Digital Communications Overview
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Questions and Answers

A digital signal is degraded by noise at each repeater.

False (B)

Error correcting codes cannot fix bit errors in digital signals.

False (B)

Multiplexing is the process of combining multiple signals onto a single channel.

True (A)

Digital signal processing (DSP) is a technique used in analog circuits.

<p>False (B)</p> Signup and view all the answers

Analog systems use electrical signals that vary continuously, not having discrete values.

<p>True (A)</p> Signup and view all the answers

Parallel data transmission is slower than serial data transmission.

<p>False (B)</p> Signup and view all the answers

Digital signals are electrical representations of signals from nature like pressure, light, and sound.

<p>False (B)</p> Signup and view all the answers

Serial transmission requires one wire for each bit of information.

<p>False (B)</p> Signup and view all the answers

Digital circuits operate on analog signals performing logic and arithmetic functions.

<p>False (B)</p> Signup and view all the answers

Serial buses are being replaced by parallel buses in high-speed applications.

<p>False (B)</p> Signup and view all the answers

The migration to digital systems started around 25 years ago.

<p>True (A)</p> Signup and view all the answers

The least significant bit (LSB) is transmitted last in serial transmission.

<p>False (B)</p> Signup and view all the answers

One of the main advantages of digital communications is its immunity to noise.

<p>True (A)</p> Signup and view all the answers

Analog signals are received, amplified and retransmitted at each microwave relay station, making the noise stronger with each transmission.

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Digital signals can be completely stripped of noise through a process called signal regeneration.

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The primary advantage of using digital communications is that the signals can be easily amplified and retransmitted without noise.

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The number of levels used by a telephone is $2^8$ because it uses 8-bit encoding.

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The number of levels used by a CD is $2^{16}$ which equates to 32,768 levels.

<p>False (B)</p> Signup and view all the answers

The number of levels used by a CD is $2^{12}$, which is 4,096 levels.

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The number of levels in a signal is the number of data levels used.

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Pulse rate defines the number of pulses per second and the bit rate describes the period of a bit.

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If a one-millisecond pulse is sent, its pulse rate is 1000 pulses per second.

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Pulse rate is calculated by multiplying the pulse duration by log base 2 of the signal level.

<p>False (B)</p> Signup and view all the answers

A signal with a bit rate of 1000 bits per second would send two data levels with a pulse duration of 1ms.

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Digital modulation can be used for wireless communication.

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Pulse modulation is a method of converting analog information to a digital format.

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Pulse Amplitude Modulation (PAM) is a type of digital pulse modulation.

<p>False (B)</p> Signup and view all the answers

Pulse Width Modulation (PWM) is a type of digital pulse modulation.

<p>False (B)</p> Signup and view all the answers

Pulse Position Modulation (PPM) is a type of digital pulse modulation.

<p>False (B)</p> Signup and view all the answers

Sampling is the process of taking periodic samples of a waveform, and the more samples that are taken, the more the final outcome will resemble the original waveform.

<p>True (A)</p> Signup and view all the answers

Increasing the signal power can improve noise immunity in digital modulation.

<p>True (A)</p> Signup and view all the answers

The pulse rate is calculated by taking the number of pulses per time, and dividing by 1000.

<p>True (A)</p> Signup and view all the answers

The bit rate is independent of the pulse rate.

<p>False (B)</p> Signup and view all the answers

The DC offset in a signal is undesirable because a signal with a DC offset is generally considered to be more stable.

<p>False (B)</p> Signup and view all the answers

NRZ-I is a line coding scheme that uses a change in voltage level to represent a logic 1 and no change in voltage level to represent a logic 0.

<p>True (A)</p> Signup and view all the answers

Self-synchronization is the process of ensuring that the sender and receiver clocks are synchronized in a digital transmission, even if there is a small difference in their speeds.

<p>False (B)</p> Signup and view all the answers

In NRZ-L (NRZ-Level), a positive voltage level typically represents a logic 1, and a negative voltage level typically represents a logic 0.

<p>False (B)</p> Signup and view all the answers

Manchester encoding is a type of unipolar line coding.

<p>False (B)</p> Signup and view all the answers

In a digital transmission with a data rate of 1 Mbps, if the receiver clock is 0.1% faster than the sender clock, the receiver will receive approximately 1000 extra bits per second.

<p>True (A)</p> Signup and view all the answers

The bit rate of a digital signal is determined by the sampling rate and the number of bits per sample.

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Parallel transmission offers a faster data transfer rate but requires more complex and expensive wiring.

<p>True (A)</p> Signup and view all the answers

Synchronous transmission is ideal for applications where data transfer is intermittent, like keyboard input.

<p>False (B)</p> Signup and view all the answers

In asynchronous transmission, the receiver relies on start and stop bits to synchronize with the incoming data stream.

<p>True (A)</p> Signup and view all the answers

Asynchronous transmission is generally faster and more efficient than synchronous transmission, especially for high-speed data transfer.

<p>False (B)</p> Signup and view all the answers

The main advantage of parallel transmission is its low cost.

<p>False (B)</p> Signup and view all the answers

Data link layer is responsible for byte synchronization in synchronous transmission.

<p>True (A)</p> Signup and view all the answers

The human voice typically contains frequencies ranging from 0 to 8000 Hz.

<p>False (B)</p> Signup and view all the answers

Flashcards

Analog systems

Systems that use continuous electrical signals to represent information.

Digital signals

Signals that represent data using discrete voltage levels, typically 0 and 1.

Binary values

The two discrete values (1 and 0) used in digital signals.

Signal regeneration

The process of removing noise from digital signals for clear transmission.

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Noise immunity

The ability of digital systems to differentiate between binary 0 and 1 despite noise.

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Analog vs Digital

Analog signals vary continuously; digital signals use discrete values.

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Examples of analog systems

Systems like AM/FM radio, cassettes, and standard television that use analog signals.

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Migration to digital systems

The ongoing transition from analog to digital communication methods over the last 25 years.

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Digital Signal Advantages

Digital signals can be regenerated and retransmitted, reducing noise at each repeater.

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Error Correcting Codes

Allow recovery of data from minor errors, improving reliability of digital signals.

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Multiplexing

A method enabling multiple signals to transmit over the same channel.

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Digital Signal Processing (DSP)

Operations on digital signals for filtering, equalization, and compression without analog circuits.

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Serial Transmission

Transmits bits one at a time in sequence, starting from least significant bit (LSB).

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Parallel Transmission

Sends all bits of a data word simultaneously using multiple wires.

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LSB and MSB

Least Significant Bit (first bit) and Most Significant Bit (last bit) in a serial data sequence.

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Serial vs Parallel Transmission

Serial is slower but longer distance; parallel is faster but limited distance due to wire count.

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Pulse Rate

The number of pulses per second in a transmission system.

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Bit Rate

The number of bits transmitted per second, calculated using pulse rate.

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DC Component

Unwanted direct current energy in a signal that can distort transmission.

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Self-Synchronization

Embedding timing information within a digital signal to ensure accurate interpretation.

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Unipolar Encoding

A simple line coding scheme using one voltage level for representation.

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Polar Encoding

A line coding scheme that uses both positive and negative voltage levels.

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Nonreturn to Zero (NRZ)

A binary encoding scheme where signal level remains constant for duration of a bit.

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Return to Zero (RZ)

A line coding scheme where the signal returns to zero between bits.

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8-bit encoding

Method to encode telephone audio with 256 levels.

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16-bit encoding

Method for CD audio with 65,536 audio levels.

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Line coding

Conversion of binary data into a digital signal.

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Signal Level vs. Data Level

Signal level is allowed values. Data level is symbols used.

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Transmission modes

Methods to send data: Parallel or Serial (Synchronous/Asynchronous).

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Digital modulation

The process of transmitting digitally modulated analog signals in communication systems.

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Advantages of digital modulation

Improves signal processing, multiplexing, and noise immunity compared to analog systems.

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Applications of digital modulation

Used in modems, high-speed data systems, microwave, satellite, and personal communication systems.

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Sampling

The process of taking periodic samples from a waveform for transmission purposes.

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Pulse Modulation

Methods of converting information into pulse form for transmission over a medium.

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Analog Pulse Modulation (APM)

A method of pulse modulation that samples analog signals into pulses.

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Digital Pulse Modulation (DPM)

A method of pulse modulation that uses digital signals and converts them into pulses.

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Pulse modulation methods

Includes PAM (Pulse Amplitude Modulation), PWM (Pulse Width Modulation), PPM (Pulse Position Modulation), and PCM (Pulse Code Modulation).

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Sampling Rate

The number of samples taken per second, crucial for digitizing audio.

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Asynchronous Transmission

Data transfer with variable time intervals and start/stop bits.

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Synchronous Transmission

Data transmission that requires constant timing for sending bits.

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Start/Stop Bits

Bits added to identify the beginning and end of a data unit.

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Byte Synchronization

Achieving synchronization of bytes in data transmission.

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Study Notes

Analog Systems

  • Analog systems use electrical signals that change continuously.
  • Signals are representations of natural phenomena like pressure, light, and sound.
  • Examples include AM/FM radio, cassettes, telephones, VCRs, and standard televisions.
  • The voltage varies over time in a continuous manner.

Digital Signals

  • Binary digital signals use two distinct voltage levels to represent 0s and 1s.
  • Combining multiple bits creates larger values.
  • Digital circuits perform logic and arithmetic functions using digital signals.
  • The voltage is discrete and changes at specific points in time.

Analog Examples

  • The images show examples of analog devices like a VCR, a telephone, and an 8-track stereo tape player.

Digital Examples

  • Modern devices like digital cameras, mobile phones, DVDs, and high-definition televisions represent digital technology.

Advantages of Digital Signals

  • Noise immunity: Digital circuits can distinguish between binary 1s and 0s, even in the presence of noise, while analog signals are affected by noise.
  • Signal regeneration: Digital signals can be regenerated, removing noise, by relay stations along a transmission path. This is not possible with analog signals, which get progressively noisier the further they travel.
  • Error correction: Error-correcting codes can fix many bit errors within a digital signal.
  • Multiplexing: Digital signals are easier to multiplex, allowing multiple signals to share one transmission channel.
  • Native format for computers: Digital is the natural format for computers and permits signal processing and storage more efficiently than analog formats

Transmission of Digital Data

  • Two methods:
    • Parallel: All bits of a word transmitted simultaneously (using multiple wires).
    • Serial: One bit transmitted at a time.

Serial Transmission

  • Bits transmitted sequentially.
  • Least significant bit (LSB) transmitted first, most significant bit (MSB) last.
  • Data transmitted faster over longer distances than parallel data.

Parallel Transmission

  • Data transmitted simultaneously over multiple wires, leading to high speed.
  • Impractical for long distances due to cost of multi-wire cables and signal attenuation over long wires. High-frequency signals are distorted by inductance/capacitance, significantly shortening the lengths required.

Serial-Parallel Conversion

  • Shift registers and sequential logic circuits, often made of flip-flops, achieve conversion between serial and parallel formats easily.

Conversion from Analog to Digital

  • Digital transmission requires translating analog signals into digital format, known as analog-to-digital (A/D) conversion.
  • Digitization involves sampling the analog signal at regular intervals and converting each sample to a binary representation.

A/D Conversion

  • Sampling: Taking the analog signal's voltage or current value at regular time intervals.
  • Sampling frequency, $f_{s}$: How frequently the analog signal is sampled.
  • Sampling period, $T_s$: The time interval between samples.
  • Nyquist Theorem: To accurately reconstruct the original signal, the sampling frequency ($f_s$) must be twice the highest frequency component ($f_{max}$) of the analog signal. The formula to find Nyquist rate is $f_s \ge 2f_{max}$.
  • Quantization: Mapping the sampled analog voltage levels to discrete binary values. A/D converters have a limited number of discrete voltage values that can be represented.

Line Coding

  • Converting binary data to a digital signal
  • Unipolar, Polar (NRZ-L, NRZ-I, RZ, Manchester, Differential Manchster), and Bipolar (AMI, BNZS) are types of line coding schemes. These schemes define how a sequence of 1s and 0s in digital data are converted into voltage levels for transmission

Common Block Codes

  • 4B/5B (4 Bits to 5 Bits): Four input bits converted to five bits for transmission.
  • 8B/10B (8 Bits to 10 Bits): Standard in high-speed serial communication; used to improve transmission properties.
  • 8B/6T (8 Bits to 6 Symbol): Improves bandwidth efficiency by reducing the amount of required bandwidth due to multiple signal level usage.

Sampling

  • The process of obtaining amplitudes of a signal at regular time intervals.
  • Pulse Amplitude Modulation (PAM): Analog pulse modulation, in which the amplitude of each pulse is varied in accordance to the amplitude of the modulating signal
  • Pulse Width Modulation (PWM): Analog pulse modulation in which the width of each pulse is varied in accordance to the amplitude of the modulating signal
  • Pulse Position Modulation (PPM): Analog pulse modulation in which the position of each pulse in the time slot is varied in accordance to the amplitude of the modulating signal
  • Pulse Code Modulation (PCM): Digital pulse modulation
  • Different methods exist for converting analog waveforms to digital signals.

Synchronous/Asynchronous Transmission

  • Synchronous:
    • Constant timing relationship between sender and receiver
    • Frames combine bit streams and possibly contain multiple bytes.
  • Asynchronous:
    • Start/stop bits delineate data units.
    • Variable time intervals between units
    • Cost-effective for low-speed transmissions, but slower

Digital Modulation

  • The process of transforming a digital signal into an analog signal for transmission.
  • Several important criteria for choosing the appropriate digital modulation method includes spectral efficiency, power efficiency, robustness to multipath signals, ease of implementation, low carrier interference and out-of-band radiation.

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Introduction to Digital Communication PDF

Description

Explore the key concepts of digital communication in this quiz. Understand the roles of error correcting codes, multiplexing, and the differences between analog and digital systems. Test your knowledge on signal processing, data transmission, and the advantages of digital communications.

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