Wireless topology

By. P. Victer Paul

Dear, We planned to share our eBooks and project/seminar contents for free to all needed friends like u.. To get to know about more free computerscience ebooks and technology advancements in computer science. Please visit....

http://free-computerscience-ebooks.blogspot.com/

http://recent-computer-technology.blogspot.com/

http://computertechnologiesebooks.blogspot.com/

Please to keep provide many eBooks and technology news for FREE. Encourage us by Clicking on the advertisement in these Blog.

Topologies Relevant for Wireless Networking

• Star  Yes, standard wireless topology

• Tree Yes (a combination of star and line)

• Line Yes, with two or more elements (PtP)

• Mesh  Yes, mainly partial mesh

• Ring  Possible, but rarely found

• Bus  Not applicable. Why?

Basic Network Topologies Most wireless implementations are based on

– Star, tree or line topology Real life wireless networks are very often

combinations of more than one topology

Wireless transmission

Overview Characteristics and quality determined by:

Medium Signal

Medium Guided - wire Unguided - wireless

For Guided Medium The medium is more important

For Unguided The bandwidth produced by the antenna is more important

Key concerns are data rate and distance

Design Factors

• Bandwidth— Higher bandwidth gives higher data rate

• Transmission impairments— Attenuation

• Interference— Issue especially in case of unguided medium

• Number of receivers— Unicast (one sender, one receiver)

— Multicast (multiple receivers can introduce more errors)

Electromagnetic Spectrum

Wireless Transmission

Frequencies 2GHz to 40GHz (Microwave Frequency)

Highly directional

Point to point devices

Microwave communications

30MHz to 1GHz (Radio Frequency) Omnidirectional

Broadcast radio

300GHz – 400THz(Infrared) Short-range communication in a closed area using line-of-sight

propagation (ex, wireless keyboard)

Antennas By definition

Is a electrical device

Transmission Radio frequency energy from transmitter

Converted to electromagnetic energy

By antenna

Radiated into surrounding environment

Reception Electromagnetic energy impinging on antenna

Converted to radio frequency electrical energy

Fed to receiver

Same antenna often used for both

An isotropic antenna radiates power in all

directions. Parabolic reflective antenna are used to focus

received signals and to transmit a parallel beam without dispersion

Terrestrial Microwave (TMW)Terrestrial Microwave (TMW)

1. Parabolic antenna

2. Small beam

3. Line of sight

4. Use especially for P2P applications

5. Usually use for long distance communication

6. Loss = 10 log10(4d/)² dB.

7. Rain attenuates the signal.

Satellite Microwave (SM)Satellite Microwave (SM)

1. Satellite is relay station

2. Satellite • receives on one frequency

• amplifies or repeats signal

• transmits on another frequency

3. Requires geo-stationary orbit— Height of 35,784km

4. Applications• Television

• Long distance telephone

• Private business networks

Used to link two or more ground stations.

A single satellite might have several transponders.

Optimal transmission frequencies are 1-10 G Hz

but saturation is causing higher frequencies to be

used.

Satellite Point to Point Satellite Point to Point LinkLink

ground based microwave transmitter

ground based microwave receiver

Satellite Broadcast LinkSatellite Broadcast Link

Broadcast RadioBroadcast RadioOmnidirectional (travel in all directions)

Line of sight is not required

Doesn’t need parabolic antenna

Example FM radio

Typical frequencies range from 30 MHz to 1GHz.

Less sensitive to rain than microwave radio.

Multipath interference is a prime source of impairment.

• Infrared

Transceivers modulate noncoherent infrared light.

Transceivers must be within line-of-sight or each

other or must be able to receive reflections of the

signal.

Infrared does not penetrate walls, and hence has a

security advantage over radio waves.

Wireless Propagation• Signal travels along three routes

1.1. Ground waveGround wave• Follows contour of earth• Up to 2MHz• AM radio

2.2. Sky waveSky wave• Signal reflected from ionize layer of upper atmosphere• BBC world service, Voice of America

3.3. Line of sightLine of sight• Above 30Mhz• Antennas must be physically aligned• Atmosphere can reflect the microwave signal

LOS Impairments Free Space Loss--the signal disperses with

distance

Atmospheric Absorption

Multipath Reflection

Refraction (bending of waves through the

atmosphere.)

Sky Wave Propagation

Ground Wave Propagation

Line of Sight Propagation

Multipath Interference

Transmission Impairments in Wireless Transmission Impairments in Wireless TransmissionTransmission

• Free Space Loss1. Signal dispersion is a function of distance2. Ratio between power-radiated to power-received 3. Greater for lower wavelength4. Antenna gain can be used to compensate the losses5. Also known as near far problem

• Refraction1. Each wireless medium has its own density2. Propagation speed is a function of density of the medium3. When medium changes, the result is refraction 4. Refraction means change of direction