Overview of Data Link Layer Concepts

Oct 9, 2024

Cisco Introduction to Networks: Module 6

Topic: Data Link Layer

Objectives

  • Explain how media access control in the data link layer supports communication across networks.
  • Understand the purpose of the data link layer, topologies, and the data link frame.
  • Describe the purpose and functions of the data link layer.
  • Compare characteristics of media access control methods in WAN and LAN topologies.
  • Describe characteristics and functions of the data link frame.

OSI Model Context

  • The Data Link Layer is Layer 2 in the OSI model, which handles MAC addresses and error-free data transmission.
  • It interfaces the Network Interface Card (NIC) with the network.
  • Prepares network data for the physical network.

Functions of the Data Link Layer

  • Enables access to media for upper layers.
  • Accepts and encapsulates Layer 3 packets into Layer 2 frames.
  • Controls data placement and receipt on the media.
  • Performs error detection and rejects corrupt frames.

Sub-layers

  1. Logical Link Control (LLC)

    • Handles communication between upper and lower layers.
    • Implemented in software.

  2. Media Access Control (MAC)

    • Responsible for data encapsulation and media access.
    • Manages frame access to network media (copper, fiber optics, wireless).
    • Controls NIC and related hardware for sending/receiving data.

Network Topologies

  • Physical Topology: Arrangement of devices and their physical connections.
  • Logical Topology: Virtual connections using device interfaces and IP addressing.

Types of Topologies

  • WAN Topologies: Point-to-point, Hub-and-Spoke, Mesh.
  • LAN Topologies: Star, Extended Star (common modern usage), Bus, Ring (legacy).

Duplex Communication

  • Half Duplex: One device sends or receives at a time (used in WLANs and legacy systems).
  • Full Duplex: Simultaneous transmission and reception.

Access Control Methods

  • Contention-Based Access: In half duplex competing for medium (e.g., CSMA/CD, CSMA/CA)
  • Control Access: Deterministic, used in legacy networks like token ring.

Frame Structure

  • Components: Header, Data (Packet), Trailer.
  • Header: Frame start, addressing, type, control.
  • Trailer: Error detection (Frame Check Sequence), frame stop.

Important Concepts

  • CSMA/CD: Used in Ethernet LANs to avoid collisions.
  • CSMA/CA: Used in WLANs to prevent collisions.
  • Error Detection: CRC used to verify data integrity.
  • Layer 2 Addressing: Known as physical addresses, updated at each routing hop.

Summary

  • The Data Link Layer is critical for facilitating NIC communications and ensuring data integrity.
  • Understanding topology types and access methods is essential for network design and troubleshooting.
  • This module prepares you for subsequent learning about LAN/WAN technologies and protocols.

Additional Notes

  • Data link protocols are defined by organizations like IEEE, ITU, ISO, and ANSI.
  • For exams, focus on understanding key concepts such as duplex communications, topology differences, and access control methods.

Full transcript