Module 3.1: Telephony and Modems
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Conversion Schemes
Voice Digitization
Why 56k modems, not more?
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Different Conversion Schemes
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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
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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
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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
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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
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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)
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According to Nyquist
theorem, the sampling
rate must be at least two
times the frequency to
ensure the accurate
reproduction of the
original signal.
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
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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.
ITU-T modem
Baud Rate
Bit Rate
Modulation
V.21
300
300
FSK
V.22
600
1200
4-PSK
V.29
2400
9600
16-QAM
V.34
2400
28800 or (33600)
4096-QAM
(data compression)
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
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