Basic Frame Structure of SDLC (original) (raw)

Last Updated : 17 Dec, 2025

Synchronous Data Link Control (SDLC) is an IBM-developed data link layer protocol that ensures reliable communication using structured frames and a primary–secondary model.

• Developed by IBM in 1974 for Systems Network Architecture (SNA)
• Operates at the data link layer
• Supports point-to-point, multipoint, and packet-switched networks
• Uses a primary–secondary station communication model
• Organizes data and control information into SDLC frames

Purpose of an SDLC Frame

An SDLC frame enables the receiving station to:

• Identify the start and end of a frame
• Determine whether the frame is intended for it
• Understand what action to perform on the received data
• Detect transmission errors
• Acknowledge successful frame reception

Frame Format :

Transmission frame of each of SDLC has specific frame format. Each frame is made up of following fields :

**SDLC Frame Format

Each SDLC frame consists of the following fields:

**1. Flag Field

• Marks the beginning and end of a frame
• Helps detect frame boundaries and synchronization

**2. Address Field

• Identifies the secondary station involved
• Supports individual, group, and broadcast addressing

**3. Control Field

• Defines the function and type of the frame
• Supports three formats:
• **Unnumbered (U) Frame: Link setup, disconnection, and control functions
• **Supervisory (S) Frame: Acknowledgments, flow control, and error reporting
• **Information (I) Frame: Data transfer and sequence control

**4. Information Field

• Optional field containing user data to be transmitted

**5. Frame Check Sequence (FCS)

• Used for error detection
• Ensures the integrity of the received frame

**6. Ending Flag Field

• Indicates the end of the transmission frame

Key Advantages of SDLC

Below are some of the key advantages of SDLC:

• Reliable data transmission
• Strong error detection and flow control
• Supports multiple network configurations
• Structured and efficient frame management

SDLC laid the foundation for many modern data link protocols by introducing robust framing, error control, and communication management techniques.