Figure 11.45 shows a state diagram to simulate the behavior...

Figure 11.45 shows a state diagram to simulate the behavior of Stop-and-Wait $\triangle \mathrm{RQ}$ at the receiver site. (figure can't copy) The states have a value of $R_n(0$ or 1$)$. The arrows shows the transitions. Explain the events that cause the two transitions labeled $A$ and $B$.

Figure 11.45 shows a state diagram to simulate the behavior of Stop-and-Wait $\triangle \mathrm{RQ}$ at the receiver site. (figure can't copy)
The states have a value of $R_n(0$ or 1$)$. The arrows shows the transitions. Explain the events that cause the two transitions labeled $A$ and $B$.

Key Concepts

Stop-and-Wait ARQ Protocol

Stop-and-wait Automatic Repeat reQuest (ARQ) is a fundamental error-control protocol used in data communication, where the sender transmits one frame and then waits for an acknowledgment from the receiver before sending the next frame. This protocol is critical for ensuring reliable data transmission, as it helps detect lost or incorrectly received frames, ensuring that data integrity is maintained through a simple yet effective communication scheme.

Receiver Finite-State Machine

A finite-state machine is used at the receiver end to manage its behavior based on the incoming frames. In the context of stop-and-wait protocols, the receiver maintains a state that indicates which sequence number it is expecting (typically represented as 0 or 1). Transitions between states occur based on the events that happen, such as the reception of a correctly sequenced frame or a duplicate frame, thus controlling the acknowledgment process.

Sequence Numbers

Sequence numbers are small, often binary (0 or 1) values used in stop-and-wait protocols to identify frames uniquely. Each transmitted frame carries a sequence number which allows the receiver to determine whether the received frame is new or a duplicate. This mechanism is essential for distinguishing between successive frames and ensuring that the protocol correctly handles retransmissions and avoids processing the same frame multiple times.

Event-Driven Transitions in the Receiver

In the state diagram of a receiver in a stop-and-wait ARQ system, transitions are triggered by specific events. For example, one event may occur when a frame with the expected sequence number is successfully received, prompting the receiver to switch its state and prepare for the next frame. Conversely, another event may occur when an unexpected or duplicate frame is received, which influences the receiver’s behavior, such as ignoring the duplicate while still resending the acknowledgment. These distinct events drive the state transitions, ensuring that the communication flow remains synchronized and reliable.

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