Voice Transmission Basic Concepts

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Voice Transmission --Basic Concepts--

• Voice---is analog in character and moves in the
  form of waves.
• 3-important wave-characteristics
• Amplitude
• Frequency
• Phase

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Telephone Handset (has 2-parts)

• 1. Transmitter (i.e mouthpiece)
• consists of a movable speaker diaphragm that is
  sensitive to both amplitude and frequency
• The diaphragm contains carbon particles that can
  conduct electricity.
• As the human voice spoken into the transmitter
  varies, the amount of carbon granules that strike
  the electrical contacts in the mouthpiece also
  variesthereby sending varying analog electrical
  signals out into the voice network.

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Telephone Handset (has 2-parts)

• 2. Receiver (i.e mouthpiece)
• Acts in an opposite direction to the mouthpiece.
• The electrical signal/waves produced by the
  transmitter are received at an electromagnet in
  the receiver.
• Varying levels of electricity produce varying
  levels of magnetismthat, in turn, cause the
  diaphragm to move in direct proportion to the
  magnetic variance.
• The moving diaphragm produces varying sound that
  corresponds to the sound waves that were input at
  the transmitter.

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Figure 2-1 Getting Voice Onto and Off the Network
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Figure 2-2 Voice Bandwidth
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Figure 2-3 Basic Telecommunications
Infrastructure
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Telephone Call Characteristics

• Telephone calls can be
• Local-LATA
• Inter-LATA
• Intra-LATA
• LATA Local Access Transport Areas
• Local loop---isanalog in character.
• Trunk line----isdigital in character.
• Interexchange circuit----digital in character.

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Figure 2-4 Representative Voice Network Heirarchy
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Telephone Number Plans

• 3 Basic parts of US-calls
• 3-digit area code---(817)
• 3-digit exchange---(496)
• 4-digit subscriber number---(3650)
• 4 Basic parts of an International call
• 011
• Country code
• City code
• City number

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Telephone Number

• Each area code can support
• 1000 exchanges
• Each exchange can support
• 10,000 telephone numbers
• Each area code can support
• 103 X 104 107 10 million phone numbers

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Voice Digitization in the POTS

• Traditional POTS was analog.
• The current telephone system of the POTS combines
  both analog and digital transmission
  technologies.
• Why Voice digitization
• Ensures better quality (than analog)
• Provides higher capacity (than analog)
• Deals with longer distance (than analog)

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Voice DigitizationHow!

• Digitization is just a discrete electrical
  voltage.
• Electrical pulses can be varied to represent
  characteristics of an analog voice signal.
• 5-different VD-techniques
• PAM pulse amplitude modulation
• PDM pulse duration modulation
• PPM Pulse position modulation
• PCM Pulse code modulation
• ADPCM Adaptive differential PCM

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Figure 2-7 Voice Digitization PAM, PDM, PPM
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What is a Codec!

• Codec Coder Decoder
• Modem Modulator Demodulator
• Codec is a voice-connector device that samples
  the analog POTS transmission into a stream of
  binary digits using PCM.
• For quality digital transmission over a Codec
• 8000 samples/sec
• Each sample 1 byte 8 bits
• 8000 X 8 64, 000 bits/sec----( DS-0 circuit)
• Uses T-1 circuit1.544 Mbps---(equivalent to 24
  DS-0)

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Codec and Modem!

• Codec is a voice-connector device
• Modem is a data-connector device.
• All high speed Codecs operate at multiples of
  DS-0 speed
• DS-0 provides 64 Kbps
• T-1 DS-1 provides 1.544 Mbps
• T-2 DS-2 provides 6.312 Mbps
• T-3 DS-3 provides 33.375 Mbps
• T-4 DS-4 provides 274.176 Mbps

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Figure 2-8 Voice Digitization Pulse Code
Modulation (PCM)
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Voice Compression!

• ADPCM--adaptive differential PCM-is also known as
  a voice compression technique, because of its
  ability to transmit 24 digitized voice
  conversations in half the bandwidth required by
  the PCM.
• More advanced VCs usedigital signal processors
  that take the digitized PCM and further
  manipulate and compress it.
• Each voice compression technique seeks to reduce
  the amount of transmitted voice-info in one way
  or another.