30 wireless topology

8/3/2019 30 Wireless Topology

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Wireless topology



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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 focusreceived signals and to transmit a parallel beam

without dispersion



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(4 d/ )² dB.

7. Rain attenuates the signal .



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 Link

ground basedmicrowavetransmitter

ground basedmicrowavereceiver



Satellite Broadcast Link



Broadcast Radio

Omnidirectional (travel in all directions)Line of sight is not required

Doesn’t need parabolic antenna

Example FM radioTypical 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. Ground wave• Follows contour of earth

• Up to 2MHz

• AM radio 2. Sky wave

• Signal reflected from ionize layer of upper atmosphere

• BBC world service, Voice of America

3. Line 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 theatmosphere.)



Sky Wave Propagation



Ground Wave Propagation



Line of Sight Propagation



Multipath Interference



Transmission Impairments in Wireless Transmission

• Free Space Loss1. Signal dispersion is a function of distance

2. Ratio between power-radiated to power-received

3. Greater for lower wavelength

4. Antenna gain can be used to compensate the losses

5. Also known as near far problem

• Refraction1. Each wireless medium has its own density

2. Propagation speed is a function of density of the medium 3. When medium changes, the result is refraction

4. Refraction means change of direction

30 wireless topology

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