Data Communications Module II

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Questions and Answers

What is the formula to determine the bandwidth of a composite signal?

Bandwidth = (Lowest frequency - Highest frequency) / 2

Bandwidth = Lowest frequency + Highest frequency

Bandwidth = Highest frequency - Lowest frequency (correct)

Bandwidth = Highest frequency / Lowest frequency

In a periodic signal composed of sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is the bandwidth?

800 Hz (correct)

700 Hz

900 Hz

500 Hz

Which of the following statements about digital signals is true?

Digital signals can only represent two levels.

Digital signals can have multiple levels, each of which can represent bits. (correct)

Digital signals are always represented by negative voltage.

Digital signals always have a periodic structure.

If a digital signal has eight levels, how many bits are needed per level?

3 bits Signup and view all the answers

What is the significance of the term 'bit rate' in digital signals?

It indicates the number of bits sent per second. Signup and view all the answers

What can be concluded about a composite signal that is nonperiodic?

It may have a wide bandwidth. Signup and view all the answers

Given a composite signal with a bandwidth of 200 kHz, what are the extreme frequencies if the middle frequency is 140 kHz?

40 kHz and 240 kHz Signup and view all the answers

Which of the following is NOT a characteristic of digital signals?

They are always represented by a continuous wave. Signup and view all the answers

What type of data takes on continuous values?

Analog data Signup and view all the answers

What is the primary function of the physical layer in data communications?

To move data as electromagnetic signals Signup and view all the answers

Which of the following best describes digital data?

Information with discrete states Signup and view all the answers

Which of these attributes is associated with analog signals?

Period Signup and view all the answers

What does transmission impairment refer to?

The degradation of signal quality Signup and view all the answers

Which of the following signals is defined as discrete?

A binary representation of data Signup and view all the answers

In data communications, which parameter affects how many bits per second can be sent?

Bandwidth Signup and view all the answers

Which characteristic is NOT associated with digital signals?

Continuous values Signup and view all the answers

What decibel value indicates a loss of power reduced to one-half?

-3 dB Signup and view all the answers

Which expression is correct for calculating gain in decibels when power is increased 10 times?

dB = 10 log10(P2/P1) Signup and view all the answers

In the expression dBm = 10 log10(Pm), what does Pm represent?

Power in milliwatts Signup and view all the answers

How is the total loss in a cable calculated when given a loss of -0.3 dB/km over a distance of 5 km?

-1.5 dB Signup and view all the answers

What is the primary cause of distortion in a composite signal?

Phase shift caused by frequency components Signup and view all the answers

What type of noise is caused by the movement of electrons in a wire?

Thermal noise Signup and view all the answers

What is a common effect of crosstalk noise?

Interference from nearby circuits Signup and view all the answers

Why do engineers prefer to use decibels for measuring signal strength?

Decibels can be added or subtracted Signup and view all the answers

What is the main issue caused by a long string of 0s or 1s in digital transmission?

It causes baseline wandering. Signup and view all the answers

What does DC component refer to in a digital signal context?

Very low frequencies around zero. Signup and view all the answers

Which characteristic is highly desirable in a line coding scheme?

Immunity to noise and interference. Signup and view all the answers

In polar NRZ encoding, how are binary values represented?

By positive and negative voltage levels. Signup and view all the answers

What can happen if the receiver's clock is not synchronized with the sender's clock?

The receiver might misinterpret the signals. Signup and view all the answers

What is a key advantage of the return-to-zero (RZ) scheme?

It allows signal changes during the bit. Signup and view all the answers

What does built-in error detection in a coding scheme allow?

It helps to detect some or all transmission errors. Signup and view all the answers

Which of the following is a drawback of more complex line coding schemes?

They are more costly to implement. Signup and view all the answers

What is the relationship between bit rate and bit length?

Bit rate equals the inverse of bit length. Signup and view all the answers

Which type of channel allows transmission of frequencies in the range from some non-zero frequency f1 to another non-zero frequency f2?

Band pass channel Signup and view all the answers

In baseband transmission, which of the following is true regarding the signal's bandwidth?

The bandwidth of the signal must be less than the bandwidth of the channel. Signup and view all the answers

What happens if the channel bandwidth is less than the bandwidth of the signal being transmitted?

The transmitted signal may experience distortion or attenuation. Signup and view all the answers

Which of the following describes the bandwidth of a channel in terms of digital signals?

It is the maximum data rate that can be transmitted per second. Signup and view all the answers

What defines a low pass channel?

It allows frequencies from 0 to a maximum frequency f1. Signup and view all the answers

Which kind of transmission does not modify the signal?

Baseband transmission Signup and view all the answers

What is the calculated bandwidth for a baseband channel with a lower frequency of 0Hz and a higher frequency of 100Hz?

100Hz Signup and view all the answers

Study Notes

Physical Layer Overview

The primary role of the physical layer is to transmit data as electromagnetic signals over a medium.
Data and signals can be categorized as analog (continuous) or digital (discrete).

Data and Signals

Data: Entities conveying meaning, e.g., computer files, music.
Signals: Electric or electromagnetic representations of data; essential for communication systems.

Analog vs. Digital Data

Analog Data: Continuous values, such as sound waves from a voice.
Digital Data: Discrete values stored in binary form (0s and 1s).

Bandwidth

Defined as the difference between the highest and lowest frequencies in a signal.
Adequate channel bandwidth must exceed signal bandwidth to avoid attenuation or distortion.

Digital Signals

Digital signals can use higher voltage for binary 1 and zero voltage for binary 0.
Multiple levels increase data capacity; e.g., 8 levels require 3 bits per level.

Channel Types

Low-pass Channel: Passes frequencies from 0 to a non-zero frequency.
Band-pass Channel: Passes frequencies between two non-zero frequencies.

Transmission Methods

Baseband Transmission: Signal transmitted without modulation; requires signal bandwidth less than channel bandwidth.
Broadband Transmission: Involves modulation of the signal for transmission.

Signal Attenuation and Gain

Attenuation of a signal to half its power corresponds to -3 dB.
Signal gain calculated as dBm = 10 log10(Pm), where Pm is power in milliwatts.

Distortion and Noise

Distortion arises from varying propagation speeds of signal components leading to phase differences.
Noise can corrupt signals, including induced noise from electrical devices, thermal noise from electron movement, and crosstalk between wires.

DC Components and Synchronization

Constant voltage levels in digital signals produce low frequencies, causing issues in systems unable to pass low frequencies.
Self-synchronization is crucial; receiver and sender bit intervals must match for accurate interpretation.

Error Detection and Code Complexity

Built-in error-detection capabilities help identify transmission errors.
More complex encoding schemes (e.g., four signal levels) increase implementation costs and decoding difficulty.

Line Coding Schemes

Unipolar NRZ: Uses a single side of the time axis.
Polar NRZ-L and NRZ-I: Assign positive and negative voltages to 0 and 1; differentiation between levels or inversions defines signal values.
Polar RZ: Uses three voltage levels to signal transitions within the bit period, addressing clock synchronization issues.

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Description

This quiz focuses on the physical layer of data communications, specifically regarding the transmission of data through electromagnetic signals. It covers the key differences between analog and digital signals and their significance in data transmission. Understanding these concepts is essential for students in computer science and engineering.