Transmission MediaCEN 220/CIS 192 Advanced Data Communications and NetworkingData and Computer Communications, W. Stallings 9/E, Chapter 4

2

Data Link Control Protocols “Communication channels in the animal world include

touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression.”

—Mind of the Raven, Bernd Heinrich

3

Overviewtransmission medium is the physical path between transmitter and receiverguided media – guided along a solid mediumunguided media – atmosphere, space, water characteristics and quality determined by medium and signal

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

is more important

key concerns are data rate and distance

4

Factors Determining Data Rate and Distance

5

Electromagnetic Spectrum

6

Transmission Characteristics of Guided Media  

  Frequency Range

Typical Attenuation

Typical Delay

Repeater Spacing

Twisted pair (with loading)

0 to 3.5 kHz

0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 to 1 MHz 0.7 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable

0 to 500 MHz

7 dB/km @ 10 MHz

4 µs/km 1 to 9 km

Optical fiber

186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

7

Guided Transmission Media

8

Twisted Pair

Twisted pair is the least expensive and most widely used guided transmission medium.

– consists of 2 insulated copper wires in a regular spiral pattern

– a wire pair acts as a single communication link

– pairs are bundled together into a cable

– most commonly used in the telephone network and for communications within buildings

9

Twisted Pair - Transmission Characteristics

susceptible to interference and noise

10

Unshielded vs. Shielded Twisted Pair

11

Twisted Pair Categories and Classes

12

Near End Crosstalkcoupling of signal from one pair of conductors to another

occurs when transmit signal entering the link couples back to the receiving pair -

(near transmitted signal is picked up by near receiving pair)

13

Signal Power Relationships

14

Coaxial Cable

Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.

– consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor

– is a versatile transmission medium used in a wide variety of applications

– used for TV distribution, long distance telephone transmission and LANs

15

Coaxial Cable – Transmission Characteristics

16

Optical Fiber

Optical fiber is a thin flexible medium capable of guiding an optical ray.

– various glasses and plastics can be used to make optical fibers

– has a cylindrical shape with three sections – core, cladding, jacket

– widely used in long distance telecommunications

– performance, price and advantages have made it popular to use

17

Optical Fiber - Benefitsgreater capacity

data rates of hundreds of Gbps

smaller size and lighter weight considerably thinner than coaxial or twisted pair cable reduces structural support requirements

lower attenuation

electromagnetic isolation not vulnerable to interference, impulse noise, or crosstalk high degree of security from eavesdropping

greater repeater spacing lower cost and fewer sources of error

18

Optical Fiber - Transmission Characteristicsuses total internal reflection to transmit light

– effectively acts as wave guide for 1014 to 1015 Hz (this covers portions of infrared & visible spectra)

light sources used:– Light Emitting Diode (LED)

● cheaper, operates over a greater temperature range, lasts longer

– Injection Laser Diode (ILD)● more efficient, has greater data rates

has a relationship among wavelength, type of transmission and achievable data rate

19

Optical Fiber Transmission Modes

20

Frequency Utilization for Fiber Applications

Wavelength (invacuum) range

(nm)

FrequencyRange (THz)

BandLabel

Fiber Type Application

820 to 900 366 to 333 Multimode LAN

1280 to 1350 234 to 222 S Single mode Various

1528 to 1561 196 to 192 C Single mode WDM

1561 to 1620 192 to 185 L Single mode WDM

WDM (wavelength division multiplexing)

21

Attenuation in Guided Media

22

Wireless Transmission Frequencies

23

Antennaselectrical conductors used to radiate or collect electromagnetic energy

same antenna is often used for both purposes

24

Radiation Patternpower radiated in all directions

does not perform equally well in all directions as seen in a radiation pattern diagram

an isotropic antenna is a point in space that radiates power

in all directions equally with a spherical radiation pattern

25

Parabolic Reflective Antenna

26

Antenna Gainmeasure of the directionality of an antenna

power output in particular direction verses that produced by an isotropic antenna

measured in decibels (dB)

results in loss in power in another direction

effective area relates to physical size and shape

27

Terrestrial Microwave

28

Terrestrial Microwave Applicationsused for long haul telecommunications, short point-to-point links between buildings and cellular systemsused for both voice and TV transmissionfewer repeaters but requires line of sight transmission1-40GHz frequencies, with higher frequencies having higher data ratesmain source of loss is attenuation caused mostly by distance, rainfall and interference

29

Microwave Bandwidth and Data Rates

30

Satellite Microwavea communication satellite is in effect a microwave relay stationused to link two or more ground stationsreceives on one frequency, amplifies or repeats signal and transmits on another frequency

frequency bands are called transponder channels

requires geo-stationary orbit rotation match occurs at a height of 35,863 km at the equator need to be spaced at least 3° - 4° apart to avoid interfering

with each other spacing limits the number of possible satellites

31

Satellite Point-to-Point Link

32

Satellite Broadcast Link

33

Satellite Microwave Applicationsprivate business networks

– satellite providers can divide capacity into channels to lease to individual business users

television distribution– programs are transmitted to the satellite then broadcast down

to a number of stations which then distributes the programs to individual viewers

– Direct Broadcast Satellite (DBS) transmits video signals directly to the home user

global positioning– Navstar Global Positioning System (GPS)

34

Transmission Characteristicsthe optimum frequency range for satellite transmission is 1 to 10 GHz

– lower has significant noise from natural sources

– higher is attenuated by atmospheric absorption and precipitation

satellites use a frequency bandwidth range of 5.925 to 6.425 GHz from earth to satellite (uplink) and a range of 3.7 to 4.2 GHz from satellite to earth (downlink)

– this is referred to as the 4/6-GHz band

– because of saturation the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GHz

35

Broadcast Radioradio is the term used to encompass frequencies in the range of 3 kHz to 300 GHz

broadcast radio (30 MHz – 1 GHz) covers– FM radio

– UHF and VHF television

– data networking applications

omnidirectional

limited to line of sight

suffers from multi-path interference– reflections from land, water, man-made objects

36

Infraredachieved using transceivers that modulate non-coherent infrared lighttransceivers must be within line of sight of each other directly or via reflectiondoes not penetrate wallsno licenses requiredno frequency allocation issuestypical uses: TV remote control

37

Frequency Bands

38

Wireless Propagation Ground Wave

ground wave propagation follows the contour of the earth

and can propagate distances well over the visible horizon

this effect is found in frequencies up to 2MHz

best example of ground wave communication is AM radio

39

Wireless Propagation Sky Wave

sky wave propagation is used for amateur radio, CB radio, and international broadcasts, e.g., BBC, Voice of America

a signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere back down to earth

sky wave signals can travel through a number of hops, bouncing back and for the between the ionosphere and the earth’s surface

40

Wireless Propagation Line of Sight

ground and sky wave propagation modes do not operate above 30-MHz

communication must be by line of sight

41

Refractionvelocity of electromagnetic wave is a function of the density of the medium through which it travels

– ~3 x 108 m/s in vacuum, less in anything else

speed changes with movement between media

index of refraction (refractive index) is– sine (incidence) / sine (refraction)

– varies with wavelength

gradual bending– density of atmosphere decreases with height, resulting in

bending of radio waves towards earth

42

Line of Sight Transmission

43

Free Space Loss

44

Multipath Interference

45

Summarytransmission Media

– physical path between transmitter and receiver

– bandwidth, transmission impairments, interference, number of receivers

guided Media– twisted pair, coaxial cable, optical fiber

wireless Transmission– microwave frequencies

– antennas, terrestrial microwave, satellite microwave, broadcast radio

wireless Propagation– ground wave, sky wave, line of sight