COE341_Chapter04

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					COE 341: Data & Computer Communications
Dr. Marwan Abu-Amara




        Chapter 4:
        Transmission Media
Agenda
    Overview
    Guided Transmission Media
        Twisted Pair
        Coaxial Cable
        Optical Fiber
    Wireless Transmission
        Antennas
        Terrestrial Microwave
        Satellite Microwave
        Broadcast Radio
        Infrared
                     COE 341 – Dr. Marwan Abu-Amara   2
Overview
   Media
       Guided - wire
       Unguided - wireless
   Transmission characteristics and quality
    determined by:
       Medium
       Signal
   For guided, the medium is more important
   For unguided, the bandwidth produced by the
    antenna is more important
   Key concerns are data rate and distance
                      COE 341 – Dr. Marwan Abu-Amara   3
    Design Issues
   Key communication objectives are:
       High data rate
       Low error rate
       Long distance
       Bandwidth: Tradeoff - Larger for higher data rates
                         - But smaller for economy
   Transmission impairments
       Attenuation: Twisted Pair > Cable > Fiber (best)
       Interference: Worse with unguided… (medium is shared!)
   Number of receivers
       In multi-point links of guided media:
        More connected receivers introduce more attenuation
                         COE 341 – Dr. Marwan Abu-Amara      4
Electromagnetic Spectrum




            COE 341 – Dr. Marwan Abu-Amara   5
Study of Transmission Media

   Physical description
   Main applications
   Main transmission characteristics




                    COE 341 – Dr. Marwan Abu-Amara   6
Guided Transmission Media

   Twisted Pair
   Coaxial cable
   Optical fiber




                    COE 341 – Dr. Marwan Abu-Amara   7
   Transmission Characteristics of Guided Media


                  Frequency            Typical                   Typical     Repeater
                    Range            Attenuation                  Delay      Spacing
Twisted pair     0 to 3.5 kHz       0.2 dB/km @              50 µs/km      2 km
(with loading)                      1 kHz

Twisted pairs    0 to 1 MHz         0.7 dB/km @              5 µs/km       2 km
(multi-pair                         1 kHz
cables)
Coaxial cable    0 to 500 MHz       7 dB/km @ 10             4 µs/km       1 to 9 km
                                    MHz
Optical fiber    186 to 370         0.2 to 0.5               5 µs/km       40 km
                 THz                dB/km




                                COE 341 – Dr. Marwan Abu-Amara                         8
Twisted Pair




               COE 341 – Dr. Marwan Abu-Amara   9
UTP Cables




             COE 341 – Dr. Marwan Abu-Amara   10
UTP Connectors




           COE 341 – Dr. Marwan Abu-Amara   11
Note: Pairs of Wires

   It is important to note that these wires work in
    pairs (a transmission line)
   Hence, for a bidirectional link
       One pair is used for TX
       One pair is used for RX




                      COE 341 – Dr. Marwan Abu-Amara   12
Twisted Pair - Applications

   Most commonly used guided medium
   Telephone network (Analog Signaling)
       Between house and local exchange (subscriber
        loop)
   Within buildings (Digital Signaling)
       To private branch exchange (PBX)
   For local area networks (LAN)
       10Mbps or 100Mbps
       Range: 100m
                     COE 341 – Dr. Marwan Abu-Amara    13
Twisted Pair - Pros and Cons

   Pros:
       Cheap
       Easy to work with
   Cons:
       Limited bandwidth
       Low data rate
       Short range
       Susceptible to interference and noise


                      COE 341 – Dr. Marwan Abu-Amara   14
Twisted Pair - Transmission Characteristics
   Analog Transmission
       Amplifiers every 5km to 6km
   Digital Transmission
       Use either analog or digital signals
       Repeater every 2km or 3km
   Limited distance
   Limited bandwidth (1MHz)
   Limited data rate (100Mbps)
   Susceptible to interference and noise

                       COE 341 – Dr. Marwan Abu-Amara   15
Attenuation in Guided Media




             COE 341 – Dr. Marwan Abu-Amara   16
    Ways to reduce EM interference


   Shielding the TP with a metallic braid or sheathing
   Twisting reduces low frequency interference
   Different twisting lengths for adjacent pairs help
    reduce crosstalk




                      COE 341 – Dr. Marwan Abu-Amara   17
Unshielded and Shielded TP
   Unshielded Twisted Pair (UTP)
       Ordinary telephone wire
       Cheapest
       Easiest to install
       Suffers from external EM interference
   Shielded Twisted Pair (STP)
       Metal braid or sheathing that reduces interference
       More expensive
       Harder to handle (thick, heavy)


                       COE 341 – Dr. Marwan Abu-Amara   18
STP: Metal Shield




             COE 341 – Dr. Marwan Abu-Amara   19
UTP Categories
   Cat 3
       up to 16MHz
       Voice grade found in most offices
       Twist length of 7.5 cm to 10 cm
   Cat 4
       up to 20 MHz
   Cat 5
       up to 100MHz
       Commonly pre-installed in new office buildings
       Twist length 0.6 cm to 0.85 cm
   Cat 5E (Enhanced) –see tables
   Cat 6
   Cat 7
                         COE 341 – Dr. Marwan Abu-Amara   20
    Near End Crosstalk (NEXT)
   Coupling of signal from one wire pair to another
   Coupling takes place when a transmitted signal
    entering a pair couples back to an adjacent
    receiving pair at the same end
   i.e. near transmitted signal is picked up by near
    receiving pair
Transmitted Power, P1
                                                                  Disturbing pair

    Coupled Received                                            Disturbed pair
    Power, P2
               “NEXT” Attenuation = 10 log P1/P2 dBs
               The larger … the smaller the crosstalk (i.e. the better the performance)
           NEXT attenuation is a desirable attenuation - The larger the better!
                                 COE 341 – Dr. Marwan Abu-Amara                     21
    Transmission Properties for Shielded & Unshielded TP
            Undesirable Attenuation- Smaller is better                        Desirable Attenuation- Larger is better!


                  Signal Attenuation (dB per 100 m)                    Near-end Crosstalk Attenuation (dB)



Frequency       Category           Category          150-ohm           Category           Category            150-ohm
  (MHz)          3 UTP              5 UTP              STP              3 UTP              5 UTP                STP

1              2.6                2.0               1.1               41                 62                 68?


4              5.6                4.1               2.2               32                 53                 58


16             13.1               8.2               4.4               23                 44                 50.4


25                    —           10.4              6.2                       —          41                 47.5


100                   —           22.0              12.3                      —          32                 38.5


300                   —                  —          21.4                      —                —            31.3

                                             COE 341 – Dr. Marwan Abu-Amara                                              22
 Twisted Pair Categories and Classes
             Category 3   Category 5             Category      Category 6   Category 7
              Class C      Class D                  5E           Class E      Class F




Bandwidth    16 MHz       100 MHz               100 MHz        200 MHz      600 MHz




Cable Type   UTP          UTP/FTP               UTP/FTP        UTP/FTP      SSTP




Link Cost    0.7          1                     1.2            1.5          2.2
(Cat 5 =1)




                              COE 341 – Dr. Marwan Abu-Amara                       23
Coaxial Cable
Physical Description:




                COE 341 – Dr. Marwan Abu-Amara   24
Physical Description




             COE 341 – Dr. Marwan Abu-Amara   25
Coaxial Cable Applications
   Most versatile medium
   Television distribution
       Cable TV
   Long distance telephone transmission
       Can carry 10,000 voice calls simultaneously
        (though FDM multiplexing)
       Being replaced by fiber optic
   Short distance computer systems links
   Local area networks (thickwire Ethernet
    cable)
                      COE 341 – Dr. Marwan Abu-Amara   26
Coaxial Cable - Transmission Characteristics

   Analog
       Amplifiers every few km
       Closer if higher frequency
       Up to 500MHz
   Digital
       Repeater every 1km
       Closer for higher data rates




                       COE 341 – Dr. Marwan Abu-Amara   27
Attenuation in Guided Media




             COE 341 – Dr. Marwan Abu-Amara   28
Optical Fibers
   An optical fiber is a very thin strand of silica glass
       It is a very narrow, very long glass cylinder with special
        characteristics. When light enters one end of the fiber it travels
        (confined within the fiber) until it leaves the fiber at the other end
   Two critical factors stand out:
       Very little light is lost in its journey along the fiber
       Fiber can bend around corners and the light will stay within it and be
        guided around the corners
   An optical fiber consists of three parts
       The core
           Narrow cylindrical strand of glass with refractive index n1
       The cladding
           Tubular jacket surrounding the core with refractive index n2
           The core must have a higher refractive index than the cladding for the
            propagation to happen
                                     n1 > n 2
                                COE 341 – Dr. Marwan Abu-Amara                   29
Optical Fibers (Contd.)
    Protective outer jacket
         Protects against moisture, abrasion, and crushing




                                                                       Individual Fibers:
                                                                       (Each having core & Cladding)




         Single Fiber Cable                     Multiple Fiber Cable




                              COE 341 – Dr. Marwan Abu-Amara                                           30
    Reflection and Refraction
   At a boundary between a denser (n1) and a rarer (n2)
    medium, n1 > n2 (e.g. water-air, optical fiber core-
    cladding) a ray of light will be refracted or reflected
    depending on the incidence angle
                                        Increasing Incidence angle, 1

rarer             2
                            v2 = c/n2
 n2                                                     90

                                                               Sin(90)        n
denser                 Sin( 2 ) n1                                            1
                                                              Sin( critical ) n2                1     2      1   2
                                
  n1                   Sin(1 ) n2                                                   n2
                                                              critical  sin 1 (      )
n1 > n2        1                                critical                           n1
                             v1 = c/n1
        Refraction                       Critical angle                                     Total internal
                                          refraction                                          reflection
          1   critical                                                                  1   critical
                                                         critical
                                          COE 3411– Dr. Marwan Abu-Amara                                             31
Optical Fiber


                         Refraction at boundary
                         for i < critical . Escaping light is absorbed in jacket

          Rarer                   n2
          Denser
          Denser          n1           n1
          Rarer




                                                              i
                                                   Total Internal Reflection at boundary
                                                   for i > critical
           n1 > n2


                  COE 341 – Dr. Marwan Abu-Amara                                 32
Attenuation in Guided Media




             COE 341 – Dr. Marwan Abu-Amara   33
Optical Fiber - Benefits
   Greater capacity
       Data rates of hundreds of Gbps
   Smaller size & weight
   Lower attenuation
       An order of magnitude lower
       Relatively constant over a larger frequency interval
   Electromagnetic isolation
       Not affected by external EM fields:
          No interference, impulse noise, crosstalk

       Does not radiate:
          Not a source of interference

          Difficult to tap (data security)

   Greater repeater spacing
       10s of km at least
                             COE 341 – Dr. Marwan Abu-Amara    34
Optical Fiber - Applications

   Long-haul trunks
   Metropolitan trunks
   Rural exchange trunks
   Subscriber loops
   LANs




                  COE 341 – Dr. Marwan Abu-Amara   35
Optical Fiber - Transmission Characteristics
   Act as wave guide for light (1014 to 1015 Hz)
       Covers portions of infrared and visible spectrum
   Light Emitting Diode (LED)
       Cheaper
       Wider operating temp range
       Last longer
   Injection Laser Diode (ILD)
       More efficient
       Greater data rate


                       COE 341 – Dr. Marwan Abu-Amara      36
Optical Fiber Transmission Modes                                                        Dispersion:
                                                                                        Spread in arrival
     n2   Refraction             Deep reflection                  Shallow reflection    time
           i < critical
n1




              Core                                                                        Large
                            Cladding                                          2 ways:
     n2
n1




                                                                                         Smaller
          • v = c/n
          • n1 lower away from center…this speeds up deeper rays
           and compensates for their larger distances, arrive together with shallower rays




                                                                                        Smallest


                                       COE 341 – Dr. Marwan Abu-Amara                              37
    Optical Fiber – Transmission modes
   Spread of received light pulse in time (dispersion) is bad:
       Causes inter-symbol interference  bit errors
       Limits usable data rate and usable distance
   Caused by propagation through multiple reflections at
    different angles of incidence
   Dispersion increases with:
       Larger distance traveled
       Thicker fibers with step index
   Can be reduced by:
       Limiting the distance
       Thinner fibers and a highly focused light source
         Single mode: High data rates, very long distances
       Graded-index thicker fibers: The half-way solution

                              COE 341 – Dr. Marwan Abu-Amara   38
Optical Fiber – Wavelength Division Multiplexing (WDM)

   A form of FDM (channels sharing the medium
    by occupying different frequency bands)
   Multiple light beams at different frequencies
    (wavelengths) transmitted on the same fiber
   Each beam forms a separate communication
    channel
   Example:
    256 channels @ 40 Gbps each  10 Tbps
    total data rate

                   COE 341 – Dr. Marwan Abu-Amara   39
Optical Fiber – Four Transmission bands (windows)
in the Infrared (IR) region
   Selection based on:
       Attenuation of the fiber
       Properties of the light sources                         S
                                                                       C L
       Properties of the light receivers


                                                                Bandwidth, THz
                                                                     33
                                                                     12
                                                                      4
                                                                      7


 Note: l in fiber = v/f = (c/n)/f = (c/f)/n = l in vacuum/n
 i.e. l in fiber < l in vacuum

                               COE 341 – Dr. Marwan Abu-Amara             40
Attenuation in Guided Media




             COE 341 – Dr. Marwan Abu-Amara   41
Wireless Transmission

   Free-space is the transmission medium
   Need efficient radiators, called antenna, to
    take signal from transmission line (wireline)
    and radiate it into free-space (wireless)
   Famous applications
       Radio & TV broadcast
       Cellular Communications
       Microwave Links
       Wireless Networks

                     COE 341 – Dr. Marwan Abu-Amara   42
Wireless Transmission Frequencies
   Radio: 30MHz to 1GHz
       Omni-directional
       Broadcast radio
   Microwave: 2GHz to 40GHz
       Microwave
       Highly directional
       Point to point
       Satellite
   Infrared Light: 3 x 1011 to 2 x 1014
       Localized communications

                       COE 341 – Dr. Marwan Abu-Amara   43
Antennas
   Electrical conductor (or system of..) used to
    radiate/collect electromagnetic energy
   Transmission
       Radio frequency electrical energy from transmitter
       Converted to electromagnetic energy by antenna
       Radiated into surrounding environment
   Reception
       Electromagnetic energy impinging on antenna
       Converted to radio frequency electrical energy
       Fed to receiver
   Same antenna often used for both TX and RX in 2-
    way communication systems


                         COE 341 – Dr. Marwan Abu-Amara      44
Radiation Pattern
   Power radiated in all directions
   Not same performance in all directions
                                                Radiation Patterns
   Isotropic antenna is (theoretical) point in
    space
       Radiates in all directions equally
       Gives spherical radiation pattern
       Used as a reference for other antennae
   Directional Antenna                                        Isotropic
       Concentrates radiation in a given desired
        direction
            Used for point-to-point, line of sight
             communications
        Gives “gain” in that direction
        relative to isotropic                                  Directional


                              COE 341 – Dr. Marwan Abu-Amara                 45
Parabolic Reflective Antenna
   Used for terrestrial and satellite microwave
   Source placed at focus will produce waves reflected
    from parabola parallel to axis
       Creates (theoretical) parallel beam of light/sound/radio
       In practice, some divergence (dispersion) occurs, because
        source at focus has a finite size (not exactly a point!)
   On reception, signal is concentrated at focus, where
    detector is placed
   The larger the antenna (in wavelengths) the better
    the directionality



                         COE 341 – Dr. Marwan Abu-Amara         46
Parabolic Reflective Antenna




                                              Axis




             COE 341 – Dr. Marwan Abu-Amara          47
Antenna Gain, G
   Measure of directionality of antenna
   Power output in particular direction compared
    with that produced by isotropic antenna
   Measured in decibels (dB)
   Increased power radiated in one direction
    causes less power radiated in another
    direction (Total power is fixed)
   Effective area, Ae, relates to size and shape
    of antenna
        Determines antenna gain
                                              4 Ae         4 f Ae
                                                               2
                                   G                     
                                                 l    2
                                                               c 2

                     COE 341 – Dr. Marwan Abu-Amara                   48
Antenna Gain, G: Effective Areas
                      4 Ae         4 f 2 Ae
                G                
                        l   2
                                       c 2

   An isotropic antenna has a gain G = 1 (0 dBi)
   i.e.      l2
         Ae       ( 0.1 cm 2 at 30 GHz - a ' Point Source" )
              4
   A parabolic antenna has:
                                   A = Actual Area =  r2
          Ae  0.56 A
                                           4 (0.56 A)           7A
   Substituting we get:         G                          
                                                   l     2
                                                                 l2
   Gain in dBi = 10 log G
   Important: Gains apply to both TX and RX antennas
                        COE 341 – Dr. Marwan Abu-Amara                49
Terrestrial Microwave
   Parabolic dish
   Focused beam
   Line of sight
       Curvature of earth limits maximum range  Use
        relays to increase range (multi-hop link)
   Long haul telecommunications
   Higher frequencies give higher data rates but
    suffers from larger attenuation



                     COE 341 – Dr. Marwan Abu-Amara     50
    Terrestrial Microwave: Propagation Attenuation

   As signal propagates in space, its power drops
    with distance according to the inverse square law
          1
     Pd  2                               While with a guided medium, signal
         d                               drops exponentially with distance…
                                          giving larger attenuation and lower
     d’ = distance in l’s                 repeater spacing


i.e. loss in signal power over distance traveled, d
      L  d 2                                  • Show that L increases by 6 dBs for
                                                every doubling of distance d.
                      4 d 
                                2
                                                • For guided medium, corresponding
            10log10 
                        l 
     LdB                                        attenuation = a d dBs, a = dBs/km
                           
                                    COE 341 – Dr. Marwan Abu-Amara                     51
Satellite Microwave
   Satellite is relay station
   Satellite receives on one frequency (uplink),
    amplifies or repeats signal and transmits on
    another frequency (downlink)
   Requires geo-stationary orbit
       Height of 35,784km
   Applications
       Television
       Long distance telephone
       Private business networks


                      COE 341 – Dr. Marwan Abu-Amara   52
Satellite Point to Point Link

                 Relay




        Uplink                                     Downlink




                  COE 341 – Dr. Marwan Abu-Amara              53
Satellite Broadcast Link




              COE 341 – Dr. Marwan Abu-Amara   54
Broadcast Radio
   Omni-directional
     No dishes
     No line of sight requirement

     No antenna alignment

   Applications
       FM radio
       UHF and VHF television
   Suffers from multipath interference
       Reflections (e.g. TV ghost images)


                      COE 341 – Dr. Marwan Abu-Amara   55
Infrared

   Modulate non-coherent infrared light
   Line of sight (or reflection)
   Blocked by walls
   No licensing required for frequency allocation
   Applications
       TV remote control
       IRD port



                      COE 341 – Dr. Marwan Abu-Amara   56

				
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