frequency band.
2. Spread Spectrum Transmission: With this transmission technology, there are two methods
used by wireless LAN products : frequency hopping and direct sequence modulation.
(a) 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.
(b) 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.
3. Infrared Transmission: This method uses infrared light to carry information. There are
three types of infrared transmission : diffused, directed and directed point-to-point.
(a) 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.

(b) Directed: The infrared light is focused before transmitting the signal. This method
increases the transmission speed.
(c) 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.
!

Caution 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.

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