Transmission Media and Data Transmission

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• Transmission Media and Data Transmission

2G1316 Data Communications and Computer
Networks 2E1623 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
T
Wave guide
Wave guide
R
Transmitter
Amplifier, Signal regenerator
Receiver

• Guided media
• Electrical
• Twisted pair cable
• Coaxial cable
• Optical
• Single-mode and multimode

• Unguided media
• Electromagnetic waves in air
• Radio
• Microwaves (terrestrial and satellite)

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Attenuation
10 log10 Pin/Pout

• No link is perfect
• Attenuation
• Power loss between sender and receiver
• Relationship between incoming and outgoing power
• Measured in decibel dB
• Example
• Pin 120 mW
• Pout 30 mW
• Attenuation 10 log10 4 6 dB

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Power and Sensitivity

• Measured in decibel watt dBW or decibel
  milliwatt dBm
• PdbW 10 log10 P
• PdBm 10 log10 P/110-3
• For example, transmitter output power and
  receiver input sensitivity
• Note absolute power measures!

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Transmission QualityDistortion

• Signal changes form or shape
• Each frequency component has its own speed
  through the medium

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Transmission QualityNoise

• Undesired signal added to the transmitted signal
• Thermal noise
• Random motion of electrons
• Independent of frequency (white noise) and
  amplitude
• Added to the signal

• Signal-to-noise ratio, SNR
• S/N, where S is signal power, 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
  (överhörning)
• 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

C R log2 L C capacityL number of levelsR
baud rate
L 2 C R
L 4 C 2R
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Nyquist Bit Rate
Cmax 2B log2 L

• Harry Nyquist (1889 1976)
• Also Nyquists/Hartleys Law
• Nyquist bit rate Cmax is the maxiumum bit rate on
  an ideal channel
• So maximum baud rate is 2B

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Channel CapacityShannons Formula
C B log2 (1 S/N)

• Claude Shannon (1916 2001)
• Father of information theory
• Highest possible bit rate in a channel with white
  noise
• B is channel bandwidth
• S/N is signal to noise ratio

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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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Cathegories of Unshielded Twisted Pair
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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 l 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 sky propagation
• 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 medium
• One channel
• Line coding (Basbandsmodulering)

• Band-pass channel
• Analog transmission
• Multiple channels in the same medium
• Bärvågsmodulering

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Line Coding

• Turn binary data into digital signal
• Dedicated medium
• Full spectrum
• Fourier transformation of square wave is infinite
  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
4B/5B
0110
0010
1101
0111
11010
01010
10010
00101

• 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

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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
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8B/6T Encoding

• Substitute an 8-bit group with a 6-symbol code
• 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
Amplitude
Original signal
Regenerated signal

• Sampling below the Nyquist rate (undersampling)
  distorts the spectrum
• Vikningsdistortion

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Amplitude Quantization

• From analog to digital data
• Binary coding

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Pulse Code Modulation (PCM)

• Signal levels are represented by a fixed number
  of bits
• 8 bit values -127 to 127
• Quantization noise introduced by rounding errors

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PCM Examples
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From Analog to Digital
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Modulation

• Sine wave fully described by amplitude A,
  frequency f and phase f
• s(t) A sin(2pft f)
• Vary one (or more) to represent symbols

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Amplitude Shift Keying (ASK)

• Signal level is varied to represent symbols
• Amplitude sensitive to noise

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Frequency Shift Keying (FSK)

• Signal frequency is varied to represent symbols
• Bandwidth limitations

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Phase Shift Keying (PSK)

• Signal phase is varied to represent symbols
• Limited by receivers ability to detect phase
  changes

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PSK Constellation Diagram

• Phase-state diagram

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4-PSK (Q-PSK)

• Four different phases
• Each phase represents two bits

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8-PSK Constellation Diagram
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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

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4-QAM and 8-QAM Constellations
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Bit and Baud Rates
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Data Transmission Over Telephone Lines
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Modems

• Modulator/demodulator

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V-series Modem Standards

• V.32
• 9600 b/s 32-QAM, baud rate 2400, 41 data
  bits/baud (trellis-coded)
• V.32 bis
• 14400 b/s 128-QAM, 61 data bits/baud
• V.34 bis
• 28800 33600 b/s 960 to 1664 points
  constellations

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56K modems (PCM modems)

• Quantization noise from PCM 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
• 1 bit for control

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Data Transmission Modes
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Parallel Transmission

• High capacity
• But costly, if it requires multiple cables

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Serial Transmission

• Need for synchronization at bit level
• External clock, such as GPS
• Separate link for clock signal
• Line coding with embedded clock
• Manchester coding, for example
• Receiver resynchronization

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Asynchronous Transmission

• 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

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Synchronous Transmission

• 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
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Multiplexing

• 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

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Multiplexing

• Analog multiplexing
• Frequency division multiplexing (FDM)
• Multiple frequency channels
• Band pass modulation
• TV and radio broadcast
• Wavelength division multiplexing (WDM)
• Similar to FDM but for optical transmission
• Digital multiplexing
• Time division multiplexing (TDM)
• Access according to time slots
• Synchronous TDM
• Statistical TDM

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Time Division Multiplexing for Telephony

• Carries PCM voice channels
• T1 (North America, Japan)
• 24 channels, 1.544 Mb/s
• E1
• 30 channels, 2.048 Mb/s

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Synchronous Time Division Multiplexing

• Access according to time slots
• Time slots grouped into frames
• If n 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

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Hierarchical Multiplexing
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E Line Rates
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Example SDH/SONET

• ANSI Synchronous Optical NETwork (SONET)
• ITU-T Synchronous Digital Hierarchy (SDH)
• TDM system
• Synchronous network
• A single, common clock allows channel
  multiplexing
• Fiber-optic transmission system
• Can carry tributaries
• DS-0, DS-1, E1

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SONET/SDH Equipment
STS Synchronous Transport Signal
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Frame Format

• Organized as a matrix with 9 90 octets
• Three columns of administration overhead
• Payload is called Synchronous Payload Envelope
  (SPE)

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SONET/SDH Rates
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Virtual Tributaries

• Carry lower rate data
• Partial payload
• VT1.5 for DS-1 service (1.544 Mb/s), VT2 for E1
  service (2.048 Mb/s), etc

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Example Digital Subscriber Link (DSL)

• 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
• Cable distance
• Size of cable
• Signalling

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Asymmetrical DSL (ADSL)

• 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

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Discrete Multitone Technique (DMT)

• Combination of QAM and FDM
• 4 kHz channels and 15 bits/baud Þ 60 kb/s
  channels

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ADSL Modems and DSLAMs
Digital Subscriber Line Access Multiplexer
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ADSL2/ADSL2

• ADSL2
• Improved rate and reach
• Improvements in modulation, framing, coding,
  signal processing, etc.
• About 12 Mb/s downstream and 1 Mb/s upstream
• Slightly increased reach (200 m)
• Higher rates on long lines
• Higher capacity by bonding two or more phone
  lines
• Channelized voice
• 64 kb/s DS-0 channels for TDM voice traffic
• All-digital mode
• Use voice channel for data
• ADSL2
• 2.2 Mhz bandwidth
• Up to 26 Mb/s downstream and 1.5 Mb/s upstream

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Other DSL Technologies

• 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

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Summary

• 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

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Reading Instructions

• Behrouz A. Forouzan, Data Communications and
  Networking, third edition
• 3 Signals
• 3.4 Analog versus Digital
• 3.5 Data Rate Limits
• 3.6 Transmission Impairment
• 4 Digital Transmission
• 4.1 Line Coding
• 4.2 Block Coding
• 4.4 Transmission mode
• 5 Analog Transmission
• 5.1 Modulation of Digital Data
• 5.2 Telephone Modems
• 6 Multiplexing
• 7 Transmission Media
• 9 High-Speed Digital Access
• 9.1 DSL Technology
• 9.3 SONET

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