Transmission Media No. 1 Seattle Pacific University Transmission Media: Wires, Cables, Fiber...

Transmission Media No. 1Seattle Pacific University

Transmission Media:Wires, Cables, Fiber Optics, and

MicrowavesBased on Chapter 4 of William Stallings, Data and

Computer Communication

Kevin BoldingElectrical Engineering

Seattle Pacific University


Transmission Media

• A signal must be transmitted through some medium

• Guided Media determine the path of the signal

• Wires (cables, twisted pair, coax)

• Fiber Optics

• Other things…

• Signals Propagate in all directions in Unguided Media

• The medium is usually free space (air), but the signal type gets the name

• Refers to transmitting signals through passive media that does not change the signal’s direction

• Microwaves, broadcast radio waves

• Lasers, Infrared


Media Issues

• Frequency range

• Some media support higher frequencies than others

• Impairments

• Different media deform signals differently

• Some are more susceptible to noise and distortion

• Cost

• We’re in the real world…

• Number of receivers

• Broadcast vs. point-to-point


How Fast/How Far can a Signal be Sent?

• The question:

• Given a source signal with a given power, how far can it go before it is attenuated so much that the SNR is too low to be usable?

• As far as media is concerned, the main issue is attenuation• Attenuation increases with distance. Usually

expressed in dB/m, dB/100ft, etc.

• Attenuation usually increases with frequency.


Attenuation CurvesAttenuation per 100ft for UTP/Coax

0

5

10

15

20

25

1 10 100 1000

MHz

Att

enu

atio

n p

er

100f

t (d

B) Cat-5 UTP

RG58 Coax

RG6 Coax

Attenuation is very dependent on conductor size Cat-5: 0.21 mm2

RG58: 0.64 mm2

RG6: 1.0 mm2


Frequency of various signals

Power/Telephone

Radio Microwave InfraredVisibleLight

Twisted Pair

Coax

AM Radio FM Radio/TV

MicrowaveTrans.

OpticalFiber

106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6

102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015

Frequency (Hz)

Wavelength (Meters)Source: Stallings, Fig. 4.1


Guided Media

• Guided media control the path of the signal wave

• Electrical – Signal needs conductor and ground• Differences are in how ground/conductor interact

• Twisted pair

• Coax

• Striplines on PCBs

• Optical – Signal is sent using internal reflection

• Differences are in light sources and fiber diameter


Differential SignalingEvil Noise(0.6v)

Source

Signals gather noise when travelling in a cable

5.0V 5.6V

Receiver

GND

If the signal and GND are both sent together, they both experience the same noise. Computing the difference removes the noise.

5.0V

Evil Noise(0.6v)

5.6VSource Receiver

GND0.6V 5.0V

Differential signaling works best when the two signal conductors are routed as close as possible to each other so they experience the same external noise.


Electrical Cables

• Electromagnetic interference (EMI)

• Loops make great antennas• Antenna strength

proportional to the area inside of the loop

• Worse for shorter wavelengths

signal

return

Interference prop. to area

signal

return Better…

• Common ground systems (such as PCBs with ground planes)

• Return path directly below signal

• Minimizes loop area

Trace on PCB

Ground return

• Keep the two parts of the signal close together


Twisted Pair Cables

• Twist the signal and ground together

• Both sides experience similar noise effects

• Loop size proportional to twist size

• Adjacent twists are 180 degrees out of phase

• Tend to cancel out

• Varying the twist size helps to minimize crosstalk

Signal

Return

Adjacent LoopsOut of phase

• Data rates

• Over long distances, about 1-3 Mbps

• Short distances: 1Gbps and higher


Shielding• Twisted pair usually comes bundled with several pairs in a

cable• Unshielded – Just a plastic (teflon) jacket

• For distances of around 100m -• Cat-3 UTP: <16Mbps, Cat-5 UTP: 100Mbps, Cat-6 UTP:

1000Mbps

• Shielded – Includes a grounded shield

(source: Microsoft Networking Essentials)


Coaxial Cables

• Concentric mesh wire for ground

• Acts as an excellent shield

• Very little interference or radiation

• Center conductor can be large (low resistance), reducing attenuation

• Better data rates over long distances than twisted pair

• The downside

• Expensive to manufacture

• More difficult to install


Optical Fiber• Relies on total internal

reflection

• Light waves bounce of edge of fiber

• Channels waves to destination

• Varieties

• Multi-mode (wide fiber)

• Light waves bounce off at different angles

• Some have shallow angles (straight path), while others have steeper angles (crooked path)

• Results in pulse spreading

• Single-mode (narrow fiber)

• Only a straight shot down the middle is allowed

• Requires a laser source

(Source: Stallings, Fig. 4.4)


Fiber has its advantages

• Advantages

• No electromagnetic interference

• Very little attenuation

• Extremely high bandwidth (THz)

• Small, lightweight

• Disadvantages

• More expensive transceivers

• More difficult to install


Wireless (Unguided) Media

• Omnidirectional

• Signal radiates in all directions

• Good for broadcast

• Inexpensive antenna

• Directional

• Signal radiates in a single direction

• Usually requires parabolic (dish) antenna

• 2-40 GHz (microwave)

• Also works with lasers


Unguided Media Attenuation

2

2

2

2 )4()4(

c

fdd

P

P

rcv

trans

dBdfc

fddBloss

c

fd

P

P

r

t

56.147log20log204

log20)(

)4(log10log10

101010

2

2

1010

f = frequency (Hz)d = distance (m)= wavelength (m)c = speed of light (m/s)

4

2

SPR

Transmitted wave spreads out over a spherical surface

Power density at receiver:

Received power depends on the receiver antenna’s aperture:

44

2

2

d

PP TR

Thus

Free Space Path Loss =

24 d

PS T


Terrestrial Radio (All forms)

Ionosphere

• Ground-wave propagation follows the curvature of the earth

• Frequencies below 2MHz

• AM radio (550-1600KHz)

• Sky-wave propagation relies on the ionosphere and the surface of the earth to refract waves back-and-forth

• Frequencies 2MHz-30MHz

• Short-wave Radio, HAM radio

• Line of site is point-to-point in a nearly straight line

• Frequencies 30MHz and up

• FM radio, TV, Mobile phones, etc.

• Max distance between antennas with height h1 and h2

21 3

4

3

457.3)( hhkmd


Satellite Radio

• Requires satellite in geosynchronous orbit

• 35,784 km

• Delay of ¼ second (round-trip)

• Satellites spaced 4 degrees apart

• Above 10GHz, signal is attenuated by atmosphere

• Higher frequencies use smaller dishes, though

http://www.mike-willis.com/Tutorial/gases.htm

Transmission Media No. 1 Seattle Pacific University Transmission Media: Wires, Cables, Fiber...

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