What is pipelining?

In computer networking, pipelining is the method of sending multiple data units without waiting for an acknowledgment for the first frame sent. Pipelining ensures better utilization of network resources and also increases the speed of delivery, particularly in situations where a large number of data units make up a message to be sent.

How Pipelining Works

In traditional stop-and-wait protocols, the sender transmits one frame and waits for its acknowledgment before sending the next frame. This approach leads to inefficient use of available bandwidth, especially in high-latency networks. Pipelining allows multiple frames to be transmitted continuously without waiting for individual acknowledgments, significantly improving throughput.

Data Link Protocols that use Pipelining

Two data link layer protocols use the concept of pipelining:

Go-Back-N Protocol

Go-Back-N protocol provides for pipelining of frames, allowing multiple frames to be sent before receiving acknowledgment for the first frame. The frames are sequentially numbered and a finite number of frames are sent depending upon the size of the sending window. If the acknowledgment of a frame is not received within the time period, all frames starting from that frame are retransmitted. The receiving window size is 1 in this case.

Selective Repeat Protocol

This protocol also incorporates pipelining with a receiver window size greater than 1. In selective repeat, only the erroneous or lost frames are retransmitted, while the good frames are received and buffered. When the sender times out, the oldest unacknowledged frame is retransmitted. If the retransmitted frame is received correctly, the receiver delivers all buffered frames starting with the retransmitted frame.

Advantages of Pipelining

• Improved throughput − Multiple frames can be transmitted simultaneously, maximizing bandwidth utilization.

• Reduced idle time − The sender doesn't wait for individual acknowledgments before transmitting subsequent frames.

• Better performance in high-latency networks − Particularly effective when round-trip time is significant.

Conclusion

Pipelining significantly improves network efficiency by allowing multiple frames to be transmitted without waiting for individual acknowledgments. Both Go-Back-N and Selective Repeat protocols implement pipelining to achieve higher throughput and better bandwidth utilization compared to stop-and-wait protocols.