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analog

IIUC Notes - Old School Voice Stuff

These are the notes I’ve taken as I read through the study materials.  Feel free to correct anything you see.

  • Analog phone signaling
    • Misc
      • Ground = positive = tip
      • Battery = negative = ring
      • Signaling uses specific frequencies for specific events
    • Loop start signaling
      • When a circuit in the phone is completed (i.e., you take it off-hook), the CO detects it and provides services.
      • Susceptible to glare, where the phone requests dialtone at the same time that the CO sends a call.
        • Can connect two different calls if in a business with multiple lines
    • Ground start signaling
      • The circuit is temporarily completed to signal the CO for services
      • Doesn’t connect any call to any phone directly
      • Used in PBXes.
    • Supervisory signaling
      • On-hook:  Circuit is open
      • Off-hook:  Circuit is completed
      • Ringing:  AC current generated by CO to tell the phone to ring
    • Informational signaling
      • Gives information for the caller to use
      • Dial tone
      • Busy
      • Ringback: the ring you hear when you call
      • Confirmation:  the call is being attempted
      • Congestion:  no lines available to make the call
      • Receiver off-hook
      • Reorder:  can’t make the call
      • No such number:  can’t find the endpoint
    • Address signaling
      • Used to send digits
      • Dual-tone multifrequency (DTMF):  uses two electrical signals to indicate a digit; touch tone
      • Pulse:  flashes the circuit to indicate a digit; rotary dial
    • Disadvantages of analog signaling
      • Attenuation
      • Repeaters can’t differentiate between call and noise
      • One cable pair for each call; think about a pair for each call taking place in Manhattan right now
  • Digitizing voice
    • Steps
      • Sampling: taking samples of the voice
        • Nyquist method:  sample rate = 2 x highest frequency
        • Human voices usually stay below 4000Hz, so a good sampling rate is 8000 samples/second.
        • Pulse-amplitude modulation (PAM)
      • Quantization:  assigning values to the sample
        • Assignment based on amplitude of the signal
        • Logarithmic scale for better accuracy at the more common amplitudes
      • Encoding:  converting quantization to binary
        • Pulse-code modulation (PCM)
        • 8 bits/sample * 8k samples/second = 64k bpbs
      • Compression:  optionally compress the binary information
    • Advantages
      • Transmitting numbers is less susceptible to attenuation
      • Multiple digital voice signals can use same pair
        • Time division multiplexing (TDM)
  • Digital signaling
    • 24 channels * 8 bits/sample = 192 bits of voice
    • The T1 frame sends all 24 channels in one T1 frame with 1 bit for framing bit, so the T1 frame = 193 bits
    • 193 bits/frame * 8k frames/second = 1.544 Mbps
    • Channel associated signaling (CAS):  steals bits in a channel for signaling
      • The 8th bit of every 6th sample is stolen for signaling
      • Super frame (SF) uses 12 frames to synchronize a signal, so 12 samples are required to be received for synchronization (12/8000 second).
      • Extended super frame (ESF) uses 24 frames; 2000 bps for sync, 2000 bps for errors, 4000 bps for control and reporting
    • Common channel signaling (CAC):  uses a dedicated channel for signaling
      • Q.931 is a CAC signaling standard.
  • The PSTN
    • Phone companies connect together using SS7 signaling (a CAC method), which is responsible for routing the call.
    • E.164 is an ITU standard for phone numbers.
      • Country code
      • National destination code
      • Subscriber number

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