ICSA 341 (Updated 12/2001)

Encoding

• There are four types of encoding possible.– Digital Encoding of Digital Data– Digital Encoding of Analog Data

– Analog Encoding of Digital Data– Analog Encoding of Analog Data

ICSA 341 (Updated 12/2001)

PolarUnipolar Bipolar

Digital Encoding Schemes

Digital Encoding of Digital Data

Digital Data of 0s and 1s

ICSA 341 (Updated 12/2001)

TIME

1 1 1 10 0 0 1 0

+ V

0 V

AM

PL

ITU

DE

UNIPOLAR (positive voltage = 1; zero voltage = 0)

Unipolar Encoding Unipolar – Single voltage - one value Zero voltage – another value

Polar Encoding Schemes

POLAR

RZNRZ BIPHASE

NRZ-L NRZ-I ManchesterDifferentialManchester

ICSA 341 (Updated 12/2001)

0 0 0 01 1 1 0 1

- V

+ V

TIME

0 V

AM

PL

ITU

DE

POLAR: NRZ-L (positive voltage = 0; negative voltage = 1)

ICSA 341 (Updated 12/2001)

1 1 1 10 0 0 1 0

- V

+ V

TIME

0 V

AM

PL

ITU

DE

POLAR: NRZ-I (change if next bit is a 1)

ICSA 341 (Updated 12/2001)

1 1 1 10 0 0 1 0

- V

+ V

TIME

0 V

AM

PL

ITU

DE

POLAR: RZ (basically NRZ-L with return to zero at midbit)

Manchester Code

• Transition in the middle of each bit period• Transition provides clocking and data• Low-to-high=1, high-to-low=0• Used in Ethernet

ICSA 341 (Updated 12/2001)

1 1 1 10 0 0 1 0

- V

+ V

TIME

0 V

AM

PL

ITU

DE

BI-PHASE: Manchester - shift to opposite pole at midbit (neg-to-pos = 1 while a pos-to-neg = 0)

Differential Manchester

• Midbit transition is only for clocking• Transition at beginning of bit period=0• Transition absent at beginning=1• Has added advantage of differential encoding• Used in token-ring

ICSA 341 (Updated 12/2001)

1 1 1 10 0 0 1 0

- V

+ V

TIME

0 V

AM

PL

ITU

DE

BI-PHASE: Differential Manchester - shift to opposite pole at midbit (transition = 0 and no transition = 1)

Bipolar Encoding Schemes

BIPOLAR

B8ZSAMI HDB3

AMI - Alternate Mark InversionB8ZS - Bipolar 8 Zero Substitution (North America)HDB3 - High Density Bipolar 3 (Europe & Japan)

ICSA 341 (Updated 12/2001)

Encoding

– Digital Encoding of Digital Data– Digital Encoding of Analog Data

– Analog Encoding of Digital Data– Analog Encoding of Analog Data

ICSA 341 (Updated 12/2001)

Analog to Digital Encoding

• Pulse Amplitude Modulation (PAM)– sample analog signal (measure amplitude of

signal at equal intervals)– uses sample and hold technique– generate pulses based on sampling

• First step in PCM - Pulse Code Modulation– Quantize PAM Pulse - assign an integral value

in a specific range to sampled instances.

ICSA 341 (Updated 12/2001)

Sampling Rate

• How much should you sample?

• The sampling theorem (Nyquist Theorem): If a signal is sampled at regular intervals of time and at a rate higher at least twice the significant signal frequency, the samples contain sufficient information to ensure accurate reproduce of the original signal.

• 8000 samples/sec sufficient for 4000hz

ICSA 341 (Updated 12/2001)

Encoding

– Digital Encoding of Digital Data– Digital Encoding of Analog Data

– Analog Encoding of Digital Data– Analog Encoding of Analog Data

ICSA 341 (Updated 12/2001)

Analog Encoding of Digital Data

• Bit Rate - The number of bits transmitted in 1 second

• Baud Rate - The number of signal units per second required to represent those bits

• Baud rate is less than or equal to bit rate

• Carrier Signal ?

ICSA 341 (Updated 12/2001)

Methods of Modulation

• amplitude shift keying (ASK)

• frequency shift keying (FSK)

• phase shift keying (PSK)

• differential phase shift keying(DPSK)

• quadrature amplitude modulation (QAM)

ICSA 341 (Updated 12/2001)

1 0 0 1

ASK Illustration

ICSA 341 (Updated 12/2001)

1 1 0 1

FSK Illustration

ICSA 341 (Updated 12/2001)

0 0 1 1

PSK Illustration

ICSA 341 (Updated 12/2001)

Complex Modulations• Combining modulation techniques allows us to

transmit multiple bit values per signal change (baud)

• Increases information-carrying capacity of a channel without increasing bandwidth

• Increased combinations also leads to increased likelihood of errors

• Typically, amplitude and phase modulation are combined

ICSA 341 (Updated 12/2001)

Quadrature Amplitude Modulation (QAM)

• the most common method for quadbit transfer

• combination of 8 different angles in phase modulation and two amplitudes of signal

• provides 16 different signals, each of which can represent 4 bits

ICSA 341 (Updated 12/2001)

90

45

0

135

180

225

270

315

amplitude 1

amplitude 2

Quadrature Amplitude Modulation Illustration

ICSA 341 (Updated 12/2001)

Bit & Baud Rate ComparisonEncoding Units Bits/Baud Baud Bit RateASK,FSK,2-PSK

Bit 1 N N4-PSK,4-QAM Dibit 2 N 2N8-PSK,8-QAM Tribit 3 N 3N16-QAM Quadbit 4 N 4N32-QAM Pentabit 5 N 5N64-QAM Hexabit 6 N 6N128-QAM Septabit 7 N 7N256-QAM Octabit 8 N 8N

ICSA 341 (Updated 12/2001)

Encoding

– Digital Encoding of Digital Data– Digital Encoding of Analog Data

– Analog Encoding of Digital Data– Analog Encoding of Analog Data

ICSA 341 (Updated 12/2001)

Methods of Modulation

• amplitude modulation (AM)

• frequency modulation (FM)

• phase modulation (PM)

ICSA 341 (Updated 12/2001)

Codec

• Coder/Decoder

• converts analog signals into a digital form and

• converts back digital data to analog signals

• e.g., hi-fi music, television pictures, the output of copying machine, videoconferencing