CS Free MCQ Bank

[ Option D ]

Industrial, Scientific, and Medical (ISM) bands are portions of the radio spectrum reserved internationally for non-commercial purposes like industrial heating, medical equipment, and scientific experimentation. Over time, they have also been widely used for unlicensed wireless communication, such as Wi-Fi and Bluetooth.

[ Option C ]

Manchester Encoding is a digital line‑coding scheme used at the physical layer to transmit data along with clock information. Each bit period is split into two phases, and there is always a transition in the middle of the bit, which provides self‑clocking and helps the receiver stay synchronized.
So, Manchester Encoding is used in the Physical Layer and provides signal synchronization through frequent transitions. It is called a Biphase Code because each bit has two phases, and its required transition occurs at the center of the bit, not necessarily at the bit boundary.

[ Option A ]

A scrambler is used in line coding schemes to modify the original data before transmission.

• To remove long sequences of 0s or 1s which can cause synchronization problems.
• To make the transmitted data appear random, even if the original data has patterns.
• Helps in maintaining clock synchronization between sender and receiver.

[ Option B ]

A coaxial cable consists of multiple layers designed to transmit signals efficiently with minimal interference. Its construction includes:

• A solid inner conductor (usually copper) to carry the signal.
• An insulating layer (dielectric) surrounding the conductor.
• An outer conductive sheath (shield) to protect against noise and interference.
• Often an outer protective jacket.

[ Option D ]

Digital Data Transmission is considered more secure because the data can be encrypted before transmission. Encryption converts the original information into a coded form so that unauthorized users cannot easily understand or access the data.
In digital communication systems, techniques such as encryption, encoding, and authentication are commonly used to improve security during data transfer.

[ Option C ]

Encoding schemes in Ethernet convert data bits into transmittable symbols over physical media (fiber/copper). They ensure clock recovery (frequent transitions), DC balance (equal 0s/1s), and error detection by adding overhead bits.
In Gigabit Ethernet (1000BASE-X), data transmission uses an encoding scheme to ensure reliable communication, proper synchronization, and error detection. The scheme used is 8B/10B encoding.
In this technique, 8 bits of actual data are converted into 10-bit code groups before transmission. This extra 2 bits help in:

• Maintaining signal balance (DC balance).
• Providing clock synchronization.
• Detecting transmission errors.

Therefore, 1000BASE-X uses 8B/10B encoding, where 8-bit data is converted into 10-bit groups for reliable high-speed transmission.

[ Option D ]

In satellite communication, signals travel between the earth station and the satellite using different frequencies. There are two main communication links:

• Uplink : Transmission from earth station to satellite.
• Downlink : Transmission from satellite to earth station.

The frequency used for sending signals from the satellite to the earth station receiver is called the Downlink Frequency.
For example, a TV broadcast satellite sends signals to home dish antennas using downlink frequency.

[ Option B ]

In digital communication, Modulation is the process of varying a carrier signal (sine wave) to transmit data. Digital modulation schemes often use two or more carriers, which can vary in amplitude, phase, or frequency to represent binary information.

A Constellation Diagram is a graphical representation of these modulated signals on a 2D plane, where the x-axis represents the in-phase component (I) and the y-axis represents the quadrature component (Q). Each point in the diagram corresponds to a unique amplitude and phase combination, allowing engineers to visualize the transmitted symbols and detect errors.

[ Option A ]

In data communication, Simplex Mode is a type of transmission where data flows in only one direction with no possibility of data flowing back, like traffic on a one-way street. One device acts as the sender, and the other only receives.
In Computer to Printer communication, data flows only from the computer to the printer. The printer does not send data back to the computer. This is a perfect example of simplex communication.

[ Option B ]

A Walkie-Talkie uses the Half-Duplex communication method. In half-duplex communication, data can travel in both directions, but not at the same time.
In a walkie-talkie:

• One person speaks while the other listens.
• Both persons cannot speak simultaneously.
• Users press a button to talk and release it to receive messages.

[ Option C ]

In digital communication, the quality or error performance of transmission is measured using BER (Bit Error Rate).
BER represents the number of bits received incorrectly compared to the total number of bits transmitted.

• It tells how reliable the communication system is.
• BER = Number of Error Bits / Total Number of Transmitted Bits

For example, if 1000 bits are transmitted and 10 bits are received incorrectly, then:
BER = 10/1000 = 0.01
A lower BER means better transmission quality.

[ Option B ]

[ Option B ]

Data communication is the process of transferring data between two or more devices through communication channels such as cables, optical fibers, or wireless networks.
Communication modes define the direction of data flow between sender and receiver. The three main communication modes are:

• Simplex.
• Half Duplex.
• Full Duplex.

In Simplex Communication, data flows only in one direction. One device only sends data and the other device only receives data.
In Half Duplex Communication, data can flow in both directions, but not at the same time. Only one device can transmit data at a particular moment. Therefore, half duplex communication is bidirectional, not multidirectional.
In Full Duplex Communication, data can flow in both directions simultaneously. Both devices can send and receive data at the same time. Therefore, full duplex communication is bidirectional communication.

[ Option B ]

[ Option C ]

A Useless Channel is a communication channel where there is no correlation between input and output symbols. This means the output is completely independent of the input, the received symbol gives no information about what was sent. In this, the channel capacity is zero because the output does not carry any meaningful information about the input.
A noiseless channel, output is exactly the same as input, so there is a perfect correlation between input and output.
A lossless channel, input can be uniquely determined from the output, so correlation still exists.
A deterministic channel, each input symbol corresponds to exactly one output symbol, hence strong correlation.

[ Option C ]

In Manchester Encoding, each bit period must have a transition in the middle, Low to High for bit 1 and High to Low for bit 0. This transition helps maintain synchronization between sender and receiver. If the line stays high throughout a bit period (High to High), it means no transition occurred, violating the Manchester rule. Therefore, the receiver interprets it as a bit error or loss of synchronization.

[ Option C ]

To digitize a human voice signal, the Pulse Code Modulation (PCM) method is used, which requires sampling the analog signal. 
According to the Nyquist theorem, the sampling rate must be at least twice the highest frequency present in the signal. 
Since the human voice ranges up to 4000 Hz, the signal must be sampled at 8000 samples per second. If each sample is represented by 8 bits, the total bit rate becomes 8000 × 8 = 64,000 bits per second, or 64 kbps.

[ Option C ]

According to Nyquist’s theorem, the maximum data rate of a noiseless channel depends on its bandwidth (B) and the number of discrete signal levels (V) used in transmission.
Maximum Data Rate = 2B log₂V bits per second

• B is bandwidth of the channel in hertz (Hz).
• V is number of discrete signal levels.

[ Option D ]

In Time Division Multiplexing (TDM), multiple channels share a single communication link, and the total bit rate is the sum of the bit rates of all channels.
Each channel sends 100 bytes/second. Since 1 byte = 8 bits, we first convert bytes to bits:
One channel: 100*8 = 800 bits/second
Now, there are 4 channels, so total bit rate: 800*4 = 3200 bits/second (3200 bps)

[ Option C ]

In computer networks, binary data cannot always be transmitted directly over text-based protocols like email (SMTP) or web (HTTP). To solve this problem, Base64 encoding is used. Base64 takes every 24 bits of input data (3 bytes) and divides it into four groups of 6 bits each. Each 6-bit value is then mapped to a printable ASCII character.

Since each Base64 output is an ASCII character, it requires 8 bits of storage. Therefore, four characters × 8 bits = 32 bits of output are generated from the original 24 bits. This explains why Base64 increases the size of the encoded data by about 33%.

When encoding the string "SRK" into Base64, the process begins by converting each character into its ASCII binary form. The character S has an ASCII value of 83, which is 01010011 in binary. The character R has an ASCII value of 82, represented as 01010010. Finally, the character K has an ASCII value of 75, represented as 01001011. Together, the string "SRK" produces a 24-bit binary stream:

01010011 01010010 01001011

Next, these 24 bits are divided into groups of 6 bits each:

010100 110101 001001 001011

Each 6-bit group is then converted into its decimal equivalent:

• 010100 = 20
• 110101 = 53
• 001001 = 9
• 001011 = 11

Using the Base64 index table, these decimal values are mapped to their respective characters:

• 20 = U
• 53 = 1
• 9 = J
• 11 = L

Thus, the final Base64 encoded output of the string "SRK" is: U1JL

[ Option A ]

For a Noiseless communication channel, the theoretical maximum bit rate is determined by the Nyquist bit rate formula. The Nyquist formula calculates the highest possible data rate based on the channel bandwidth (B) and the number of discrete signal levels (M) used in transmission.
Maximum bit rate = 2 × B × log₂(M)

In contrast, the Shannon bit rate formula considers noisy channels and accounts for signal-to-noise ratio (SNR), making it applicable when noise is present.

[ Option A ]

To carry huge data over long distances with low delay (Latency), a transmission medium must combine high bandwidth, low signal loss (Attenuation), and little interference.
Optical Fiber Cables use light signals to transmit data, which allows them to carry very large amounts of data over long distances with minimal loss and very low latency.

[ Option B ]

A Modem (Modulator-Demodulator) is a device that allows computers to communicate over telephone lines or other analog transmission media.

• The data originates as digital (0s and 1s) form from the computer.
• The modem converts this digital data into analog signals suitable for transmission over traditional telephone lines or other analog communication media.
• At the receiving end, the modem converts the analog signals back into digital data for the receiver.

This process is called Modulation (Digital to Analog) and Demodulation (Analog to Digital), which is why the device is called a MODEM (Modulator-Demodulator).

[ Option A ]

Quantizing noise occurs during the quantization process in Pulse Code Modulation (PCM). PCM converts an analog signal into a digital signal by sampling the amplitude of the analog signal at regular intervals and then quantizing these samples to the nearest value among a finite set of levels. The difference between the actual analog value and the quantized digital value is called quantization error or quantizing noise.

• PCM (Pulse Code Modulation): Quantization is a necessary step, hence quantizing noise appears.
• TDM (Time Division Multiplexing): No quantization; it only multiplexes signals in the time domain.
• FDM (Frequency Division Multiplexing): Signals are separated by frequency bands, no quantization noise.
• PPM (Pulse Position Modulation): The noise mainly comes from timing errors, not quantization.

[ Option B ]

The maximum data rate of a noiseless channel is determined by the Nyquist Theorem. It defines the theoretical upper limit for data transmission rate in an noiseless channel.
Maximum bit rate = 2×B×log₂(M)
Where:
B = Bandwidth of the channel.
M = Number of discrete signal levels (symbols) used.
Given Values:
B=3000 Hz, M=2 (says binary signal and binary means two levels, 0 and 1)
Maximum bit rate = 2 × 3000 × log₂(2) and log₂(2)=1
So, Maximum bit rate = 2 × 3000 × 1 = 6000 bps.