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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jac

Aaron does lots of stuff and, therefore, does nothing well. He's trying, though.

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