GBNs

GBNs stands for Go-Back-N Automatic Repeat reQuest, a sliding-window protocol used for reliable data transfer over unreliable networks. It enables a sender to transmit multiple frames without waiting for an individual acknowledgment, up to a predefined window size. The receiver, however, only acknowledges in-order receipt and does not buffer out-of-order frames; any out-of-order or corrupted frames are discarded. Frames are numbered within a finite sequence space, typically using modulo arithmetic.

In operation, the sender maintains two primary variables: a base (the oldest unacknowledged frame) and a next sequence number (the next frame to send). A window of size W allows the sender to transmit up to W frames ahead of the base. Each transmitted frame starts a timer. When the receiver obtains the next in-sequence frame, it acknowledges the last in-order frame received. ACKs are cumulative, meaning an acknowledgment for a higher-numbered frame implicitly acknowledges all preceding frames. If the sender’s timer expires before an ACK is received for the base frame, it retransmits the base frame and all subsequent frames in the current window, hence the “go back” behavior.

To avoid ambiguity with wraparound, the window size is typically restricted to be less than the size of the sequence space. GBN is often contrasted with Selective Repeat ARQ, which retransmits only erroneous frames. Go-Back-N performs well with moderate error rates and larger round-trip times, but performance degrades as loss probability increases or the window size is poorly tuned. It remains a foundational model in networking for illustrating reliable transmission concepts.