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Networks and Communication Department

Transmission Media Data and Computer Communications by William Stallings

Eighth Edition

Overview

transmission medium:

is the physical path between transmitter and receiver.

transmission media can be classified as :

1- guided media- wire :

electromagnetic waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable, and optical fiber

2- unguided media – wireless:

wireless transmission occurs through the atmosphere, outer space, or water.

Overview

characteristics and quality determined by medium and signal

 in unguided media - bandwidth produced by the antenna is more important  in guided media - medium is more important

key concerns are data rate and distance

Design Factors

1.Bandwidth

higher bandwidth gives higher data rate

2.transmission impairments

eg. attenuation, limit the distance. For guided media, twisted pair generally suffers more impairment than coaxial cable, which in turn suffers more than optical fiber.

Design Factors

3.Interference

Interference from competing signals in overlapping frequency bands can distort or wipe out a signal. Interference is of particular concern for unguided media. For guided media, interference can be caused by emanations from nearby cables. For example, twisted pairs .

4.number of receivers in guided media

-in guided media -each attachment introduces some attenuation and distortion on the line, limiting distance and/or data rate .

-more receivers (multi-point) introduces more attenuation, limiting distance and data rate.

Electromagnetic Spectrum

Transmission Characteristics of Guided Media

Twisted pair (with loading) Twisted pairs (multi-pair cables) Coaxial cable Optical fiber

Frequency Range

0 to 3.5 kHz 0 to 1 MHz 0 to 500 MHz 186 to 370 THz

Typical Attenuation

0.2 dB/km @ 1 kHz 0.7 dB/km @ 1 kHz

Typical Delay

50 µs/km 5 µs/km 7 dB/km @ 10 MHz 0.2 to 0.5 dB/km 4 µs/km 5 µs/km

Repeater Spacing

2 km 2 km 1 to 9 km 40 km

Twisted Pair

physical description:

• A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. • The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. • Twisted pair is much less expensive than the other • easier to work with.

Twisted Pair

Application:

• Most common medium • Telephone network • Between house and local exchange -Within buildings .

• For local area networks (LAN)

Twisted Pair

Transmission Characteristics:

• analog  needs amplifiers every 5km to 6km • digital  can use either analog or digital signals  needs a repeater every 2-3km • limited distance • limited bandwidth (1MHz) • limited data rate (10 to 100MHz) • susceptible to interference and noise

Unshielded vs Shielded TP

Twisted pair comes in two varieties: 1.

unshielded Twisted Pair (UTP)

 ordinary telephone wire  cheapest  easiest to install  suffers from external Electromagnetic interference (EMI)

2.

shielded Twisted Pair (STP)

 metal braid or sheathing that reduces interference  more expensive  harder to handle (thick, heavy)

UTP Categories

Cat 3 (up to 16 MHz) — Twist length of 7.5 cm to 10 cm Cat 5 (up to 100MHz) — Twist length 0.6 cm to 0.85 cm — Tighter twist -> more expensive, But better performance Cat 5E (Enhanced), Cat 6 & Cat 7

Bandwidth Cable Type Link Cost (Cat 5 =1) Category 3 Class C

16 MHz UTP 0.7

Category 5 Class D

100 MHz UTP/FTP 1

Category 5E

100 MHz UTP/FTP 1.2

Category 6 Class E

200 MHz UTP/FTP 1.5

Category 7 Class F

600 MHz SSTP 2.2

Coaxial Cable

physical description:

• like twisted pair, consists of two conductors. • It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor • Coaxial cable can be used over longer distances.

Coaxial Cable

Application:

• Television distribution • Long distance telephone transmission • Short distance computer systems links • Local area networks

Coaxial Cable

Transmission Characteristics:

• • • Coaxial cable is much less susceptible to interference and crosstalk than twisted pair • Coaxial cable is used to transmit both analog and digital signals.

-analog signals • • amplifiers every few km closer if higher frequency • up to 500MHz -digital signals repeater every 1km closer for higher data rates

Optical Fiber

physical description:

An optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers

Optical Fiber

Application:

• Long-haul trunks • Rural exchange trunks • LANs

Optical Fiber

Benefits:

• greater capacity  data rates of hundreds of Gbps • smaller size & weight • lower attenuation than for coaxial cable or twisted pair • electromagnetic isolation • greater repeater spacing

Optical Fiber

Transmission Characteristics:

uses total internal reflection to transmit light can use several different light sources :  Light Emitting Diode (LED)  cheaper, lasts longer  Injection Laser Diode (ILD)  more efficient, has greater data rate

Optical Fiber Transmission Modes

There are three fundamental types of fiber optic cable in use: 1. step-index multimode

Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are reflected and propagated along the fiber. variety of angles will reflect. With multimode transmission, multiple propagation paths exist, each with a different path length

2. graded-index multimode

light in the core curves helically because of the graded index, reducing its travel distance. Graded-index fibers are often used in local area networks.

3. single-mode

there is a single transmission path with single-mode transmission. Single mode is typically used for long-distance applications, including telephone and cable television

Optical Fiber Transmission Modes

Difference between Guided Media

Wireless Transmission techniques

Microwave

 Frequency :2GHz to 40GHz  Used for point to point communication  Used for satellite communication

Radio wave

 omnidirectional  30MHz to 1GHz

Infrared wave

 Used for local point-to-point and multipoint applications  3x10^11Hz to 2x10^14 Hz

Antennas

Electrical conductor used to radiate or collect electromagnetic energy -transmission antenna

 radio frequency energy from transmitter  converted to electromagnetic energy by antenna  radiated into surrounding environment

-reception antenna

 electromagnetic energy impinging on antenna  converted to radio frequency electrical energy  fed to receiver

-Same antenna is often used for both purposes

Types of Antenna

1- Horn antenna 2-Dish antenna

Wireless propagation methods

• • •

1-Ground propagation

Follows contour of earth This effect found in freq below 2MHz Example :AM radio

Wireless propagation methods

• • • •

2-Sky propagation

Signal reflected from ionosphere layer of upper atmosphere Frequency from 2 to 30 MHz Useful for long distance communication Example : BBC world service, Voice of America

Wireless propagation methods

• • •

3-Line of sight propagation

Point to point Frequency above 30Mhz communication Example : satellite ,TV

Terrestrial Microwave

Physical description:

• The most common type of microwave .

• used for : 1- long haul telecommunications 2- short point-to-point links • line of sight transmission • use a parabolic dish to focus a narrow beam onto a receiver antenna • an alternative to coaxial cable or optical fiber • requires far fewer amplifiers or repeaters than coaxial cable over the same distance

Terrestrial Microwave

Transmission characteristic:

• At higher frequency more attenuation • Attenuation increase with rainfall

Satellite Microwave

• Satellite is relay station • Used to link two or more earth stations •

Satellite communication configuration:

1-Point to Point Link 2-Broadcast Link

Satellite Point to Point Link

Satellite Broadcast Link

Broadcast Radio

• use broadcast radio, 30MHz - 1GHz, for:  FM radio • is omnidirectional • Communication is line of sight • Can penetrate walls • Less sensitive to rainfall • suffers from multipath interference

Infrared

• infrared light • within line of sight (or reflection) • are blocked by walls , Thus the security and interference problems encountered in microwave systems are not present.

• typical uses  TV remote control

Refraction

• • • • velocity of electromagnetic wave is a function of density of material C = 3 x 10 8 m/s in vacuum, less in anything else • When an electromagnetic wave moves from a medium of one density to a medium of another density, its speed changes.

The effect is to cause a bending of the direction of the wave.

Wavelength get smaller

Refraction

Index of refraction (refractive index) is:

 n=sin(incidence)/sin(refraction)  also n= C/V

Summary

looked at data transmission issues frequency, spectrum & bandwidth analog vs. digital signals transmission impairments