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					Transmission Media and
Data Transmission
    2G1316 Data
    Communications and
    Computer Networks

    2E1623 Data Links and
    Local Area Networks
Transmission and Physical Infrastructure


 • Transmission Media
     Attenuation and link budget
     Signal distortion
     Capacity limitations

 • Modulation and line coding
 • Synchronization and framing
 • Multiplexing
 • Capacity requirements
 • Examples—TDM, ADSL, SDH

                                           2
Transmission Media

        T                Wave guide
                                                                Wave guide              R
    Transmitter                        Amplifier,                                    Receiver
                                       Signal regenerator




•     Guided media                                  •   Unguided media
           Electrical                                         Electromagnetic waves in air
              o   Twisted pair cable                             o   Radio
              o   Coaxial cable                                  o   Microwaves (terrestrial and
                                                                     satellite)
           Optical
              o   Single-mode and multimode




                                                                                                   3
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]
       o   Example:
       o   Pin = 120 mW
       o   Pout = 30 mW
       o   Attenuation = 10 log10 4  6 dB



                                                        4
Power and Sensitivity

• Measured in ‖decibel watt‖ dBW or
  ‖decibel milliwatt‖ dBm
   PdbW = 10 log10 P

   PdBm= 10 log10 P/1×10-3

• For example, transmitter output power
  and receiver input sensitivity

• Note: absolute power measures!


                                          5
Transmission Quality—Distortion




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


                                       6
Transmission Quality—Noise




• Undesired signal added to           • Signal-to-noise ratio, SNR
  the transmitted signal                  S/N, where S is signal
    Thermal noise                         power, N is noise power
       o   Random motion of
           electrons
       o   Independent of frequency
           (‖white noise‖) and
           amplitude
       o   Added to the signal

                                                                     7
Transmission Quality—Counter 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



                                                                  8
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: capacity
L: number of levels
R: baud rate



                             L=2                  L=4
                             C=R                  C = 2R


                                                           9
Nyquist Bit Rate

          Cmax = 2B log2 L

• Harry Nyquist (1889 – 1976)
• Also Nyquist’s/Hartley’s Law
• Nyquist bit rate Cmax is the
  maxiumum bit rate on an ideal
  channel
      So maximum baud rate is 2B


                                    10
 Channel Capacity—Shannon’s 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


                                         11
Shannon’s 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?)




                                          12
Bandwidth for Different Media




                                13
Guided Media

• Wires, cables

• Twisted pair cables

• Coaxial cables

• Optical fibers




                        14
Twisted Pair Cable




• Separately insulated

• Pair of cables twisted together
    Even out external disturbances

    Receiver operates on signal differences


                                               15
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

                                                    16
Cathegories of Unshielded Twisted Pair

Category    Bandwidth   Data Rate    Digital/Analog     Use

   1         very low   < 100 kbps      Analog        Telephone

             <2
   2                     2 Mbps      Analog/digital   T-1 lines
            MHz

   3          16 MHz     10 Mbps        Digital        LANs

   4          20 MHz     20 Mbps        Digital        LANs

   5         100 MHz    100 Mbps        Digital        LANs

6 (draft)    200 MHz    200 Mbps        Digital        LANs

7 (draft)    600 MHz    600 Mbps        Digital        LANs


                                                              17
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


                                             19
Optical Fibre




• Core of glass or plastic
• Cladding with lower index of refraction
• Light Emitting Diode (LED) or laser



                                            20
Fiber Transmission Modes




                           21
Loss vs Wavelength




• Wavelength l = c/f
   c is propagation speed, f is frequency
                                             22
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




                                                      23
Propagation Methods for Unguided Signals




                                     24
Radio Waves

• Radio, television,
  etc

• Up to 1 GHz

• Ground and sky
  propagation

• Omnidirectional
  antennas


                       25
Microwaves

• 1-300 GHz

• Cellular phones, satellite networks, wireless LANs

• Line of sight propagation

• Unidirectional antennas




                                                       26
Infrared

• 300 GHz – 400 THz

• Line-of-sight propagation

• Closed areas
   Interference from sun rays

• Short distances


                                 27
Analog and Digital Signals
Analog and Digital Signals




• Low-pass channel            • Band-pass channel
    Digital transmission         Analog transmission
    Dedicated medium             Multiple channels in the
      o   One channel              same medium
    Line coding                  ‖Bärvågsmodulering‖
     (‖Basbandsmodulering‖)
                                                         29
  Line Coding

• Turn binary data into
  digital signal




• Dedicated medium
   Full spectrum
   Fourier transformation
    of square wave is
    infinite serie


                             30
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




                                                                                               31
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
                                                    32
Return to Zero (RZ) Encoding




•   Synchronization even for long strings of 1s or 0s
•   Two signal-changes per bit—more bandwidth
•   ―Differential RZ‖



                                                        33
Manchester Encoding




• Two signal levels
• Higher pulse rate requires larger bandwidth



                                                34
Differential Manchester Encoding




• Need only detect ‖transition‖ or ‖no transition‖



                                                35
Block Coding

  0110   0010   1101   0111   4B/5B   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




                                                                  36
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


                                                                       37
4B/5B Control Characters

               Data        Code

Q (Quiet)                  00000
I (Idle)                   11111
H (Halt)                   00100
J (start delimiter)        11000
K (start delimiter)        10001
T (end delimiter)          01101
S (Set)                    11001
R (Reset)                  00111
                                   38
Transmission of Analog Signal
Sampling
• Coding of analog signals
   For example voice and video
• Analog signal is measured at equal intervals
   Sampling
   Quantization in time
   PAM—pulse amplitude modulation




                                                 41
Sampling Rate—The 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


                                                                                 42
Aliasing


  Amplitude
                         Original signal




                                Regenerated signal



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

                                                     43
Amplitude Quantization




• From analog to digital data

• Binary coding
                                44
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

                                              45
PCM Examples

            Sample rate   Sample size   Bit rate
            (kHz)         (bits)        (kb/s)
Telephony   8             8             64


CD          44.1          16            705.6


DVD Audio   44.1, 48,     16, 20, 24    9600
            88.2, 96,                   (max)
            176.4, 192


                                                46
From Analog to Digital




                         47
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




                                              48
Amplitude Shift Keying (ASK)




• Signal level is varied to represent symbols

• Amplitude sensitive to noise

                                                49
Frequency Shift Keying (FSK)




• Signal frequency is varied to represent symbols
• Bandwidth limitations


                                                    50
Phase Shift Keying (PSK)




• Signal phase is varied to represent symbols
• Limited by receiver’s ability to detect phase
  changes

                                                  51
PSK Constellation Diagram




• Phase-state diagram




                            52
4-PSK (Q-PSK)




• Four different phases

• Each phase represents two bits
                                   53
Qadrature Amplitude Modulation (QAM)




• Combination of ASK and PSK
    Allows for more combinations—more bits per baud

• Maximum contrast between signal units

                                                       55
Bit and Baud Rates

                                                      Bit
   Modulation      Units     Bits/Baud   Baud rate
                                                     Rate
ASK, FSK, 2-PSK     Bit         1           N         N
4-PSK, 4-QAM       Dibit        2           N        2N
8-PSK, 8-QAM       Tribit       3           N        3N
16-QAM            Quadbit       4           N        4N
32-QAM            Pentabit      5           N        5N
64-QAM            Hexabit       6           N        6N
128-QAM           Septabit      7           N        7N
256-QAM           Octabit       8           N        8N

                                                          57
Data Transmission Over Telephone Lines




                                     58
Modems




• Modulator/demodulator




                          59
V-series Modem Standards




•   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




                                                                                60
    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
       o   1 bit for control




                                 61
Data Transmission Modes

               Transmission


          Parallel       Serial


              Asynchronous    Synchronous




                                            62
Parallel Transmission




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

                                               63
Serial Transmission




•   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




                                                64
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




                                                                                                   65
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




                                                                66
Multiplexing
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


                                                         68
Multiplexing

• 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


                                                         69
Time Division Multiplexing for Telephony




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

                                           70
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


                                                                         71
Hierarchical Multiplexing




                            72
E Line Rates

                Rate      Voice
    E Line
               (Mbps)    Channels

     E-1        2.048       30


     E-2         8.448     120


     E-3        34.368     480


     E-4       139.264    1920



                                    73
Example: SDH/SONET

• 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



                                                               74
SONET/SDH Equipment




STS: Synchronous Transport Signal
                                    75
SONET/SDH Rates

Optical   Electrical   SDH        Line rate   Payload
level     level        equivalent (Mb/s)      rate
                                              (Mb/s)
OC-1      STS-1        -          51.84       50.112

OC-3      STS-3        STM-1      155.52      150.336

OC-12     STS-12       STM-4      622.08      601.344

OC-48     STS-48       STM-16     2488.32     2405.376

OC-192    STS-192      STM-64     9953.28     9621.604

OC-768    STS-768      STM-256    39813.120   38486.016

                                                          77
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
     o   Cable distance
     o   Size of cable
     o   Signalling


                                             79
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



                                                                     80
Discrete Multitone Technique (DMT)




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

                                              81
ADSL Modems and DSLAMs




   Digital Subscriber Line Access Multiplexer
                                                82
ADSL2/ADSL2+
•   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




                                                                                    83
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



                                                              84
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




                                          85
Reading Instructions

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




                                                                   86
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