The Physical Layer Read Chapter 2 CSC 3352 Computer Communications Chapter 22 Putting the bits on...
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Transcript of The Physical Layer Read Chapter 2 CSC 3352 Computer Communications Chapter 22 Putting the bits on...
CSC 3352 Computer Communications Chapter 2 3
Terminology (1)
TransmitterReceiverMedium Guided medium
e.g. twisted pair, optical fiber Unguided medium
e.g. air, water, vacuum
CSC 3352 Computer Communications Chapter 2 4
Terminology (2)
Point-to-point Direct link
Only 2 devices share link
Multi-point More than two devices share the link
CSC 3352 Computer Communications Chapter 2 5
Terminology (3)Simplex One direction
e.g. Television
Half duplex Either direction, but only one way at a
time e.g. police radio
Full duplex Both directions at the same time
e.g. telephone
CSC 3352 Computer Communications Chapter 2 6
Continuous Signals
Continuous signal: Intensity Varies in a smooth way over time, no breaks...
CSC 3352 Computer Communications Chapter 2 7
Discrete signal: Maintains a constant level then changes to another constant level
Discrete Signals
CSC 3352 Computer Communications Chapter 2 8
Periodic Signals
Periodic signal: The same pattern is repeated over time...
CSC 3352 Computer Communications Chapter 2 9
Amplitude (A) maximum strength of signal…Volts
Frequency (f) Rate of change of signal, Hertz (Hz) or
cycles per second
The period Period = time for one repetition (T) T = 1/f
Phase () Relative position in time
Amplitude, Frequency and Period...
CSC 3352 Computer Communications Chapter 2 11
Wavelength ()
: Distance occupied by one cycle. It is also the distance between two points of corresponding phase in two consecutive cyclesIn a vacuum space: c = 3*108 ms (speed of light in free
space) = cT f = c
CSC 3352 Computer Communications Chapter 2 12
Baud
Definition: the unit baud measures the number of changes per second in the signal.
OR, how many times the signal changes its amplitude per second...
CSC 3352 Computer Communications Chapter 2 13
Spectrum & Bandwidth
Spectrum range of frequencies contained in
signal
Absolute bandwidth width of spectrum
Effective bandwidth Often just bandwidth Narrow band of frequencies containing
most of the energy
CSC 3352 Computer Communications Chapter 2 14
Data Rate and Bandwidth
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
CSC 3352 Computer Communications Chapter 2 15
The Maximum data rate of a channel
Nyquist:maximum data rate = 2 H log2 V bits/s
Shannon (Nyquist is an optimist)maximum data rate = H log2 (1+S/N)S/N…..
CSC 3352 Computer Communications Chapter 2 16
Analog and Digital Data Transmission
Data Entities that convey meaning
Signals Electric or electromagnetic
representations of data
Transmission Communication of data by
propagation and processing of signals
CSC 3352 Computer Communications Chapter 2 17
Data
Analog Continuous values within some interval e.g. sound, video
Digital Discrete values e.g. text, integers
CSC 3352 Computer Communications Chapter 2 18
SignalsMeans by which data are propagatedAnalog Continuously variable Various media: wire, fiber optic, space Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz
Digital Use only two levels of voltage
CSC 3352 Computer Communications Chapter 2 19
Data and SignalsUsually use digital signals for digital data and analog signals for analog dataCan use analog signal to carry digital data Modem
Can use digital signal to carry analog data Compact Disc audio
CSC 3352 Computer Communications Chapter 2 20
Transmission Impairments
Signal received may differ from signal transmittedAnalog - degradation of signal qualityDigital - bit errorsCaused by: Attenuation Distortion Noise
CSC 3352 Computer Communications Chapter 2 21
AttenuationSignal strength falls off with distance
Loss of energy due to the propagation of bits in long distance…
Depends on medium
Received signal strength: must be enough to be detected must be sufficiently higher than noise to
be received without error
CSC 3352 Computer Communications Chapter 2 22
Distortion
Only in guided mediaPropagation velocity varies with frequencyBits do not arrive in the initial order...
CSC 3352 Computer Communications Chapter 2 23
NoiseAdditional signals inserted between transmitter and receiver
Thermal: due to thermal agitation of electrons
Crosstalk: a signal from one line is picked up by another
Impulse: high amplitude
CSC 3352 Computer Communications Chapter 2 24
What To Do Against Attenuation, Distortion and Noise?
A media adapted to the distance/bit rate
Encoding schemes (we will see it later)
CSC 3352 Computer Communications Chapter 2 25
Media for Networks
Twisted pair 2 twisted insulated copper wires Twesting waires reduces interferences Ex.: the telephone system Category 3: Some megabits per sec. (for only
few Kms, may need repeaters for longer distances)
Category 5: more twists per centimeter, less crosstalk, longer distance, more suitable for high speed computer communications…
CSC 3352 Computer Communications Chapter 2 26
Media for NetworksCoaxial cable See fig. 2-3 on page 84 Has a better shielding so it can span longer
distance… Higher speed… Good noise immunity… Baseband: 2Gbps for 1km, less speed for
more kms (amplifiers are requiered), used in both voice and data transmission…
Broadband: oriented cable TV, mainly analog transmission, accuracy is not required, long distance…
CSC 3352 Computer Communications Chapter 2 27
Media for NetworksFiber optics See page 87 for an introduction to optical
transmission system… 3 main elements: the light source, the transmission
medium and the light detector A pulse of light=1 bit, absence=0bit The transmission media is an ultra-thin fiber of
glass How to transmit… Several Gbps for 30kms, 1 repeater per 30kms…
CSC 3352 Computer Communications Chapter 2 28
Future is With Fiber
Bigger bandwidths
Attenuation/distance is lower than copper
Not sensitive to electromagnetic interference
Lighter and thinner than copper
Difficult to tap
CSC 3352 Computer Communications Chapter 2 29
Wireless Links: The spectrum of telecommunication
UV
100 1014104 108 1012
Infrared
1016 1022
GammaRadio Microwa. X-Ray
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016F(Hz)
Band LF MF HF VHF UHF
Twisted pair
Coax
Satellite
Microwave
Fiber
Maritime
AMRadio
FMRadio
TV
CSC 3352 Computer Communications Chapter 2 30
Different Bands of the Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
CSC 3352 Computer Communications Chapter 2 31
Relation Spectrum, Frequency, Data rate...
The frequency band is derived from .f = c
Deriving gives the frequency band corresponding to a wavelength and wavelength interval is then :…
CSC 3352 Computer Communications Chapter 2 32
The Spectrum: A cake to Share
WARC (International)
FCC ( U.S. agency)
CSC 3352 Computer Communications Chapter 2 33
Radio Transmission
Easy to generateTravel long distance, penetrate buildingsOmnidirectionalProperties depend on frequency low frequencies, penetrates buildings
well but sensitive to distance (1/r3). High frequencies, travel in straight
lines, absorbed by rain
Sensitive to interferences
CSC 3352 Computer Communications Chapter 2 34
Microwave Transmission
Travel long distance, (Does not penetrate buildings)Not Omnidirectional. Travel in straight lines (transmitter and receiver must be carefully aligned)Absorbed by rain
ISM band
CSC 3352 Computer Communications Chapter 2 35
Infrared and Millimiter Waves
Do not go thru solid walls
Good for indoor wireless LANs
CSC 3352 Computer Communications Chapter 2 36
Using the Telephone System for Computer Networks
Why?
What is the telephone system?
How does it work?
How can computers use it for communicating?
CSC 3352 Computer Communications Chapter 2 37
The Telephone System
Phone
Local Loop
End office
Toll office (We may have other levels of hierarchy…)
CSC 3352 Computer Communications Chapter 2 38
The Telephone SystemLocal loop analog twisted pair one per user Low bandwidth
Inter switches trunks Use to be analog but is now digital (see
page 109) fiber High bandwidth Shared
CSC 3352 Computer Communications Chapter 2 39
Computer Modem
End office
CodecDigital
Short digital connectionRS 232
Local loopAnalog
Local loop for a Computer
CSC 3352 Computer Communications Chapter 2 40
Transmission impairments
(Local Loop)
Attenuation
Delay distorsion
Noise
CSC 3352 Computer Communications Chapter 2 41
Modulation
Amplitude modulation
Frequency modulation (FSK)
Phase modulation (PSK)
Constellation patterns (see ex. in page 111)
CSC 3352 Computer Communications Chapter 2 42
Full/Half duplex
Other problems with telephone lines: Echo effects: a person hears its own words
after a short delay… Echo suppressors: a device that detects
human voice from one end and suppresses signals from the other end… switching… half duplex connection…
Echo cancelers… Estimates the echo and subtracts it from the original signal…Full duplex connection
CSC 3352 Computer Communications Chapter 2 43
Interface Computer (DTE)-Modem (DCE)
The RS 232 is a good example of physical layer protocol
DataTerminal Equipment
DataCircuit-Term.
Equipment
Protective ground (1)Transmit (2)
Receive(3)Request To Send (4)
Clear To Send(5)Data Set Ready(6)
Common Ground (7)
Carrier Detect(8)Data terminal Ready(20)
CSC 3352 Computer Communications Chapter 2 44
Interface Computer (DTE)-Computer (DTE)
With a null modem device we can use RS 232 to connect 2 DTE…
DTE DTEShield (1)
Transmit (2)Receive(3)
Request To Send (4)Clear To Send(5)
Data Set Ready(6)
Common Ground (7)
Carrier Detect(8)Data terminal Ready(20)
CSC 3352 Computer Communications Chapter 2 45
Computer Modem
End office
Codec
Digital
Digital
RS 232Local loopAnalog
Review
We looked in detail into thisWe focus now on trunks
CSC 3352 Computer Communications Chapter 2 46
Trunks: transporting many conversations on the same “wire”
Frequency Division Multiplexing Wavelength Division Multiplexing
(Fiber)
Time Division Multiplexing
CSC 3352 Computer Communications Chapter 2 47
Frequency Division Multiplexing
Let the signals we need to transmit working in frequency.
All signals are raised in frequency by n.’ such that ’ > . The extra is called guard band.
All signals are added
CSC 3352 Computer Communications Chapter 2 48
Frequency Division Multiplexing(Example)
The phone voice is limited to Hz frequency.
All signals are raised in frequency by n.’ with ’=4000 Hz (two 500Hz guard band)
All signals are added
CSC 3352 Computer Communications Chapter 2 49
Frequency Division Multiplexing
12
4000Hz
1 group (60 to 108KHz)
51 super group
51 mastergroup
CSC 3352 Computer Communications Chapter 2 50
Time Division Multiplexing
Works only for digital signals
Allocate time slots to every signal
Each signal has the exclusive use of the line for a given time slot
CSC 3352 Computer Communications Chapter 2 52
PCM : Transporting Digital Voice
A codec digitalizes 24 voice channels, sends-
receives 8 bits per channel,
If a standard voice sampling ratio (8000bps) is
used, the codec will perform 192bits every 125Msec
In fact it is 192bits+1checkbit=193bits/125Msec
What should be the data rate to transport the
samples of the 24 channel? 1.544Mbps (this is the
capacity of a T1 career)
CSC 3352 Computer Communications Chapter 2 53
A word on ISDN
Read section 2.5
Idea : integrated voice and digital service Voice channel 2 and more digital channels in
multiples of 8, 16, or 64 Kbps
CSC 3352 Computer Communications Chapter 2 54
Cellular Radio :the American system
Mobile phones: Two way system The AMPS was invented by Bell Labs in 1982 Based on cell architecture (see figure in next slide) Moving in the space… Security issues
CSC 3352 Computer Communications Chapter 2 55
Cellular Radio :the European system
The Global Systems for Mobile communications (GSM). The standard of digital mobile systems in Europe (and north africa)
Incompatible with USA and Japan
A new frequency (1.8GHz)
Uses FDM and TDM
Uses a smart card to store the serial number and phone number — » less vandalsime…
56Chapter 2CSC 3352 Computer Communications
Cellular Radio : Architecture
Public Switching Telephone System
A cellulaire area
MSCMSC