All questions of Data Link Layer for Computer Science Engineering (CSE) Exam

In two-way communication, wherever a frame is received, the receiver waits and does not send the control frame (acknowledgement or ACK) back to the sender immediately.

The receiver waits until its network layer passes in the next data packet. The delayed acknowledgement is then attached to this outgoing data frame.

This technique of temporarily delaying the acknowledgement so that it can be hooked with next outgoing data frame is known as piggybacking.

A packet-switching network:

Packet-switching is a method of transmitting data over a network in which messages are divided into small packets. These packets are then individually sent across the network and reassembled at the destination. This approach is different from circuit-switching, where a dedicated communication path is established for the entire duration of the transmission.

Advantages of a packet-switching network:

A packet-switching network offers several advantages over other networking approaches, such as:

1. Reduced cost: Packet-switching networks can significantly reduce the cost of using an information utility. This is because they allow multiple users to share the same communication channels, resulting in more efficient use of network resources.

2. Shared channels: In a packet-switching network, communications channels can be shared among more than one user. This means that multiple users can send packets simultaneously over the same channel, increasing the overall network capacity and improving efficiency.

3. Flexibility: Packet-switching networks are highly flexible and can handle various types of data, including voice, video, and text. This versatility makes them suitable for a wide range of applications and allows for efficient transmission of different types of information.

4. Error detection and correction: Packet-switching networks often incorporate error detection and correction mechanisms. Each packet is typically accompanied by checksums or other error detection codes, which allow the receiver to verify the integrity of the data. If errors are detected, the receiver can request retransmission of the affected packets, ensuring reliable data transmission.

5. Scalability: Packet-switching networks can easily accommodate a growing number of users and increasing data traffic. As more users join the network, additional packets can be transmitted simultaneously, providing scalability and adaptability to changing network demands.

6. Efficient use of network resources: Packet-switching networks make efficient use of network resources by dynamically allocating bandwidth to users as needed. This allows for optimal utilization of available resources and prevents underutilization or congestion.

In conclusion, a packet-switching network offers numerous benefits, including reduced cost, shared channels, flexibility, error detection and correction, scalability, and efficient use of network resources. These advantages make packet-switching networks a preferred choice for modern communication systems.

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Error Detection and Correction
Error detection and correction is done by adding redundancy bits to the data being transmitted. These extra bits help in identifying and correcting errors that may occur during transmission.

Adding Redundancy Bits
- When data is transmitted, additional bits are added to the original data to create a codeword.
- These extra bits are calculated based on the original data using specific algorithms.
- The receiver then uses these redundancy bits to check for any errors in the received data.

Error Detection
- The receiver performs error detection by checking the received data against the redundancy bits.
- If any errors are detected, the receiver can request the sender to retransmit the data.
- Common error detection techniques include checksums, parity bits, and cyclic redundancy checks (CRC).

Error Correction
- In addition to error detection, some systems also incorporate error correction capabilities.
- Error correction algorithms use the redundancy bits to correct errors in the received data without the need for retransmission.
- Examples of error correction codes include Hamming codes and Reed-Solomon codes.

Benefits of Error Detection and Correction
- Error detection and correction mechanisms help ensure data integrity during transmission.
- By detecting and correcting errors, these techniques improve the reliability of data communication systems.
- This is especially important in critical applications where data accuracy is essential.

Cyclic redundancy check (CRC) is more efficient than parity check. Parity check can only detect single-bit errors, while CRC can detect multiple-bit errors and even some types of random errors. Additionally, CRC is widely used in modern communication systems, such as Ethernet and WiFi, due to its high reliability and efficiency.

A Modem, short for modulating and demodulating device, is a crucial piece of computer hardware that allows computers to connect to the internet. It stands as a bridge between the computer and the internet service provider (ISP), enabling the transfer of data over various types of communication channels.

Modulating and Demodulating Process:
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Modulation is the process of converting digital data from a computer into analog signals suitable for transmission over telephone lines or other communication channels. Demodulation is the reverse process, converting analog signals back into digital data that the computer can understand.

Functions and Features:
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1. Signal Conversion: The primary function of a modem is to convert digital signals generated by the computer into analog signals that can be transmitted over the communication channel. It also converts analog signals received from the channel back into digital signals for the computer to interpret.

2. Connection to the ISP: Modems establish a connection with the ISP, enabling access to the internet. They communicate with the ISP's network equipment using a specific protocol, such as ADSL (Asymmetric Digital Subscriber Line) or cable.

3. Data Transmission: Modems facilitate the transmission of data between the computer and the ISP's network. They modulate the digital data into analog signals for transmission and demodulate the received analog signals into digital data for the computer to process.

Types of Modems:
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1. Dial-up Modems: These modems were prevalent in the past and used telephone lines for internet connectivity. They operated at low speeds and required a dial-up connection to establish a connection with the ISP.

2. DSL Modems: Digital Subscriber Line (DSL) modems use telephone lines to provide high-speed internet connectivity. They support faster data transfer rates compared to dial-up modems and are widely used for home and small business networks.

3. Cable Modems: Cable modems utilize the existing cable TV infrastructure to provide internet connectivity. They offer higher speeds than dial-up and DSL modems and are commonly used in residential and commercial settings.

4. Wireless Modems: These modems connect to the internet via wireless networks such as Wi-Fi or cellular data networks. They provide flexibility in terms of mobility and can be used with laptops, smartphones, and other devices.

Conclusion:
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In conclusion, a modem is a modulating and demodulating device that allows computers to connect to the internet by converting digital signals into analog signals and vice versa. It plays a crucial role in facilitating data transmission between the computer and the ISP's network, enabling access to the vast resources available on the internet.

Calculation:
- The minimum size of a frame can be calculated using the formula:
- Frame Size = 2 × Propagation Delay × Bandwidth
- Given Propagation Delay = 40 microseconds and Bandwidth = 20 × 10^6 bits per second

Calculation Steps:
1. Convert Propagation Delay to seconds: 40 microseconds = 40 × 10^-6 seconds
2. Calculate Frame Size:
Frame Size = 2 × 40 × 10^-6 seconds × 20 × 10^6 bits per second
= 2 × 40 × 20 = 1600 bits
= 1600 / 8 bytes (1 byte = 8 bits)
= 200 bytes

Conclusion:
- The minimum size of a frame in the network is 200 bytes.

Media access control (MAC) deals with transmission of data packets to and from the network-interface card, and also to and from another remotely shared channel.

Data Link Layer

The Data Link Layer is the second layer of the OSI (Open Systems Interconnection) model. It is responsible for the transmission of data between adjacent network nodes. The main functions of the Data Link Layer include:

- Framing
- Error Control
- Flow Control

Task not done by Data Link Layer

Channel Coding is not done by the Data Link Layer.

Explanation

Channel Coding is a technique used to improve the reliability of data transmission over a communication channel that is noisy or prone to errors. It involves adding redundancy to the original data so that errors can be detected and corrected at the receiver end. Channel Coding is typically performed by the Physical Layer of the OSI model, which is responsible for the transmission of raw data over a communication channel.

The Data Link Layer, on the other hand, is responsible for the transmission of data frames between adjacent nodes in a network. It is concerned with the reliable delivery of data frames, but it does not perform any coding or decoding of the data. Instead, it relies on techniques such as error control and flow control to ensure the reliable transmission of data.

Conclusion

In conclusion, the Data Link Layer is responsible for the reliable transmission of data frames between adjacent network nodes. It performs functions such as framing, error control, and flow control to ensure the reliable delivery of data. However, it does not perform channel coding, which is typically done by the Physical Layer of the OSI model.

Slotted ALOHA Network:
Slotted ALOHA is a protocol in which a station can transmit only at the beginning of a time slot. It is an algorithm for multiple access transmission.

Given Data:
Frame size = 200 bits
Bandwidth = 200 Kbps
Number of frames transmitted per second = 250

Calculation:
The throughput of the system can be calculated as follows:

1. Total possible data that can be transmitted per second:
Data transmitted = Bandwidth × Time
= 200 Kbps × 1 second
= 200,000 bits

2. Total data that can be transmitted per second considering Slotted ALOHA:
Let the efficiency of the system be 'e'.
The maximum efficiency of Slotted ALOHA is 1/e = 1/e^2.
Therefore, e^2 = Data transmitted / Total data produced
= (200,000 × 250) / (200) bits
e^2 = 250
e = 15.81%

3. Throughput of the system:
Throughput = e × Data transmitted
= 15.81% × 200,000 bits
= 31,620 bits/second or 31.62 Kbps

Answer:
The throughput of the system is 31.62 Kbps, which is closest to option 'A' (49).

TDMA (Time Division Multiple Access)
TDMA is a type of access technology that is used to increase the battery life in mobile systems.

How TDMA works:
- In TDMA, each user is assigned a specific time slot within a predefined time frame.
- Users take turns transmitting and receiving data within their designated time slots.
- This method allows multiple users to share the same frequency channel without interfering with each other.

Benefits of using TDMA for increasing battery life:
- Efficient use of resources: TDMA allows for efficient use of the available bandwidth by dividing it into time slots.
- Reduced power consumption: By only transmitting and receiving data during their designated time slots, mobile devices can conserve battery power.
- Improved battery life: The power-saving features of TDMA help to increase the battery life of mobile devices.

Conclusion:
In conclusion, TDMA is a valuable access technology for increasing battery life in mobile systems. By efficiently managing the use of resources and reducing power consumption, TDMA helps to improve the overall performance and longevity of mobile devices.

Cyclic Redundancy Check (CRC)

CRC stands for Cyclic Redundancy Check and is a method used for error detection in computer networks. It is widely used in communication protocols, such as Ethernet, to ensure the integrity of transmitted data.

How CRC Works

CRC involves the use of a polynomial division algorithm to generate a checksum for a block of data. This checksum is appended to the data and transmitted along with it. At the receiving end, the same polynomial division algorithm is applied to the received data, including the checksum. If the calculated checksum matches the received checksum, it indicates that the data has been received without errors. However, if the checksums do not match, it suggests that errors have occurred during transmission.

Polynomial Division Algorithm

The polynomial division algorithm used in CRC involves treating the data as a string of bits and dividing it by a predetermined divisor polynomial. The divisor polynomial is typically represented as a binary number and is chosen based on the specific CRC algorithm being used. The division process is performed bit by bit, with each bit being exclusive-ORed (XOR) with the corresponding bit of the divisor. The result of this XOR operation determines whether the next bit of the data should be XORed with the divisor or not. This process continues until all bits of the data have been processed.

Checksum Calculation

The CRC checksum is calculated by performing the polynomial division algorithm on the data, including the appended checksum bits. The remainder obtained from this division is the checksum. The checksum is then appended to the original data and transmitted.

Error Detection

At the receiving end, the same polynomial division algorithm is applied to the received data, including the appended checksum. If the remainder obtained from the division is zero, it indicates that no errors have occurred during transmission. However, if the remainder is non-zero, it suggests that errors have occurred.

Conclusion

CRC is a powerful error detection technique used in computer networks to ensure the integrity of transmitted data. By calculating a checksum using a polynomial division algorithm, CRC can detect errors introduced during transmission. It is a widely used and efficient method for error detection in various communication protocols.

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Popular Techniques for Error Detection

There are several techniques available for error detection in computer networks and data transmission. Among these techniques, the following are the most popular:

Cyclic Redundancy Check (CRC)
- CRC is a widely used error detection technique in communication networks, storage systems, and digital systems.
- It involves the use of mathematical algorithms to detect errors in transmitted data.
- The sender and receiver both have a pre-agreed generator polynomial, which is used to generate a checksum or CRC code.
- The sender appends this checksum to the data packet before transmission.
- The receiver performs the same calculation using the received data and the generator polynomial.
- If the calculated checksum at the receiver matches the received checksum, it indicates that the data has been received without any errors. Otherwise, an error is detected.
- CRC is highly effective in detecting errors, including both single-bit and burst errors.

Checksum
- Checksum is another popular error detection technique used in computer networks and data transmission.
- It involves the calculation of a checksum value for the data packet being transmitted.
- The sender appends this checksum value to the data packet before transmission.
- The receiver performs the same calculation using the received data packet.
- If the calculated checksum at the receiver matches the received checksum, it indicates that the data has been received without any errors. Otherwise, an error is detected.
- Checksum is simple to implement and provides a reasonable level of error detection. However, it is not as effective as CRC in detecting certain types of errors.

Simple Parity Check
- Simple Parity Check is a basic error detection technique that uses a single parity bit for error detection.
- The sender adds a parity bit to the data packet before transmission.
- The parity bit is calculated as the XOR of all the data bits in the packet.
- The receiver performs the same calculation using the received data packet.
- If the calculated parity at the receiver matches the received parity bit, it indicates that the data has been received without any errors. Otherwise, an error is detected.
- Simple Parity Check is simple to implement but has limited error detection capabilities. It can only detect odd numbers of bit errors.

Conclusion
In conclusion, all of the options (a) Cyclic Redundancy Check, (b) Checksum, and (c) Simple Parity Check are popular techniques for error detection. These techniques play a crucial role in ensuring the integrity and reliability of transmitted data in computer networks and communication systems. While CRC is highly effective in detecting errors, checksum and simple parity check offer simpler implementations but with some limitations in error detection capabilities.

Burst Error

Burst errors occur when two or more bits in a data unit have been changed during transmission. These errors are called burst errors because they tend to occur in clusters, rather than randomly.

Causes of Burst Errors

Burst errors can be caused by a variety of factors, including:

1. Electrical interference
2. Environmental factors
3. Faulty hardware or software
4. Poor quality transmission channels

Impact of Burst Errors

Burst errors can have a significant impact on the accuracy and reliability of data transmission. They can cause data corruption, data loss, and even system crashes.

Preventing Burst Errors

To prevent burst errors, various error detection and correction techniques can be used, including:

1. Checksums
2. Cyclic redundancy checks (CRC)
3. Forward error correction (FEC)

Conclusion

In summary, burst errors occur when two or more bits in a data unit have been changed during transmission. They tend to occur in clusters, rather than randomly, and can have a significant impact on data transmission. To prevent burst errors, various error detection and correction techniques can be employed.