ECE144 Data-Link Layer Chapter 3

Data-Link Layer Overview

• Node-to-node communication occurs at the data-link layer, contrasting with end-to-end communication in higher layers.
• Data units must traverse multiple networks (LANs and WANs) and are facilitated by routers.
• Nodes consist of end hosts and routers, while links connect these nodes.

Link Types

• Data-link layer manages the utilization of transmission mediums (cable, air).
• Point-to-point links dedicate resources to two devices; broadcast links share resources among multiple devices.
• Traditional home phones exemplify point-to-point links, while cellular networks illustrate broadcast links.

Sublayers of Data-Link Layer

• Divided into two sublayers:
  • Data-Link Control (DLC): Addresses issues for both point-to-point and broadcast links.
  • Media Access Control (MAC): Specifically manages issues for broadcast links.

Framing

• Bits are packed into frames for distinction; analogous to postal systems.
• Each frame includes sender and destination addresses for effective communication.
• Messages are broken into smaller frames which improves flow and error control.
• Frames can be fixed or variable in size, and may utilize different framing protocols:
  • Character-oriented Framing: Traditional approach, but limited by character encoding issues.
  • Bit-oriented Framing: Uses bits for data representation and requires delimiters to separate frames.

Bit Stuffing

• Employed to prevent flag patterns (0 1 1 1 1 1 1 0) from appearing in data.
• If five consecutive 1 bits are detected, an extra 0 is inserted, which is later removed by the receiver.

Error Control

• Incorporates error detection and correction strategies while addressing data loss and integrity during transmission.
• Two error types:
  • Single-bit Error: Occurs in a single bit.
  • Burst Error: Involves multiple bits.
• Error Detection Techniques include:
  • Exact-code coding, parity checking, and redundancy methods (Checksum, VRC, LRC, CRC).

Error Correction

• More complex than detection; focuses on identifying and correcting erroneous data.
• Two primary methods:
  • Retransmission: Involves sending data again after detection of errors.
  • Forward Error Correction: Uses techniques like Hamming codes to correct errors without retransmission.

High-Level Data-Link Control (HDLC)

• A bit-oriented protocol used for communication on point-to-point and multipoint links.
• Defined transfer modes:
  • Normal Response Mode (NRM): Unbalanced configuration, primary node initiates communication.
  • Asynchronous Balanced Mode (ABM): Balanced configuration, each node can act as primary or secondary.

HDLC Frame Types

• Information Frames (I-frames): Carry user data and control info.
• Supervisory Frames (S-frames): Convey control info without user data.
• Unnumbered Frames (U-frames): Manage session and control information.

HDLC Frame Structure

• Flag: Synchronization pattern that indicates frame boundaries.
• Address: Identifies secondary station's address.
• Control: Bytes for flow and error control; varies based on frame type.
• Information: Contains user data; length can vary.
• Frame Check Sequence (FCS): Error detection field using CRC.

Point-to-Point Protocol (PPP)

• Extends HDLC for point-to-point communication; facilitates link establishment and data transfer with various protocol layers.
• State Phases of PPP include:
  • Dead State
  • Establish State
  • Authenticate State
  • Open State
  • Terminate State

PPP Protocols

• Link Control Protocol (LCP): Configures and maintains the connection; negotiates options.
• Authentication Protocols (APs): Verify user identity through methods like PAP and CHAP.
• Network Control Protocols (NCPs): Configure links for multiple network-layer protocols.

Multilink PPP

• Allows logical frames to be distributed across multiple physical channels, enhancing link efficiency.
• Sequence numbering in PPP frames indicates the position of fragments within logical frames.

Explore the fundamentals of the Data-Link Layer in this quiz. Dive into key concepts such as framing, error control, and protocols that facilitate node-to-node communication. Perfect for students of Data Communications in course ECE144.