Unguided Media Computer networks(cn)

7.2 Unguided media

twireless tsignals are broadcasted through air

Kyung Hee University

1

contd

tRadio Frequency Allocation


2

t Propagation of radio wavesTypes of propagation


3

contd

t Type of propagationSurface Propagationq

Radio waves travel through the lowest portion of the atmosphere

TropospherePropagationq

can work two ways(x signal can be directed in a straight line from antenna to antenna. y It can be broadcast at an angle into the upper layers of troposphere where it is reflected back down to the earths surface4


contd

IonospherePropagationq

High-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth

Line-of-Sight Propagationq

Very high frequency signals are transmitted in straight lives directly from antenna to antenna

Space Propagationq

Utilizes satellite relays in place of atmospheric refraction5


contd

sPropagation of Specific SignalsThe type of propagation used in radio transmission depends on the frequency(speed) of the signal

t VLF(Very Low Frequency)is propagated as surface waves, usually through air but sometimes through seawater is used mostly for long-range radio navigation and for submarine communication


6

contd t LF(Low Frequency)is also propagated as surface waves is used for long-range radio navigation and for radio beacons or navigation locators


7

contd t MF(Middle Frequency)is propagated in the troposphere is absorbed by the ionosphere uses for MF transmissions include AM radio, maritime radio, radio direction finding(RDF), and emergency frequencies


8

contd t HF(High-Frequency)uses ionospheric propagation uses of for HF signals include amateur radio(ham radio), CB(Citizens band) radio, international broadcasting, military communication, long-distance aircraft and ship communication, telephone, telegraph, and facsimile


9

contd t VHF(Most Very High Frequency)use line-of-sight propagation uses for VHF include VHF television, FM radio, aircraft AM radio, and aircraft navigational aid


10

contd t UHF(Ultrahigh Frequency)always uses line-of-sight propagation. Uses for UHF include UHF television, mobile telephone, cellular radio, paging , and micro wave links


11

contd t SHF(SuperHigh Frequency) ~ are transmitted using mostly line-of-sight and some space propagation. ~ uses for SHF include terrestrial and satellite microwave, and radar communication


12

contd t EHF(Extremely High Frequency)use space propagation uses for EHF are radar, Satellite, and experimental communication


13

contd t Terrestrial microwavesrequire line-of-sight transmission and reception equipment. The distance coverable by a line-of-sight signal depends on a large extent on the height of the antenna : the taller the antenna, the longer the sight distance


14

contd t RepeatersTo increase the distance served by terrestrial microwave, a system of repeaters can be installed with each antenna.


15

contd t Antennastwo types of antenna

x Parabolic dishis based on the geometry of a parabola


16

contd y Horn antennalooks like a gigantic scoop


17

contd t Satellite Communicationis much like line of sight microwave transmission in which one of the station is a satellite orbiting the earth


18

contd t Geosynchronous satellites

@ the view from the North PoleKyung Hee University

19

contd t Frequency bands for satellite communicationBand Downlink Uplink

C Ku Ka

03.7 - 04.2 GHz ` 11.7 - 12.2 GHz 17.7 - 21.0 GHz

05.925 - 06.425 GHz 14.000 - 14.500 GHz 27.500 - 31.000 GHz


20

contd

t Cellular telephonyis designed to provide stable communications connections between two moving device or between one mobile unit and one stationary(land) unit.


21

t Cellular System

contd

The typical radius of a cell is 1 ~ 12 miles : Cell size is not fixed and can be increased or decreased depending on the population of the area. Each cell office is controlled by MTSO.


22

contd t Cellular BandsTraditional cellular transmission is Analog, now mostly Digital. The FCC has assigned two bands for cellular use.

Initiating from mobile phonesKyung Hee University

Initiating from land phones

23

contdt TransmittingThe caller enter a code of 7 or 10 digits(phone number) Press the send button The mobile phone then scans the band, seeking a setup channel with a strong signal Sends the data(phone number) to the closest cell office Cell office relays the data to the MTSO MTSO sends data on to the telephone central office


24

contdt Receiving

* When a land phone places a call to a mobile phoneTelephone central office sends the number to the MTSO MTSO searches for the location of the mobile phone Mobile phone is found, the MTSO transmits a ringing signal When the mobile phone is answered, assigns a voice channel to the call, allowing voice communication to begin


25

contdt HandoffDuring a conversation, the mobile phone moves from one cell to another Signal may become weak MTSO monitors the level of the signal every few seconds MTSO seeks a new cell that can accommodate the communication better Handoffs are performed so smoothly that most of the time they are not observed by the users


26

contd t DigitalAnalog cellular service Analog circuit switching cellular(ACSC) Require modem with max speed of 9600 to 19200 bps Since 1993, cellular digita packet data(CDPD)

t Integration with Satellites and PCs


27

7.3 Performance

uFive factors to evaluate the suitability of a particular mediumCost : cost of the material + installation Speed Attenuation EMI(Electromagnetic Interference) SecurityKyung Hee University

28

Performance(contd) t Transmission media performanceMediumUTP STP Coax Optical fiber Radio Microwave Satellite Cellular

CostLow Moderate Moderate High Moderate High High High

Speed1 - 100 Mbps 1 - 150 Mbps 1 Mbps - 1 Gbps 10 Mbps - 2 Gbps 1 - 10 Mbps 1 Mbps - 10 Gbps 1 Mbps - 10 Gbps 9.6 - 19.2 Kbps

AttenuationHigh High Moderate Low Low-high Variable Variable Low

EMIHigh Moderate Moderate Low High High High Moderate

SecurityLow Low Low High Low Moderate Moderate Low


29

7.3 Transmission Impairment t Transmission media are not perfect because of impairment in the signal sent through the mediumSignal at the beginning and end of the medium are not same


30

Transmission Impairment t Attenuationmeans loss of energy When signal travels trough a medium, it losses some of it energy So, to compensate for this loss, amplifiers are used to amplify the signal

t Decibel (dB)dB = 10 log10 (p2/p1)


31

Transmission Impairment t p2 = (1/2) p1 t Signal powers are increased 10 times by AMP


32

Transmission Impairment


33

Transmission Impairment t DistortionMeans that signal changes its from or shape


34

Transmission Impairment t Noise - Noise typesthermal noise, induced noise, crosstalk and impulse noise Thermal noise : random motion of electrons Induced noise : from sources such as motors, appliances Crosstalk : the effect of one wire on the other Impulse noise : a spike that comes from power lines, lightning, and so on.Kyung Hee University

35

Transmission Impairment t noise


36

7.4 Performance t Throughputis the measurement of how fast data can pass through a point


37

Performance


38

7.5 Wavelength


39

7.6 Shannon Capacity t A formula to determine the theoretical highest data rate for a channelC = B log 2 (1 + S/N)C : Shannon capacity

B - bandwidth of the channel S/N signal to noise ratio


40

7.6 Shannon Capacity t Example 7.4 (extremely noise channel) The noise is so strong that signal is faintC = B log2 (1 + S/N) = B log 2 (1 + 0) = B log 2 (1) = B x 0 = 0

t Example 7.5 (theoretical highest bit rate of a regulartelephone line) bandwidth : 3000 hzC = B: log2 (1 + S/N) = 3000 log 2 (1 + 1362) = 3000 log 2 (3163) S/N 3162 (35 dB) = 3000 x 11.62 = 34,860 bpsKyung Hee University

41

7.7 Media Comparison

uFive factors to evaluate the suitability of a particular mediumCost : cost of the material + installation Speed Attenuation EMI(Electromagnetic Interference) SecurityKyung Hee University

42

Performance(contd) t Transmission media performanceMediumUTP STP Coax Optical fiber Radio Microwave Satellite Cellular

CostLow Moderate Moderate High Moderate High High High

Speed1 - 100 Mbps 1 - 150 Mbps 1 Mbps - 1 Gbps 10 Mbps - 2 Gbps 1 - 10 Mbps 1 Mbps - 10 Gbps 1 Mbps - 10 Gbps 9.6 - 19.2 Kbps

AttenuationHigh High Moderate Low Low-high Variable Variable Low

EMIHigh Moderate Moderate Low High High High Moderate

SecurityLow Low Low High Low Moderate Moderate Low


43

Unguided Media Computer networks(cn)

Documents

Transcript of Unguided Media Computer networks(cn)