Understanding HDLC Protocol and ARQ Error Control Mechanisms

Data Link Control and Error Control Mechanisms

Error Control Techniques

Efficient data transmission involves acknowledging successful data receipt and handling errors. When a station has both data and an acknowledgment to send, they are combined into one frame to optimize communication. If only an acknowledgment is available, a separate frame (RR or RNR) is sent. In cases where data is ready but no new acknowledgment is needed, the last sent acknowledgment sequence number is repeated within the data frame.

Error control relies on several key mechanisms:

• Error Detection: Techniques like checksums or CRCs identify erroneous frames, which are then discarded.
• Positive Acknowledgment: The receiver sends a positive acknowledgment (ACK) to confirm successful frame reception.
• Retransmission After Timeout: If an ACK isn't received within a set time, the sender retransmits the frame.
• Negative Acknowledgment and Retransmission: The receiver sends a negative acknowledgment (NAK) for frames with errors, prompting retransmission.

Automatic Repeat Request (ARQ)

ARQ protocols manage error control and flow control. Three common types are:

Stop-and-Wait ARQ

The simplest ARQ type, where the sender transmits one frame and waits for an ACK before sending the next. Damaged frames are discarded, and timeouts trigger retransmissions. To avoid confusion with duplicate ACKs, alternate numbering (ACK0/ACK1) is used.

Go-Back-N ARQ

A sliding window protocol where multiple frames can be sent before receiving ACKs. If an error is detected, the receiver sends a NAK and discards that frame and all subsequent ones. The sender then retransmits the erroneous frame and all following frames.

Selective Reject ARQ

Also known as selective retransmission, only frames with errors are retransmitted. Subsequent frames are accepted and buffered by the receiver. This method requires a larger buffer at the receiver and is less commonly used.

High-Level Data Link Control (HDLC)

HDLC is a widely used data link control protocol defined by ISO standards. It serves as the basis for other protocols and defines different station types and configurations:

Station Types

• Primary Station: Controls the link's operation and issues commands.
• Secondary Station: Operates under the primary's control and issues responses.
• Combined Station: Combines primary and secondary functions, issuing both commands and responses.

Link Configurations

• Balanced: One primary station communicates with multiple secondary stations.
• Unbalanced: A combined station interacts with secondary stations.

HDLC Data Transfer Modes

• Normal Response Mode (NRM): Used in unbalanced configurations, where the primary initiates data transfer.
• Asynchronous Balanced Mode (ABM): Used in balanced configurations, allowing either station to initiate transmission without polling overhead.
• Asynchronous Response Mode (ARM): Used in unbalanced configurations, allowing secondary stations to transmit without primary permission (rarely used).

HDLC Frame Structure

HDLC frames consist of several fields:

• Address Field: Identifies the secondary station involved in the communication.
• Control Field: Defines the frame type (information, supervisory, or unnumbered) and carries control information.
• Information Field: Contains user data and is present only in I-frames and some U-frames.
• Frame Check Sequence (FCS) Field: An error-detecting code calculated from the frame's bits to ensure data integrity.

HDLC, with its various configurations and frame types, provides a robust framework for reliable data transfer across communication links.