Module 3.1: Telephony and Modems

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Module 3.1 Telephony and Modems

• Conversion Schemes
• Voice Digitization
• Why 56k modems, not more?

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Different Conversion Schemes

• Some Jargon
• D to D encoding or conversion.
• Encoding means representation of data into
  digital signal
• A to D conversion or digitizing analog signal.
• D to A conversion or modulating digital signal.
• A to A conversion or modulating analog signal.

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Digital to Digital Conversion

• Different D/D encoding
• Cost
• Voltage level
• Number of levels
• Number of signal changes
• Speed
• Synchronization
• Detection
• Reliability
• noise
• Bit errors

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Examples of Polar Encoding
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Analog to Analog Conversion

• Examples are radio, TV, and satellite
  communication

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Amplitude Modulation
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Frequency Modulation
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Structure of Telephone System

• End office, known also as local central office
• Local loops (twisted pairs, analog signaling)
• Trunks (fiber optics or microwave, mostly
  digital)
• Intermediate switches

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Need for A/D and D/A conversions

• Modem converts D/A signals
• Codec converts A/D signals

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Analog to Digital Conversion (Codec)
First Step Pulse Amplitude Modulation (PAM)

• According to Nyquist theorem, the sampling rate
  must be at least two times the frequency to
  ensure the accurate reproduction of the original
  signal.

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Second step Quantized PAM Signal
Quantizing Using Sign and Magnitude
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Third Step Pulse Code Modulation (PCM)
Remember this
Out of each sample, Telephone companies only
uses the upper 7 bits. The lower bit (bit 0) is
always assumed to be 0. For example, sampling
values 024 ? 024, 038 ? 038, 025 ? 024,
039 ? 038 This is not affecting sampling values
much. In transmission, the most significant bit
is used for control purposes, as we will see
later.
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From Analog to PCM digital signal
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Digital to Analog Modulation (Modem)
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Amplitude Shift Keying (ASK)
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
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Quadrature Amplitude Modulation (QAM)
QAM means combining ASK and PSK in such a way we
have the maximum contrast between each bit,
dibit, tribit, quadbit, and so on.
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Comments

• Bit rate is the number of bits per second. Baud
  rate is the number of signal units per second.
  The encoding/represenation of one baud is called
  symbol. Baud rate is less than or equal to the
  bit rate.

• Shanon formula and quantization limits data rate
  to 33.6kbps. How about 56K modems?
• 56K modems are achieved if we have an ISP that
  uses digital signaling. Uploading will always be
  33.6k due to PCM quantization at the CO Codec.
  However, downloading can be 56k since there is no
  PCM quantization at the CO Codec. Quantization
  always limits the data rate.
• Why only 56K in downloading? Because of the way
  voice is digitized. CO switches use PCM and
  inverse PCM at sampling rate of 8000 samples and
  7 bits per sample, which gives 8000 x 7 of data
  rate.

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Why 56k?
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Traditional Modems
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56K modems