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Here’s a detailed tutorial to help you learn the Data Link Layer chapter in the Computer Networks Technology course (02141 in the Chinese Self-Study Exam system).


Data Link Layer: A Comprehensive Guide

The Data Link Layer is the second layer in the OSI model, responsible for providing reliable data transfer between directly connected nodes. It ensures that frames (data packets at this layer) are correctly formatted, transmitted, and received.

Key Responsibilities:


2. Framing

Framing involves breaking a continuous stream of data into smaller units, called frames, which include synchronization information.

Types of Framing Methods:

  1. Character Count Method – The first field in the frame specifies the number of characters.
  2. Flag-based Framing (Bit Stuffing) – Uses special flag bits (e.g., 01111110 in HDLC) to mark the start and end.
  3. Character-based Framing (Byte Stuffing) – Uses escape sequences to differentiate control characters from data.

3. Error Detection and Correction

Error handling ensures that data transmission is accurate.

Error Detection Techniques:

Error Correction Techniques:


4. Flow Control

Flow control prevents the sender from overwhelming the receiver.

Flow Control Methods:


5.1 Ethernet (IEEE 802.3)

Ethernet is a widely used LAN technology based on the IEEE 802.3 standard.

Ethernet Frame Structure:

| Field | Description | |——–|————| | Preamble | Synchronization | | Destination Address | MAC address of receiver | | Source Address | MAC address of sender | | Type/Length | Identifies protocol type (IPv4, IPv6, etc.) | | Data | Actual payload | | CRC | Error-checking value |

Ethernet Transmission Modes:


5.2 Point-to-Point Protocol (PPP)

PPP is used in dial-up and broadband connections.

PPP Features:

PPP Frame Structure:

| Field | Description | |——–|————| | Flag | Marks the start and end of the frame | | Address | Usually 0xFF (Broadcast) | | Control | Usually 0x03 (Unnumbered Information) | | Protocol | Indicates the protocol used (IPv4, IPv6, etc.) | | Data | Actual data payload | | CRC | Error-checking |


6. Medium Access Control (MAC) Methods

6.1 Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

6.2 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)


7. Switching Techniques

Switching determines how data is forwarded in a network.

7.1 Circuit Switching

7.2 Packet Switching

7.3 Message Switching


8. Introduction to LAN Technologies

LAN (Local Area Network) technologies define how devices communicate in a local network.

Types of LAN Technologies:

  1. Ethernet (IEEE 802.3) – Uses CSMA/CD, supports speeds from 10 Mbps to 100 Gbps.
  2. Wi-Fi (IEEE 802.11) – Uses CSMA/CA, wireless transmission.
  3. Token Ring (IEEE 802.5) – Uses a token-passing method to control access.
  4. FDDI (Fiber Distributed Data Interface) – Uses fiber-optic cables for high-speed networking.

9. Summary


10. Practice Questions

  1. What is the main function of the Data Link Layer in the OSI model?
  2. Compare CSMA/CD and CSMA/CA.
  3. Explain the difference between circuit switching and packet switching.
  4. What are the key differences between Ethernet and PPP?
  5. How does error detection using CRC work?

This tutorial provides a solid foundation for mastering the Data Link Layer in your Computer Networks Technology (02141) course. If you need further explanations, practice questions, or real-world examples, feel free to ask! 🚀


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