Communication Systems Part 11

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Questions and Answers

What is the main goal of the Data Link Layer?

To establish connections between multiple networks

To encrypt data during transmission

To achieve reliable and efficient communication between adjacent machines (correct)

To manage routing of data between different network layers

Which of the following is NOT a service provided to the Network Layer by the Data Link Layer?

Framing

Error Control

Flow Control

Network Addressing (correct)

Which framing method uses special sequences of bits to delineate boundaries?

Header framing

Bit stuffing (correct)

Character count framing

Byte stuffing

Which framing technique suffers from the problem of data being mistaken for frame delimiters?

Flag bytes with byte stuffing (B) Signup and view all the answers

What type of service involves acknowledgment in the connection-oriented category?

Acknowledged connection-oriented services (A) Signup and view all the answers

What is character count framing primarily concerned with?

Identifying start and end of frames based on character numbers (B) Signup and view all the answers

Which of the following is a limitation of byte stuffing?

Data being interpreted as flag bytes (C) Signup and view all the answers

Which method of framing avoids possible errors with long characters such as UNICODE?

Bit stuffing framing (A) Signup and view all the answers

How does flow control in the Data Link Layer primarily function?

By managing the rate of data transmission between sender and receiver (B) Signup and view all the answers

Which framing technique is dependent on the physical layer for coding errors?

Physical layer coding violations (A) Signup and view all the answers

What is the reason burst errors are harder to correct?

They can affect multiple bits in close proximity. (D) Signup and view all the answers

What is preferred for error handling on fibers due to efficiency?

Error detection. (D) Signup and view all the answers

Which of the following statements about Hamming codes is true?

They rely on parity bits for error correction. (A) Signup and view all the answers

How are errors corrected in burst error scenarios using Hamming codes?

Data is reorganized in a columnar format. (B) Signup and view all the answers

What describes the relationship between message bits, redundancy, and codeword size in error-correcting codes?

Codeword size equals message bits plus redundancy. (C) Signup and view all the answers

In error-detecting codes, what form of redundancy is commonly used?

Polynomial code checksums. (A) Signup and view all the answers

What characterizes a physical layer coding violation in networks?

It involves specific high-low and low-high bit pairs. (D) Signup and view all the answers

Why is error correction essential in wireless transmission?

Many errors are likely to occur during transmission. (C) Signup and view all the answers

What happens when a non-legal codeword is received in error-correcting codes?

It gets replaced by a valid codeword. (D) Signup and view all the answers

What is the impact of an error rate of 0.001 on different block sizes?

Larger blocks tend to have more errors per block. (C) Signup and view all the answers

Study Notes

Communication Systems and Computer Networks (Part 11)

The Data Link Layer is responsible for reliable and efficient communication between adjacent machines.
Adjacent machines are physically connected (e.g., coaxial cable, telephone lines, point-to-point wireless).
Data Link Layer Design Issues provide services to the Network Layer.

Services Provided to the Network Layer

Services may vary, but generally include:
- Unacknowledged connectionless services
- Acknowledged connectionless services
- Acknowledged connection-oriented services

Framing Concept

The Data Link Layer receives packets from the Network Layer and forms frames.
Packets are encapsulated into frames which contain a header, payload field, and trailer.
The process involves sending and receiving messages via packets and frames.

Framing Techniques

Framing is achieved through various methods:
- Character counts: A count of characters in the frame is included.
- Flag bytes with byte stuffing: Flag bytes are used to delimit frame boundaries, and stuffing replaces special characters within the frame.
- Starting and ending flags with bit stuffing: Similar to byte stuffing, special bit patterns (flags) mark the beginning and end of the frame. Bit stuffing inserts extra bits to prevent confusion with flags.
- Physical layer coding violations: Special codes, such as sequences of high-low or low-high signals, define frame boundaries in some physical media.

Error Detection and Correction

The Data Link Layer performs error detection and correction.
Error-detecting codes and error-correcting codes are used to identify and correct errors.
- Errors tend to occur in bursts (clusters).
The practicality of error detection versus correction depends on the communication channel. Fiber optic communication favors detection due to its robustness, while wireless communication benefits from correction to deal with frequent errors.
Error Detection using polynomial code checksum is shown.

Burst Errors

Burst errors are difficult to correct than isolated errors.
Longer blocks have a greater chance of burst errors than small blocks, at the same error rate.

Detection or Correction

On fiber optic cables, retransmitting a block is cost-effective when an error is detected.
Wireless transmission may cause multiple errors, necessitating error correction.

Error Correcting Codes

Codewords contain a message and redundancy bits.
Not all codewords are valid.
Example: (0000000000), (0000011111), (1111100000), and (1111111111) are valid codewords.

Hamming Method

A codeword is constructed from the original message using formulas to determine parity for specific bits (e.g., 1, 2, 4, 8...).
The original message bits are placed into specific bits of the codeword.

Hamming Method (2)

Hamming codes can only correct single errors.
Burst errors of length k or less are correctable by structuring the burst error into a columnar form for easier error correction.

Example of a Hamming Code

The use of a Hamming Code to correct bit transmission errors for certain ASCII character values is illustrated.

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Description

This quiz explores the Data Link Layer, focusing on its role in facilitating communication between adjacent machines. It covers design issues, services provided to the Network Layer, and key concepts like framing and various framing techniques used to encapsulate packets. Test your understanding of these essential networking principles.