Data Communication Protocols

Framing in Networking

• Fixed-size framing

  • Utilizes size as a delimiter, eliminating the need for boundary definition.
  • Example: ATM WAN employs fixed-size frames known as cells.

• Variable-size framing

  • Common in local-area networks.
  • Requires methods to mark frame beginnings and endings.
  • Traditionally uses two approaches: character-oriented and bit-oriented.

Character-oriented Framing

• Operates with 8-bit characters, typically from ASCII coding.
• Frames begin and end with an 8-bit flag, ensuring clear frame separation.
• Components of a frame include:
  • Header: Contains source and destination addresses, control information.
  • Trailer: Includes error detection bits.
• Designed for text data exchange; now also supports diverse information types.
• Issues arise with patterns in data resembling flag patterns; leads to confusion about frame boundaries.

Byte Stuffing Technique

• Introduced to address ambiguity between flag characters and data.
• Adds an escape byte (ESC) before a character that matches the flag pattern.
• If an ESC appears in the data, another ESC precedes it to clarify that it’s data, not a flag.

Bit-oriented Framing

• Handles a sequence of bits instead of fixed-width characters, accommodating various data types (text, graphic, audio).
• Requires a delimiter for frame separation; commonly uses an 8-bit flag pattern.
• Faces similar problems as character-oriented framing; introduces bit stuffing to maintain frame integrity.
• In bit stuffing, an extra 0 is added after encountering 0 followed by five consecutive 1s to prevent flag appearance in data.

Error Control in Data-Link Layer

• Encompasses both error detection and correction.
• Allows receivers to notify senders about lost or damaged frames for retransmission.
• Types of errors include single-bit errors and burst errors.

Error Detection Techniques

• Involves monitoring data to identify transmission errors.
• Does not pinpoint specific erroneous bits; focuses on preventing undetected errors.
• Common techniques include exact-code coding, parity checks, normal response mode (NRM), and asynchronous balanced mode (ABM).

HDLC (High-Level Data Link Control)

• Defines three frame types:
  • Information Frames (I-frames): Transport user data and control info.
  • Supervisory Frames (S-frames): Transport only control information.
  • Unnumbered Frames (U-frames): Manage session and control information.
• Frame structure includes:
  • Flag: Synch pattern for frame identification.
  • Address: Secondary station address.
  • Control: Used for flow and error management.
  • Information: User data from network layer.
  • Frame Check Sequence (FCS): Error detection field utilizing CRC.

HDLC Frame Control

• Frames vary by type, identified by bit patterns (e.g., I-frame starts with 0, S-frame starts with 10).
• S-frames feature acknowledgment codes for frame transmission statuses.

Point-to-Point Protocol (PPP)

• Widely used for point-to-point access, especially for ISP connectivity.
• Services Provided:
  • Defines frame format, negotiates link establishment.
  • Supports multipoint authentication.
• Limitations:
  • Lacks flow control, sophisticated error handling, and does not guarantee frame sequencing.

PPP Frame Structure

• Flag: Singular byte with pattern 01111110 indicating frame boundaries.
• Address: Fixed broadcast address.
• Control: Constant value mimicking U-frames in HDLC.
• Protocol: Indicates the content type within data field.
• Payload: User data, max length of 1500 bytes.
• FCS: Standard CRC for error detection.

Byte Stuffing and Transition Phases in PPP

• Escape byte added whenever flag-like patterns appear in data.
• Transition Phases:
  • Dead State: No active carriers.
  • Establish State: Connection setup and option negotiation.
  • Authenticate State: Authentication verification process.
  • Open State: Active data transfer with packet exchange.
  • Terminate State: Connection closure, returning to dead state.

Multiplexing in PPP

• Uses protocols such as Link Control Protocol (LCP) and Network Control Protocols (NCPs) to manage connections.
• Authentication Protocols:
  • Password Authentication Protocol (PAP): Basic two-step authentication.
  • Challenge Handshake Authentication Protocol (CHAP): More secure three-step process to verify user identity.

Multilink PPP

• Allows logical PPP frames to split into multiple actual frames across several channels.
• Facilitates efficient data transmission by adding sequence numbers to fragments for proper reassembly.