transmission media and data transmission
TRANSCRIPT
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Transmission Media andData Transmission
2G1316 DataCommunications and
Computer Networks2E1623 Data Links and
Local Area Networks
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Transmission and Physical Infrastructure
Transmission Media
Attenuation and link budget
Signal distortion
Capacity limitations
Modulation and line coding
Synchronization and framing
Multiplexing
Capacity requirements ExamplesTDM, ADSL, SDH
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Transmission Media
Guided media
Electrical
o Twisted pair cable
o Coaxial cable
Optical
o Single-mode and multimode
Unguided media
Electromagnetic waves in air
o Radio
o Microwaves (terrestrial and
satellite)
T
R
Transmitter ReceiverAmplifier,Signal regenerator
Wave guideWave guide
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Attenuation
No link is perfect Attenuation
Power loss between sender and receiver
Relationship between incoming and outgoing power
Measured in decibel [dB]
o Example:
o Pin = 120 mW
o Pout= 30 mW
o Attenuation = 10 log10 4 6 dB
10 log10 Pin/Pout
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Power and Sensitivity
Measured in decibel wattdBW ordecibel milliwattdBm
PdbW= 10 log10 P
PdBm= 10 log10 P/110-3
For example, transmitter output powerand receiver input sensitivity
Note: absolute power measures!
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Transmission QualityDistortion
Signal changes form or shape
Each frequency component has itsown speed through the medium
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Transmission QualityNoise
Undesired signal added tothe transmitted signal
Thermal noise
o Random motion of
electronso Independent of frequency
(white noise) andamplitude
o Added to the signal
Signal-to-noise ratio, SNR
S/N, where S is signalpower, N is noise power
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Transmission QualityCounter Measures
Amplification
Compensates for attenuation and other losses
Adds noise
Regeneration (for digital signals)
Recreates the shape of the signal
Noise filters
Can attenuate the signal Protection against disturbances and crosstalk (verhrning)
For example shielding against electromagnetic fields
Protection against distortion
Equalizers
Dispersion compensation
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Bit Rate and Baud Rate
Link capacity: number of bits per second (bit rate)
Baud rate: number of signal elements per second
L = 2C = R
L = 4C = 2R
C = R log2L
C: capacity
L: number of levels
R: baud rate
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Nyquist Bit Rate
Harry Nyquist (1889 1976)
Also Nyquists/Hartleys Law
Nyquist bit rate Cmax is themaxiumum bit rate on an idealchannel
So maximum baud rate is 2B
Cmax = 2B log2L
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Channel CapacityShannons Formula
Claude Shannon (1916 2001)
Father of information theory
Highest possible bit rate in a channel withwhite noise
B is channel bandwidth S/N is signal to noise ratio
C = B log2 (1 + S/N)
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Shannons Formula
Example B = 3100 Hz
S/N = 20 dB = 100 times C = 3100 log2(1 + 100) = 20.6 kb/s
Telephone line B: 3100-3500 Hz S/N: 33-39 dB C 33-45 kb/s (What about ADSL and 56K modems?)
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Bandwidth for Different Media
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Guided Media
Wires, cables
Twisted pair cables
Coaxial cables
Optical fibers
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Twisted Pair Cable
Separately insulated
Pair of cables twisted together
Even out external disturbances
Receiver operates on signal differences
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Twisted Pair Cable
Several pairs bundled together Often with RJ-45 connector Often installed in building when built Shielded (STP) and unshielded (UTP)
Shielding protects from noise and crosstalk Bulkier and more expensive
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600 MHz
200 MHz
100 MHz
20 MHz
16 MHz
< 2
MHz
very low
Bandwidth
LANsDigital600 Mbps7 (draft)
LANsDigital200 Mbps6 (draft)
LANsDigital100 Mbps5
LANsDigital20 Mbps4
LANsDigital10 Mbps3
T-1 linesAnalog/digital2 Mbps2
TelephoneAnalog< 100 kbps1
UseDigital/AnalogData RateCategory
Cathegories of Unshielded Twisted Pair
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10Base-
T100
16 Mb/s16 MHzCAT 3
4 Mb/s
< 0.1 Mb/s
Bit rate
TelephoneDoorbell
Very lowCAT 1
T-1TokenRing
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Coaxial Cable
Solid inner connector Outer connector is braid or metal foil Separated by insulating material Higher bandwidth than twisted pair
But also higher attenuation
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Optical Fibre
Core of glass or plastic
Cladding with lower index of refraction
Light Emitting Diode (LED) or laser
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Fiber Transmission Modes
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Loss vs Wavelength
Wavelength = c/f
c is propagation speed,f is frequency
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Fiber Advantages and Disadvantages
Advantages
Very high capacity
Low attenuation
Low crosstalk: no interference between photons
Not sensitive to electromagnetic noise
Light weight
Disadvantages
Installation/maintenance
Unidirectional
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Propagation Methods for Unguided Signals
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Radio Waves
Radio, television,etc
Up to 1 GHz
Ground and skypropagation
Omnidirectional
antennas
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Microwaves
1-300 GHz
Cellular phones, satellite networks, wireless LANs
Line of sight propagation
Unidirectional antennas
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Infrared
300 GHz 400 THz
Line-of-sight propagation
Closed areas
Interference from sun rays
Short distances
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Analog and Digital Signals
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Analog and Digital Signals
Low-pass channel
Digital transmission
Dedicated mediumo One channel
Line coding(Basbandsmodulering)
Band-pass channel
Analog transmission
Multiple channels in thesame medium
Brvgsmodulering
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Line Coding
Turn binary data intodigital signal
Dedicated medium Full spectrum
Fourier transformation
of square wave isinfinite serie
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Unipolar Encoding
One signal level (and zero)
Contains DC component Distorted (blocked) in some components Extra energy
Lack of synchronization Long sequences of all ones or all zeros may cause receiver to loose synchronization
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Nonreturn to Zero (NRZ)
Polar signal (two levels)
NRZ-level (NRZ-L) and NRZ-invert (NRZ-I)
Average signal level reduced
Synchronization could still a problem
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Return to Zero (RZ) Encoding
Synchronization even for long strings of 1s or 0s
Two signal-changes per bitmore bandwidth
Differential RZ
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Manchester Encoding
Two signal levels
Higher pulse rate requires larger bandwidth
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Differential Manchester Encoding
Need only detect transition or no transition
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Block Coding
Bit stream is divided into m-bit groups
Groups are encoded as n-bit codes 4B/5B: 5-bit codes represent 4-bit groups 8B/10B: 10-bit codes represent 8-bit groups
01100110
00100010
11011101
01110111 4B/5B4B/5B 1101011010 0101001010 1001010010 0010100101
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Substitution in 4B/5B Block Coding
Chose codes in such a way that synchronization is ensured
In 4B/5B, there can never be more than three consecutive 0s
Error detection Control information Disadvantage: higher bandwidth
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4B/5B Control Characters
1000110001K (start delimiter)
0110101101T (end delimiter)
1100111001S (Set)
0011100111R (Reset)
1100011000J (start delimiter)
0010000100
1111111111
0000000000
Code
H (Halt)
I (Idle)
Q (Quiet)
Data
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8B/6T Encoding
Substitute an 8-bit group with a 6-symbolcode
Ternary symbols Limited bandwidth
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Transmission of Analog Signal
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Sampling
Coding of analog signals For example voice and video
Analog signal is measured at equal intervals Sampling Quantization in time PAMpulse amplitude modulation
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Sampling RateThe Nyquist Theorem
The sampling rate must be at least twice the highest frequency in the
analog signal This frequency is often called the Nyquist frequency, or Nyquist rate
Theoretically, no information is gained by sampling at a higher rate
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Aliasing
Sampling below the Nyquist rate
(undersampling) distorts the spectrum Vikningsdistortion
AmplitudeOriginal signal
Regenerated signal
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Amplitude Quantization
From analog to digital data
Binary coding
P l C d M d l i (PCM)
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Pulse Code Modulation (PCM)
Signal levels are represented by a fixednumber of bits
8 bit values: -127 to 127
Quantization noise introduced by roundingerrors
PCM E l
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PCM Examples
16, 20, 24
16
8
Sample size
(bits)
648Telephony
9600(max)
44.1, 48,88.2, 96,
176.4, 192
DVD Audio
705.644.1CD
Bit rate
(kb/s)
Sample rate
(kHz)
F A l t Di it l
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From Analog to Digital
M d l ti
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Modulation
Sine wave fully described by amplitude A,frequency fand phase
s(t) = A sin(2ft + )
Vary one (or more) to represent symbols
Amplit de Shift Ke ing (ASK)
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Amplitude Shift Keying (ASK)
Signal level is varied to represent symbols Amplitude sensitive to noise
Frequency Shift Keying (FSK)
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Frequency Shift Keying (FSK)
Signal frequency is varied to represent symbols Bandwidth limitations
Phase Shift Keying (PSK)
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Phase Shift Keying (PSK)
Signal phase is varied to represent symbols
Limited by receivers ability to detect phasechanges
PSK Constellation Diagram
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PSK Constellation Diagram
Phase-state diagram
4 PSK (Q PSK)
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4-PSK (Q-PSK)
Four different phases Each phase represents two bits
8-PSK Constellation Diagram
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8-PSK Constellation Diagram
Qadrature Amplitude Modulation (QAM)
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Qadrature Amplitude Modulation (QAM)
Combination of ASK and PSK Allows for more combinationsmore bits per baud
Maximum contrast between signal units
4-QAM and 8-QAM Constellations
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4 QAM and 8 QAM Constellations
Bit and Baud Rates
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Bit and Baud Rates
8N
7N
6N
5N
4N
3N
2N
N
Bit
Rate
N5Pentabit3232--QAMQAM
N6Hexabit6464--QAMQAM
N7Septabit128128--QAMQAM
N8Octabit256256--QAMQAM
N4Quadbit1616--QAMQAM
Tribit
Dibit
Bit
UnitsUnits
N388--PSK, 8PSK, 8--QAMQAM
N244--PSK, 4PSK, 4--QAMQAM
N1ASK, FSK, 2ASK, FSK, 2--PSKPSK
Baud rateBaud rateBits/BaudBits/BaudModulationModulation
Data Transmission Over Telephone Lines
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Data Transmission Over Telephone Lines
Modems
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Modems
Modulator/demodulator
V-series Modem Standards
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se es ode Sta da ds
V.32 9600 b/s: 32-QAM, baud rate 2400, 4+1 data bits/baud (trellis-coded)
V.32 bis
14400 b/s: 128-QAM, 6+1 data bits/baud
V.34 bis 28800 33600 b/s: 960 to 1664 points constellations
56K modems (PCM modems)
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( )
Quantization noise fromPCM sampling
ISPs have digital connection(no modem)
V.90 and V.92 standards
Asymmetric rate
56/33.6 Kb/s (V.90) 8000 samples per second
7 bits of data per sample
o 1 bit for control
Data Transmission Modes
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TransmissionTransmission
ParallelParallel SerialSerial
AsynchronousAsynchronous SynchronousSynchronous
Parallel Transmission
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High capacity But costly, if it requires multiple cables
Serial Transmission
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Need for synchronization at bit level
External clock, such as GPS
Separate link for clock signal
Line coding with embedded clock
o Manchester coding, for example
Receiver resynchronization
Asynchronous Transmission
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Asynchronous at the word (byte) level Start and stop bits mark the beginning and end of a byte
(Loose) synchronization at bit level
Receiver is resynchronized when start bit is detected
Clock frequencies sufficiently close to keep synchronization for the duration of a byte
Often combined with parity bit for error control (e.g. RS-232) Keyboard, serial port, etc
Synchronous Transmission
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Continuous stream of bits No extra bits or extra space between bytes
Special idle patterns to indicate absense of data
Bit stream can be divided into larger data units (frames)
Responsibility of the data link layer
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Multiplexing
Multiplexing
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Subdivision of a link into multiple channels
Multiple sender/receiver pairs can share the link
Resource sharing
Bandwidth divided into frequency channels Transmission time divided into time slots
Multiplexing
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Analog multiplexing
Frequency division multiplexing (FDM)
o Multiple frequency channels
o Band pass modulation
o TV and radio broadcast
Wavelength division multiplexing (WDM)
o Similar to FDM but for optical transmission
Digital multiplexing
Time division multiplexing (TDM)
o Access according to time slots
Synchronous TDM
o Statistical TDM
Time Division Multiplexing for Telephony
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Carries PCM voice channels T1 (North America, Japan)
o 24 channels, 1.544 Mb/s E1
o 30 channels, 2.048 Mb/s
Synchronous Time Division Multiplexing
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Access according to time slots
Time slots grouped into frames
Ifn is the number of inputs, the output link needs to be n times faster
than each input link
Frame duration is the same as the duration of a data unit on the input
Hierarchical Multiplexing
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E Line Rates
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19201920139.264139.264EE--44
34.36834.368
8.4488.448
2.0482.048
Rate
(Mbps)
480480EE--33
120120EE--22
3030EE--11
Voice
ChannelsE Line
Example: SDH/SONET
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ANSI: Synchronous Optical NETwork (SONET)
ITU-T: Synchronous Digital Hierarchy (SDH)
TDM system
Synchronous network
o A single, common clock allows channel multiplexing
Fiber-optic transmission system
Can carry tributaries
DS-0, DS-1, E1
SONET/SDH Equipment
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STS: Synchronous Transport Signal
Frame Format
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Organized as a matrix with 9 90 octets Three columns of administration overhead
Payload is called Synchronous Payload Envelope (SPE)
SONET/SDH Rates
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38486.01639813.120STM-256STS-768OC-768
9621.6049953.28STM-64STS-192OC-192
2405.3762488.32STM-16STS-48OC-48
601.344622.08STM-4STS-12OC-12
150.336155.52STM-1STS-3OC-3
50.11251.84-STS-1OC-1
Payloadrate(Mb/s)
Line rate(Mb/s)
SDHequivalent
Electricallevel
Opticallevel
Virtual Tributaries
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Carry lower rate data Partial payload
VT1.5 for DS-1 service (1.544 Mb/s), VT2for E1 service (2.048 Mb/s), etc
Example: Digital Subscriber Link (DSL)
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High-speed Digital Access to Internet
Exploit the actual bandwidth available
in twisted pair cables in local loop(subscriber access lines)
Up to 1.1 MHz
Subject to strict physical limitations
o Cable distance
o Size of cable
o Signalling
Asymmetrical DSL (ADSL)
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Adaptive Bandwidth and data rate depends on conditions
Lower rate in upstream direction (from subscriber)
For residential access
Upstream 64 kb/s to 1 Mb/s, Downstream 500 kb/s to 8 Mb/s
Bandwidth (typically) divided into 4 kHz channels
Discrete Multitone Technique (DMT)
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Combination of QAM and FDM
4 kHz channels and 15 bits/baud 60 kb/schannels
ADSL Modems and DSLAMs
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Digital Subscriber Line Access Multiplexer
ADSL2/ADSL2+
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ADSL2
Improved rate and reach
o Improvements in modulation, framing, coding, signal processing, etc.
o About 12 Mb/s downstream and 1 Mb/s upstream
o Slightly increased reach (+200 m) Higher rates on long lines
Higher capacity by bonding two or more phone lines
Channelized voice
o 64 kb/s DS-0 channels for TDM voice traffic
All-digital mode
o Use voice channel for data
ADSL2+
2.2 Mhz bandwidth
Up to 26 Mb/s downstream and 1.5 Mb/s upstream
Other DSL Technologies
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Symmetric DSL (SDSL)
Equally divided bandwidth
High-bit-rate DSL (HDSL)
Alternative to T1 access Up to 2 Mb/s
2B1Q encoding (four levels, two bits per baud)
Two twisted-pair wires for full-duplex
Very-high-bit-rate DSL (VDSL)
Similar to ADSL
DMT with up to 50-55 Mb/s downstream, 1.5-2.5 upstream
Short distances (300 to 1800 m) Fiber, coaxial cable, twisted-pair cable
Summary
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Transmission media Link budget
Capacity limitations
Transmission of digital information
Line coding
Digital modulation
Transmission of analog information
Conversion to digital signals
Sampling
Synchronization
Multiplexing
Examples
Modems
SDH/SONET
ADSL
Reading Instructions
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Behrouz A. Forouzan, Data Communications and Networking, third
edition
3 Signalso 3.4 Analog versus Digital
o 3.5 Data Rate Limitso 3.6 Transmission Impairment
4 Digital Transmissiono 4.1 Line Codingo 4.2 Block Codingo 4.4 Transmission mode
5 Analog Transmissiono 5.1 Modulation of Digital Datao 5.2 Telephone Modems
6 Multiplexing 7 Transmission Media
9 High-Speed Digital Accesso 9.1 DSL Technologyo 9.3 SONET