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					        Serial communications principles
• Serial communication principles
    – Parallel/serial & serial/parallel converters
    – Physical media: cable & optical fiber
    – Advantages of optical fiber systems
• Optical signaling
    – Intensity modulation
    – Other methods: PSK and FSK




Aussois, 26            Serial communication principles   1
November 1998
                Parallel/Serial Conversion
                                           Register      Shitf Register
                                                                                 Serial
                                           DO   Q0         DO     Q
                                                                                Data Out
                   Parallel
                   Data In

                                           D7   Q7         D7

                              Word Clock
                                                     x8 (PLL)


                   Parallel Data
                         D7:D0
                   Word Clock


                   Serial Clock


                   Serial Data
                                     D7    D6   D5    D4   D3    D2       D1   DO



•   Bits in word are time division multiplexed
•   Bit Rate = (Word Rate) x (# Bits in Word)
•   Advantage: Physically simpler system
•   HEP disadvantage: adds at least one clock cycle of latency
Aussois, 26                      Serial communication principles                           2
November 1998
                Serial/Parallel Conversion
                                              Shitf Register         Register
                    Serial
                                                D     Q0             DO   Q0
                    Data In
                                                                                     Parallel
                                                                                     Data Out

                                                      Q7             D7   Q7

                                      Clock
                                     Recovery                  1/8


                     Serial Data
                                    D7   D6     D5    D4   D3        D2   D1    DO
                     Serial Clock


                     Word Clock


                     Parallel Data
                                                                                     D7:D0



•   Serial bits are time division de-multiplexed
•   Requires:
     – Clock recovery or
     – Transmitted “bit clock”
•   HEP disadvantage: Adds at least one clock cycle of latency
Aussois, 26                    Serial communication principles                                  3
November 1998
              Physical Media: Cable & fiber
     Electrical transmission link:

          Parallel                                                       Parallel
                           P/S       E/E                 E/E       S/P
          Data In                              cable                     Data Out


     Lightwave transmission link:

          Parallel                                                       Parallel
                           P/S       E/O                O/E        S/P
          Data In                              fiber                     Data Out


•   After P/S conversion a physical medium is necessary to “transport” the data
•   An electrical link requires an E/E interface:
     –   Driving capability
     –   Impedance matching
     –   Differential signaling
     –   Ground potential differences accommodation, etc.
•   A Lightwave link requires an optoelectronics interface:
     –   Electrical/Optical conversion (E/O)
     –   Optical/Electrical conversion (O/E)
Aussois, 26                      Serial communication principles                    4
November 1998
                                     Cable Link Capacity
•   Capacity is limited by the cable attenuation and bandwidth
•   Attenuation is typically 20-30dB/km (coaxial cable)
•   The cable transfer function is inversely proportional to:    frequency
•   For high bit rates the capacity is mainly limited by the cable bandwidth:
     – 1Mbit/s => L(max) = 40km
     – 100Mbit/s => L(max) = 4km

                                        Transmission distance vs Bit rate
                      100
            Distance (km)




                            10




                             1
                                 1                    10                    100
                                            Bit Rate (Mbit/s)

Aussois, 26                            Serial communication principles            5
November 1998
                Optical fiber Link Capacity
• Optical fiber systems capacity is limited by fiber attenuation and
  bandwidth (dispersion)
• For a typical long haul 1300nm fiber system:
     – fiber attenuation: 0.4dB/km
     – Dispersion limit: 250 Gbit/s*km
•   For low to high bit rates the system span is limited by
    attenuation
     – 100Mbit/s => 120km
• For very high bit rates the system span is dispersion limited
     – 10 Gbit/s => 25km
• The capacity of optical fiber systems is orders of magnitude
  higher than that of cable systems




Aussois, 26              Serial communication principles               6
November 1998
    Advantages of Optical fiber Communications
•   Enormous potential bandwidth
•   Small size and weight (+ for HEP)
•   Electrical isolation (+ for HEP)
•   Immunity to noise and crosstalk (+ for HEP)
•   Signal security
•   Low transmission loss
•   Raggedness and flexibility




Aussois, 26         Serial communication principles   7
November 1998
                   Direct Intensity Modulation

    Intensity Modulation: Direct

    Parallel                                                               Parallel
                       P/S                                      S/P
    Data In                            Fiber                               Data Out



•    The serial data stream out of the serializer is converted into current pulses
•    The current pulses are used to directly modulate the light produced by a laser-
     diode
      –   “0” - light “OFF”
      –   “1” - light “ON”
•    The light is coupled into the optical fiber
•    The light out of the optical fiber is detected by a photo-diode, which converts the
     optical pulses into electrical pulses
•    The current pulses are converted into a voltage and amplified before they are
     feed to the serial to parallel converter
•    Direct intensity modulation can be used for data rates up to a few Gbit/s
Aussois, 26                   Serial communication principles                          8
November 1998
               External Intensity Modulation

    Intensity Modulation: External

                                                                  Parallel
                         Modulator                          S/P
                                     Fiber                        Data Out


    Parallel
                  P/S
    Data In


•    A laser diode is operated continuously and its light is coupled to an
     external modulator
•    The external modulator modulates the light in “on” / “off” according to
     the data pattern
•    External modulation reduces the spectral broadening of the light source
•    External modulation allows data rates of the order of several tens of
     Gbit/s
Aussois, 26               Serial communication principles                    9
November 1998
                             Coherent detection
                     Beam
                     Splitter



         Signal


                                          (mixer)
                 Local
                Oscillator      ~
•   In coherent detection systems the signal information is carried by the
    phase, the frequency or the intensity of the optical carrier
•   At the receiver the signal is mixed with a local oscillator signal:
    homodyne or heterodyne
•   The PIN diode is used as the mixer (square law device)
•   Coherent receivers can achieve close to quantum limited noise
    performance
•   However, they are complex and expensive and consequently not used
    in practice
Aussois, 26                         Serial communication principles      10
November 1998

				
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