lesson 5

Wireless Network Communications

Wireless Communications

Here radio (or infrared) signals are used instead of wire to pass data from computer to computer.

Hybrid Systems

(A hybrid is a mix - part one thing, part something else.)

Some networks use ordinary cables between most computers, but radio between others.

Advantages

1. Can be used in places were cables are inconvenient - for example at a temporary desk in a hotel, or trade show.

2. Can be used with laptops that are carried from place to place.

Three Types of Wireless Network

1. Local Area Networks

Looks and acts like an ordinary network, except some PCs use radio links instead of cable. All of the computers in the network must be quite close together.

2. Extended Local Area Networks

A wireless bridge is a radio link between two networks - maybe in different buildings. The networks can be up to 3 miles apart.

A long-range wireless bridge can join together networks up to 25 miles apart.

3. Mobile Computing

It enables users of laptop PCs to connect to their network wherever they are.

The Four Transmission Techniques Used in Wireless Local Area Networking

1. infrared:

Infrared is light. This system is fast (up to 10 Mbps), but can be unreliable in areas of bright light, such as next to windows, and is limited to distances of less than 100 feet.

Light travels only in straight lines, so this has implications for the layout of an infrared network. There are four common ways of using infrared:

a) Line-of-sight. All computers must be visible from all others.

b) Scatter. The light bounces of walls and ceilings, so the computers do not have to be visible to each other. (Try this with the remote control for your TV.) This is slower than line of sight.

c) Reflective Networks. Here a central station is used. All computers send to this station, and it then re-transmits the data to all computers.

d) Broadband Optical Telepoint

This is the highest performance infrared system, with speed comparable with a cable network. (No other details are given.)

2. Laser

Data is sent between devices using a laser beam. Laser light only travels in straight lines so there must be a line of sight between the devices.

3. Narrow-Band (Single Frequency) Radio

All devices are tuned to the same radio frequency. Line of sight is not necessary, but the high frequency radio used cannot pass through steel or concrete walls. Speeds are around 5 Mbps.

4. Spread-Spectrum (Multi-Frequency) Radio

Here a whole set of radio frequencies are used. The system keeps ‘hopping’ from one frequency to another. This makes it hard for any unauthorized person to listen in to our signals. This system is normally slow - about 250 Kbps (0.25 Mbps).

Mobile Computing In More Detail :

Two Types of Signal Transmission

1. Packet-Radio Communication

Data to be sent across the radio network is broken into units called packets. Each packet contains the address of its sender, plus the address of its target. The sender transmits these packets to a satellite, which then broadcasts them back down to earth. They are received by all computers using this system, but ignored by all except the one they are addressed to.

2. Cellular Networks

This uses the cellular phone system to transmit data.

Microwave Systems

Microwaves are used to move data at high speed over long distances. Microwaves are sent and received using dish antennas - often on towers. There must be a line-of-sight between the sender and receiver.

A college in the U.K. has two campuses, 50 miles apart. Each campus has two dish antennas pointing to the nearest transmission tower. These towers, maybe using other towers en-route, move data from campus to campus.

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