৪৯তম বিসিএস ⎯ কম্পিউটার সায়েন্স (CSE) [৯৭১]

Answer: C) Square wave
Detailed Explanation:
• A digital signal is represented by discrete voltage levels (commonly two: HIGH = 1, LOW = 0).
• Square waves are used to represent binary bits (0/1), hence digital.
• Human voice is analog (continuous).
• Sine wave and radio wave are also analog signals since they vary continuously.

Correct: Square wave = digital signal

Answer: B) Loss of signal strength
 
Explanation:
Attenuation = reduction in amplitude of a signal as it travels through a medium.
Example: Voice signal weakening in long telephone cables.

Distortion = signal waveform altered (different frequency components travel differently).
Noise = unwanted addition to signal.
Phase shift = timing difference.

Correct: Loss of strength = Attenuation

Answer: C

Attenuation – Loss of signal strength as it travels through a medium (e.g., a weaker Wi-Fi signal as distance increases).

Distortion – When the signal changes its shape or form, often because different frequency components travel at different speeds.

Noise – Unwanted external signals that mix with the original signal and corrupt it. Sources include crosstalk, impulse noise, thermal noise, or electromagnetic interference (EMI).

Decibel (dB) – Not an impairment; it is a unit of measurement used to express signal strength or loss.

Since the question mentions "outside source such as crosstalk corrupts a signal", that clearly refers to Noise.

 Answer: D. distance a signal or bit has traveled

Propagation Speed (v): The speed at which a signal travels through a medium.
Example: In fiber-optic cable ≈ 2×108?/?
Propagation Time (t): The time it takes for a signal to travel from the sender to the receiver.

Formula: 
t = distance/ propagation speed
​
Multiplying Them: 
Propagation Speed × Propagation Time= v × t

Since t = distance / v

Substituting: v×t = v × distance/v
                      = distance

Therefore, the product gives the distance the signal (or bit) has traveled.

Other options

Throughput → rate of successful data transfer, not related to propagation.
Wavelength → depends on frequency, not directly on time.
Distortion factor → not defined this way.

Answer C) Bandwidth

A composite signal is made up of multiple sine waves of different frequencies (Fourier analysis).

Frequency (f):
The number of cycles a wave completes in one second (measured in Hertz, Hz).

Period (T):
The time taken for one complete cycle of a signal.
                                                                                T=1/f
Amplitude (A):
The height of the signal wave; indicates the strength (or power) of the signal.

Bandwidth (BW):
The range of frequencies contained in a composite signal.
                                                                                   
Bandwidth = f_high − f_low

Example: If a composite signal contains frequencies from 100 Hz to 5000 Hz,
BW = 5000−100 = 4900Hz

Frequency → describes one particular sine wave, not the range.
Period → reciprocal of a single frequency.
Amplitude → describes signal strength, not frequency range.

Answer: D) 
Explanation:
• In satellite links, the distance (about 36,000 km for geostationary orbit) causes noticeable propagation delay (~250 ms one-way).
• Attenuation is also present but manageable by amplification.
• Crosstalk is rare due to different frequency allocations.

Correct: Propagation delay

Answer: C) bps
Data rate = bits per second (bps).
Baud = symbols per second (may carry multiple bits per symbol).
Hz = frequency unit.
dB = signal strength unit.

Answer: D) Infrared
Explanation:
• Guided media = signals travel through a physical path (twisted pair, coaxial, fiber).
• Unguided media = signals transmitted freely through space (infrared, radio, microwave).

Correct: Infrared = unguided.

 Answer: C) To reduce crosstalk and EMI

Explanation:
• Twisting pairs cancels out electromagnetic interference (EMI) and reduces crosstalk.
• Bandwidth is not increased by twisting.
• Attenuation still exists, twisting doesn’t prevent it.

Correct: To reduce crosstalk and EMI.

Answer: D) Optical fiber

Explanation:
• Optical fiber works on the principle of total internal reflection of light inside the core, even when bent.

Answer: B) Bluetooth
Explanation:
• Bluetooth, Wi-Fi (2.4 GHz band) use unlicensed ISM (Industrial, Scientific, Medical) band.
• GSM = 900/1800 MHz.
• Satellite TV = GHz bands.

Correct: Bluetooth

Answer: A) AM
Detailed Explanation:
•    Amplitude Modulation (AM): carrier amplitude changes according to message signal.
•    FM: frequency changes.
•    PM: phase changes.
•    FSK: frequency shift for digital bits.
Example: AM radio broadcasting.

Answer: C) PSK
Detailed Explanation:
•    Digital modulation: ASK, FSK, PSK, QPSK, QAM.
•    Analog modulation: AM, FM, PM.

 Correct: PSK (Phase Shift Keying)

Answer: C) Two different frequencies
Detailed Explanation:

• FSK: Frequency Shift Keying (FSK) is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. FSK is a scheme of frequency modulation.
o    0 → Lower frequency carrier
o    1 → Higher frequency carrier

• More noise-resistant than ASK.
Example: Used in modems and Bluetooth low-rate transmission

Answer: B) 2
Detailed Explanation:
•    QPSK uses 4 phase states (0°, 90°, 180°, 270°).
•    Each phase can represent 2 bits:
o    00 → 0°
o    01 → 90°
o    11 → 180°
o    10 → 270°
•    Why others are wrong:
o    1 bit → BPSK.
o    3 or 4 bits would require 8PSK or 16PSK.

Answer: D) ASK + PSK
Detailed Explanation:
•    QAM is a hybrid modulation technique combining:
o    Amplitude variations (ASK)
o    Phase variations (PSK)
•    Allows transmission of multiple bits per symbol (e.g., 16-QAM = 4 bits/symbol, 64-QAM = 6 bits/symbol).
•    Why others are wrong:
o    ASK + FSK or AM + FM don’t form QAM.
•    Real-life usage: QAM is the backbone of modern 4G/5G, DSL broadband, and Wi-Fi.

Answer: B) The difference between successive samples
Detailed Explanation:
•    Delta Modulation encodes only the difference between the current and previous sample.
•    Reduces bit rate compared to PCM but suffers from:
o    Slope overload (when step size is too small for fast-changing signals).
o    Granular noise (when step size is too large).

•    Why others are wrong:
o    (A) is PCM.
o    (C) is phase modulation.
o    (D) is FSK.

Answer: B

Explanation: Raw digital signals (square waves) are not suitable for direct long-distance transmission. Modulation shifts them to high-frequency carriers, making transmission more efficient and robust.

Answer: A) A signal must be sampled at least at twice its highest frequency

Sampling Theorem (Nyquist Theorem) When we convert an analog signal into a digital signal, we must sample it.

The theorem states:
If a signal is band-limited to a maximum frequency f_max , it must be sampled at a rate at least twice that frequency (i.e., f_s≥2f_max) to reconstruct it without distortion.

# This minimum sampling rate is called the Nyquist Rate.
# Half of the sampling rate is called the Nyquist Frequency.

 Formula    f_s ≥ 2f_max

Where:  f_s = Sampling frequency

f_max = Maximum frequency in the analog signal

Example

Suppose an audio signal has frequencies up to 20 kHz.

According to Nyquist:  fs ≥ 2 × 20,000 = 40,000Hz

So, to digitize human voice/music, we need ≥ 40 kHz sampling rate.

CDs use 44.1 kHz for this reason.

Answer C
Explanation: Half of the sampling rate is called the Nyquist Frequency.
Nyquist Frequency =f_s /2. It is the highest frequency that can be correctly represented at a given sampling rate.

Answer: A) Converting analog to binary
Detailed Explanation:

• PCM process:
1. Sampling (at Nyquist rate)
2. Quantization (discretizing amplitude)
3. Encoding (into binary digits)

• Example: PCM is the basis of digital telephony and audio CDs.
Correct: Analog → Binary

Answer B

Sampling → discrete in time
Quantization → discrete in amplitude
Encoding → mapping quantized levels to binary digits.

Answer: D) Frequency Modulation (FM)

Detailed Explanation:
• Digital-to-digital encoding = converting binary data to line codes (NRZ, NRZI, Manchester, Differential Manchester).
• FM = Analog modulation (carrier frequency variation).

Answer: C) QAM
Explanation:
DSL uses Quadrature Amplitude Modulation (QAM) because:
 It efficiently combines amplitude + phase variations to send multiple bits per symbol.
Example: 256-QAM → 8 bits per symbol.
Others options
o    ASK → too noise-sensitive for long copper wires.
o    FSK → simple, but not efficient for high-speed broadband.
o    Manchester → line coding, not modulation.
•    Application: DSL modems commonly use 64-QAM or 256-QAM depending on noise tolerance.

 Answer: C) One character at a time with start and stop bits
Explanation:
   Asynchronous transmission:
Data → transmitted character by character (usually 8 bits).
Each character → preceded by a start bit and followed by stop bit(s).
No common clock between sender and receiver.

Blocks with sync bits = synchronous transmission.
Continuous stream = synchronous.
Only clock = not possible; async relies on start/stop, not global clock.

Example: Used in RS-232 serial communication, keyboard input.

Answer: A) HDLC

Explanation: 
HDLC (High-Level Data Link Control):
   Bit-oriented synchronous protocol.
   Provides error detection (CRC) + flow control.

Others option:
 RS-232 → async serial standard.
 Manchester → encoding scheme, not protocol.
 TCP → transport layer protocol.

Answer: B) CRC (Cyclic Redundancy Check)

Explanation:
CRC is a mathematical method to detect errors in transmitted data.
The sender generates a checksum using polynomial division; receiver recalculates it.

Others option:
    Manchester/NRZI → encoding, not error detection.
    Null modem → hardware cable configuration.

Applications: Ethernet, HDLC frames, storage devices.

Answer: B) Waits for acknowledgment before sending next frame
 Explanation:
    Stop-and-Wait ARQ:
    Simple flow control mechanism.
    Sender sends one frame and waits for ACK.
    If no ACK → retransmit.

Others option:
    (A) = Go-Back-N / Sliding Window
    (C) = Sliding Window
    (D) = Parity → error detection, not flow control

    Drawback: Inefficient at high bandwidth/distance

Answer: A) HDLC
Explanation:
    Bit-oriented protocol: treats data as a stream of bits rather than characters.
    HDLC: frames marked with flags, uses bit stuffing to avoid confusion.
    Others option:
     ASCII → character-oriented
     Ethernet → frame-based, uses preamble + CRC, but not purely bit-oriented
     Manchester → line coding

Answer: C. Match the transmission rate of the sender with the receiving capacity
Explanation: 
 Flow Control is a data link layer and transport layer function.

 Its job is to ensure that the sender does not overwhelm the receiver with too much data too quickly.

If the sender transmits faster than the receiver can process/store, buffer overflow occurs, and data is lost.
Flow control = sender’s speed adjusted to receiver’s speed (like water tap & bucket analogy)

Flow Control Mechanisms:

Stop-and-Wait Protocol → Sender sends one frame, waits for ACK before sending the next.

Sliding Window Protocol → Allows multiple frames in transit before requiring ACKs.

Other options:

A) Error correction → Done by error control, not flow control.

B) Prevent congestion → That’s congestion control (network-level, e.g., TCP congestion control).

D) Security → Handled by encryption/authentication, not flow control.

Answer: A) Frame size and propagation delay
Explanation:
    

Answer: C) Allow multiple signals to share a single transmission medium
 Explanation:
    Multiplexing: Combines multiple signals for transmission over one physical medium.
 Others option:
   A) Signal strength → Amplifiers
   C) Error detection → CRC/parity
   D) Digital → Analog → Modulation, not multiplexing
•    Examples: Telephone lines, DSL, cable TV.

Answer: A) Assigning each signal a unique frequency band
Explanation:
    FDM: Each user’s signal modulates a different carrier frequency.
    Total bandwidth = sum of individual signal bandwidths + guard bands.
    Others Option:
    B) → TDM
    C) → CDMA
    D) → Amplitude modulation, not multiplexing
    Example: Cable TV, analog phone lines.

Answer: B) It assigns slots dynamically based on demand
Explanation:
   Statistical TDM: Efficient → only active users get slots → reduces idle bandwidth.
   Synchronous TDM: All slots allocated, wasted if user idle.

   Applications: Modern digital networks, high-speed LANs.

Answer: D) Divided into packets and sent independently
Explanation:
    Packet switching:
    Each packet may take different routes → efficient use of bandwidth
    Packets reassembled at destination

    Examples: Internet (TCP/IP), LANs.

    Others option:
    A) → Circuit switching
    C) → Circuit switching
    D) → Nonsense

Answer: C) Circuit switching
Explanation:
    PSTN → dedicated path between caller & receiver
    Guarantees constant bandwidth & low delay
    Others option:
    Packet switching → Internet
    Message switching → Store-and-forward (obsolete)
    Statistical TDM → multiplexing, not switching

Answer: D) All of the above
 Explanation:
    Packet switching:
    Shares bandwidth efficiently → no idle circuits
    Handles bursty traffic → dynamic routing
    No dedicated path → flexible routing → lower cost

    Examples: Internet, LANs, modern networks

Answer: C) OFDM
 Explanation:
    OFDM (Orthogonal Frequency Division Multiplexing):
    Splits available spectrum into orthogonal subcarriers
    Reduces inter-symbol interference in multipath channels
    Works with high-order QAM → high throughput

    Used in 4G LTE, 5G NR, Wi-Fi 802.11a/g/n/ac

 Answer: B) Twisted pair
 Explanation:
    Twisted pair: two insulated copper wires twisted together.
    Advantages: cheap, widely available, easy to install.
    Applications: Landline phones, DSL, Ethernet (10BASE-T).
    Others option:
    Coaxial → cable TV, broadband
    Optical fiber → high-speed backbone
    Wireless → radio, Wi-Fi, cellular

Answer: A) Internet (IP)
Explanation:
    Each IP packet → independent routing → may arrive out of order
    Reassembly → destination host responsibility

Answer: A) Packets delivered in order
Explanation:
    Logical connection established → packet order preserved

    Bandwidth not guaranteed, unlike circuit switching

Answer: A) Cost per channel
Explanation: In Multiplexing 
 Single medium → multiple signals → fewer physical channels → reduced cost
 Bandwidth & noise → depend on modulation & medium

Answer B) 3 kHz – 1 GHz
 
Explanation:

Radio waves: 3 kHz – 1 GHz (used in AM/FM radio, TV, mobile communication).

Microwaves: 1 – 300 GHz (used in satellite, Wi-Fi, Bluetooth, radar).

Infrared: 300 GHz – 400 THz (used in TV remotes, short-range wireless links).

 Remember: Radio waves are best for broadcasting & long-distance communication.

Answer C) Microwaves 
Explanation:

Satellites use microwaves (1–40 GHz range) for uplink (Earth → Satellite) and downlink (Satellite → Earth).

Reason: Microwaves can travel through the atmosphere with less attenuation than higher-frequency signals.
Example: TV broadcasting, GPS, internet via satellite.

Radio waves: 3 kHz – 1 GHz (used in AM/FM radio, TV, mobile communication).

Infrared: 300 GHz – 400 THz (used in TV remotes, short-range wireless links).

Answer B) 2.4 GHz Radio/Microwave

Explanation:

Bluetooth: 2.4 GHz ISM (Industrial, Scientific, Medical) band.

Wi-Fi: 2.4 GHz and 5 GHz (newer 6 GHz for Wi-Fi 6E).

2.4 GHz is popular because it penetrates walls and is globally license-free.
Answer: 2.4 GHz band.

Answer A) Television remote controls

Explanation:

TV remotes use infrared because it is cheap, short-range, and secure within a room.

Radio → broadcasting.

Satellite → microwave.

Fiber optics → light (but not free-space infrared).

Answer C) Attenuation 

Explanation:

Attenuation = gradual loss of signal strength as it propagates.

In wireless, attenuation happens due to distance, obstacles, and medium absorption.