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Module 3.1: Encoding and Modulating

• Conversion Schemes

• D to D Conversion

• A to A Conversion

• A to D Conversion

• D to A Conversion

• Final comments

• 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)

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.

Remember this

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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.

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