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ISDN

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					   Alternatives to Ordinary Phone
         Lines & Modems
• Faster Speeds; Called Broadband Services
• Also Higher Cost
  – Translation devices more expensive than modems
  – Faster (more expensive) transmission line to ISP
  – ISP may charge more because of heavier
    transmission load
  – Faster interface than 232 serial port may be
    needed
  Transmission Line Versus ISP
• Transmission line may be provided by a
  transmission carrier, not the ISP

• Users have to pay separate fees to carrier and ISP

• Some ISPs also provide transmission line
   – But not all do
   – Transmission line costs must always be considered

         PC                                 ISP
                   Transmission Line
          Translator Devices
• Device versus Line
             Device
                                    Transmission Line
               Translator

                       Analog Line          Digital Line

   Analog Device                              Codec

   Digital Device           Modem              DSU
             ISDN

Integrated Services Digital Network
             Codecs
              DSUs
                     ISDN
• Get ISDN line from telephone company
  – Multiplexes (mixes) three channels on one UTP
    wire pair to the desktop

             B                    D Channel:
                     ISDN        control signals
           Channel
                     Modem
        1101                   2B+D

                             Multiplexed
                              Onto One
                 B           Set of Wires
               Channel
                    ISDN
• Two B Channels are 64 kbps
  – Original idea: one for voice, one for data

            B
          Channel
       1101


                B
              Channel
                  ISDN
• Can now “bond” the two B channels for 128
  kbps data if ISP supports it
• D Channel is 16 kbps (for supervisory
  signaling)

                                  Bonding
          ISDN    B   Telephone
                                            ISP
          Modem   B    Network
                                   128
                                   kbps
               ISDN Costs
• Must install an ISDN line
  – Will cost more than telephone line

• Actually, could use your existing phone line
  – But then would need one B channel for
    telephony
              ISDN Costs
• Line is expensive
  – $60-$80/mo plus installation fee
  – ISP charge is separate; may charge more for
    ISDN access

• Dial Up: Not always connected
  – Do not have to pay for full-time use

• Need “ISDN modem” (expensive)
               ISDN Modem
• ISDN “Modem” is a Misnomer
  – Modem is for for digital device, analog line
  – ISDN line is digital

• ISDN modem really contains
  – Codec to link analog telephone to digital ISDN line
  – Data service unit (DSU) to translate between PC digital
    format and ISDN digital format (voltage levels, timing,
    etc.)
                ISDN Modem
• Codec
  – Translates analog device signal into digital
    signal for propagation
  – Constantly samples the intensity of the voice
    analog signal
                    Sample

      0110010

                    1/8,000 sec
              ISDN Modem
• Codec
  – Divides each second into 8,000 sampling periods
  – Only measures intensity of voice signal in each


      Intensity
                     Sample
        Value
      0110010

             1/8,000 sec Sampling Period
             ISDN Modem
• Codec
  – Measures voice intensity as an 8-bit intensity
    value (0-255)
  – Overall, sends 8 bits 8,000 times per second (64
    kbps)
      Intensity
                    Sample
       Value
      0110010

           1/8,000 sec Sampling Period
              ISDN Modem
• Codec
  – This is why telephone channels are 64 kbps
     • Designed for digitized voice
     • Often steal 8 kbps for supervisory signaling, so 56
       kbps

     Intensity
                      Sample
       Value
     0110010

            1/8,000 sec Sampling Period
       DSU (Data Service Unit)
• DSU translates between different digital formats
   – Device and line are both digital, but still must have
     translator
      •   Different bit rates
      •   Different number of possible states
      •   Different voltage levels for the states
      •   Different ways to represent ones and zeros

                                                       Digital Line
                               DSU

				
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