Data Link Layer Functions and Error Control

Data Link Layer Tasks and Error Handling

The Data Link Layer is responsible for media access control in shared environments and comprehensive error handling.

Communication Design and Error Management

• Data Division: Information is divided into blocks carrying a sequence number and an error detection code. If an error occurs, the block is re-received.
• Information Control: All necessary information is included to ensure the receiver can reconstruct the data.
• Error Correction: Detecting and correcting errors within the communication channel.
• Flow Control: Managing the rate of data transmission.
• Control Dialog: Managing transmission shifts and communication flow.

Design of the Link Layer

The design focuses on providing connection-oriented and reliable services among other essential networking functions.

Types of Errors and Solutions

• Incorrect Information: Data frames that contain errors. Solution: Implementation of confirmation and sequence frames.
• Incomplete Frames: Frames that are not fully received. Solution: Methods that allow the recipient to distinguish frames from one another.
• Lost Frames: Frames that are not received due to host problems. Solution: Use of acknowledgment frames and appropriate timers.

Frame Division and Structure

Asynchronous signaling involves the composition of frames consisting of a header (script, direction), info (control, data), and end (redundancy, screenplay).

Frame Components

• Union: Indicates the start and end of a frame (e.g., STX, DLE, ETX).
• Mailing Address: Physical addresses (MAC addresses) for both origin and destination.
• Control Field: Includes the type (control info), sequence (frame sequence number), and confirmation (order number of the received frame).
• Data: Information originating from the network layer.
• Redundancy: Used for error tracking and bug detection.

Flow Control Mechanisms

Flow control is necessary when the sender transmits faster than the receiver can process the data. Workarounds:

1. The emitter waits for a confirmation from the receiver before continuing the broadcast.
2. A preset number of consecutive frames are sent without requiring immediate confirmation.

Environmental Management and Addressing

1. Protocols enable the transmission of turn-based stations.
2. The OSI model includes a sublayer in the data link layer called the MAC (Media Access Control) sublayer.

Addressing

A unique ID differentiates stations; this is the address for the link layer or MAC sublayer.

Coding Theory and Hamming Distance

• Code: A limited set of symbols combined with specific rules to be understood between the transmitter and receiver.
• Distance between two words: The number of digits that must be modified in a word to transform it into another word.
• Distance of a binary code: The lowest of the distances between any two code words.
• Adjacent words: Words are considered adjacent if their distance is exactly one.
• Dense Code: An n-digit code that has 2ⁿ words.

Error Correction Formulas

• Correcting N errors: d ≥ 2N + 1
• Length: N + K
• Hamming Rule: 2^k ≥ N + K + 1