synchronous Data Link Control (SDLC) is a bit-oriented synchronous protocol developed by IBM in the 1970s. It was a foundational protocol for network communication, particularly within IBM’s Systems Network Architecture (SNA), and significantly influenced later protocols like HDLC (High-level Data Link Control). SDLC provides a robust and efficient method for transmitting data between network devices.
Key Characteristics of SDLC
- Bit-Oriented: Unlike character-oriented protocols, SDLC treats data as a continuous stream of bits, allowing for greater efficiency and flexibility in data encoding. This means it doesn’t rely on specific character codes for control information.
- Synchronous Transmission: Data is transmitted in a highly synchronized manner, meaning both the sender and receiver are synchronized by a common clock signal. This ensures that data bits are interpreted correctly at the receiving end, leading to higher data rates.
- Full-Duplex Capability: SDLC supports full-duplex communication, allowing data to be transmitted in both directions simultaneously. This significantly improves throughput and efficiency.
- Error Detection and Correction: SDLC incorporates a robust error detection mechanism using a Cyclic Redundancy Check (CRC). This helps ensure data integrity during transmission.
- Flow Control: The protocol includes mechanisms for flow control, preventing a fast Synchronous Data Link Control sender from overwhelming a slower receiver.
- Primary/Secondary Station Architecture: SDLC operates on a primary/secondary station architecture.
- Primary Station: The primary station is responsible for managing the link, initiating data transfers, and recovering from errors. It controls the secondary stations.
- Secondary Station: Secondary stations respond to commands from the primary station and transmit data when granted permission.
- Frame Structure: Data in SDLC is transmitted in frames, each with a specific structure:
- Flag Field (01111110): Marks the beginning and end of a frame.
- Address Field: Identifies the secondary station involved in the communication.
- Control Field: Contains commands, responses, and sequence numbers for flow and error control.
- Information Field: Carries the actual user data.
- Frame Check Sequence (FCS): The CRC value used for error detection.
- Flag Field (01111110): Again, marks the end of the frame.
How SDLC Works
Communication in SDLC begins with the primary station sending commands to a secondary station. The secondary station then responds to these commands. The control field within the frame is crucial for managing this interaction, including:
- Sequencing: Ensures that frames are received in the correct order and that no frames are lost or duplicated.
- Acknowledgement: Confirms the receipt of frames.
- Flow Control: Manages the rate of data transmission to prevent buffer overflows.
- Error Recovery: Handles retransmissions in case of detected errors.
One of the key features for maintaining synchronization and preventing ambiguity in the flag sequence within the data stream is bit stuffing. Whenever five consecutive ‘1’ bits appear in the data, address, control, or FCS fields, a ‘0’ bit is automatically inserted by the sender. This ‘0’ bit is then removed by the receiver. This ensures that the flag sequence (01111110) is unique and never accidentally appears within the data itself.
Applications and Legacy
SDLC was widely used in IBM mainframe environments for connecting terminals, printers, and other peripherals to host computers. It formed the backbone of many corporate networks before the widespread adoption of Ethernet and TCP/IP.
While SDLC is not commonly used in modern networking environments, its influence is significant. It served as the basis for HDLC, which became an international standard and is still used in various applications, including point-to-point links and X.25 packet-switched networks. Many of the fundamental concepts of reliable data link layer communication, such as frame structuring, error control, and flow control, were refined and popularized by SDLC. Its robust design contributed to the development of more advanced and efficient communication protocols we use today.
