What are Data Link Protocols in Computer Network?

Data Link protocols are a collection of rules and conventions that manage communication at the Data Link Layer of the network protocol stack. These protocols ensure reliable transmission of data frames between directly connected nodes over a single link.

The primary function of Data Link protocols is to provide error detection, flow control, and reliable delivery of data frames across individual network links. Different protocols may be used on different segments of a path - for example, Ethernet on one link and PPP (Point-to-Point Protocol) on another.

Simplex Stop and Wait Protocol

The Simplex Stop and Wait protocol is the simplest data link protocol designed for error-free transmission channels. It operates with the following mechanism:

• Unidirectional transmission − Data flows in only one direction from sender to receiver

• One frame at a time − The sender transmits a single frame and waits for acknowledgment before sending the next

• Flow control only − Provides flow control but no error detection or correction

The sender encapsulates network layer packets into frames and transmits them one by one. After receiving each frame, the receiver sends back a null acknowledgment frame. This process continues until all data is transmitted.

Limitations

The major drawback is inefficiency. Since only one frame can be in transit at any time, the protocol wastes bandwidth, especially on long-distance links where propagation delay is significant.

Positive Acknowledgement with Retransmission (PAR)

The PAR protocol enhances the Stop and Wait approach by adding error control capabilities:

• Sequence numbering − Each frame carries a sequence number for identification

• Timeout mechanism − Sender retransmits frames if acknowledgment is not received within a specified time

• Error detection − Handles lost or corrupted frames through retransmission

When the sender transmits a frame, it starts a timer. If a positive acknowledgment with the correct sequence number arrives before timeout, the next frame is sent. If the timer expires or an incorrect acknowledgment is received, the frame is retransmitted.

Protocol Comparison

Conclusion

Data Link protocols manage reliable communication between directly connected nodes. While simple protocols like Stop and Wait provide basic functionality, enhanced protocols like PAR add essential error control capabilities for real-world networks.