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

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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