Data Link Layer: Packets, Access Methods, and Topologies

Posted on Nov 8, 2024 in Computers

Data Link Layer: A Comprehensive Overview

Preparing Data Link Layer Packets

Packets are prepared for transmission by encapsulating them with a header and a trailer, creating a frame for transport across the local media.

Methods of Access to the Data Link Layer

Four general methods provide access to the data link layer:

Shared Media Access Control

  • Controlled Access: Each node has its own time to use the medium (e.g., ring topology).
  • Contention-Based Access: All nodes compete for medium usage (e.g., bus topology).

Point-to-Point Media Access Control

  • Half-Duplex: Nodes transmit or receive one at a time (suitable for low-bandwidth, long-distance links).
  • Full-Duplex: Nodes transmit and receive simultaneously (suitable for high-bandwidth, long-distance links).

Logical Topologies vs. Multi-Access and Point-to-Point

Point-to-Point Topology

This topology directly connects two nodes, simplifying media access control. Frames travel between these two nodes. Half-duplex links allow one-way data flow at a time, while full-duplex links enable simultaneous bidirectional communication.

Multi-Access Logical Topology

This topology enables multiple nodes to communicate using shared media. Each node sees all frames, but only the intended recipient processes the data. Media access control protocols regulate data transmission to minimize collisions.

Functions of a Ring Topology

In a logical ring topology, each node receives and forwards frames. This facilitates controlled media access using token passing. Only the node possessing the token can transmit data.

Five Layer 2 Protocols

  • Point-to-Point Protocol (PPP)
  • Ethernet
  • High-Level Data Link Control (HDLC)
  • Frame Relay
  • Asynchronous Transfer Mode (ATM)

Data Link Layer vs. Network Layer Addressing

Unlike hierarchical Layer 3 logical addresses, Layer 2 physical addresses are non-hierarchical and local to the network. They remain unchanged even if a device moves to a different network or subnet. Layer 3 addresses, however, remain consistent across networks.

Data Link Frame Header Fields

Typical frame header fields include:

  • Start Frame Field: Indicates the frame’s beginning.
  • Destination and Source Address Fields: Identify the communicating nodes.
  • Priority/Quality of Service Field: Specifies the communication service type.
  • Type Field: Indicates the upper-layer service.
  • Logical Connection Control Field: Establishes logical connections.
  • Physical Link Control Field: Manages the media link.
  • Flow Control Field: Controls media traffic flow.
  • Congestion Control Field: Indicates media congestion.

Frame Check Sequence (FCS) in the Trailer

The Frame Check Sequence (FCS) in the trailer ensures data integrity. The transmitting node generates an FCS, which is a logical summary of the frame’s content. The receiving node recalculates the FCS and compares it with the received FCS. If they match, the frame is considered valid; otherwise, it’s discarded.