Question

In the discussion of ARQ protocol in Section 3.3.3, a scenario was outlined that resulted in the receiver accepting two copies of the same frame due to a loss of acknowledgement frame. Is it possible that a receiver may accept multiple copies of the same frame when none of the frames (message or acknowledgement) are lost?

Short answer

No, if no frames are lost and sequence numbers are correctly used, receivers will not accept duplicate frames.

Step-by-step solution

Understanding ARQ Protocol

ARQ (Automatic Repeat reQuest) is a protocol for error control in data transmission. It ensures that data is reliably transmitted by using acknowledgements (ACK) and timeouts. A sender transmits a frame and waits for an ACK. If the ACK does not arrive within a certain time period, the sender retransmits the frame.

Identify the ARQ Components

For ARQ to function properly, there are three main components: the sender who sends the frames, the receiver who acknowledges receipt, and the channel that transmits both data frames and ACKs.

Consider No Frame Loss Scenario

In the scenario where no message frames or acknowledgement frames are lost, the frames should ideally be transmitted and acknowledged without duplication. The protocol typically ensures that a frame is not accepted more than once due to sequence numbers.

Role of Sequence Numbers

Sequence numbers are crucial in preventing the receiver from accepting duplicate frames. Each frame is assigned a sequence number and for every correct receipt of a frame, the receiver sends an ACK with this sequence number.

Avoiding Duplicates with Sequence Numbers

With proper handling of sequence numbers, even if there are retransmissions due to delays (but no losses), duplicates can be detected. The receiver checks the sequence numbers of frames; if the sequence number of a frame matches the number of the last correctly received and acknowledged frame, the receiver knows it's a duplicate and will not accept it again.

Final Conclusion

Given that there are no losses and the protocol effectively utilizes sequence numbers, it is not possible for the receiver to accept duplicate copies of the same frame. The correct use of sequence numbers by both the sender and receiver prevents such redundancy.

Key concepts

Error control

Error control is a crucial aspect of data transmission, ensuring that information sent from a sender reaches the receiver accurately and completely, even in the presence of errors. The ARQ protocol is instrumental in this process. It employs methods such as retransmissions and acknowledgements to detect and correct any errors that might occur during transmission.

The ARQ (Automatic Repeat reQuest) protocol works by utilizing a process where the sender expects an acknowledgment (ACK) from the receiver for every data frame sent. If an ACK is not received within a certain time cycle, the sender assumes that either the data or the acknowledgement was lost or corrupted, prompting the sender to resend the data frame. This continuous exchange ensures that data errors are detected and corrected. The constant checking and retransmitting form the backbone of error control in reliable communication protocols, ensuring that the chances of data being accurately received are maximized.

Sequence numbers

Sequence numbers are numerical identifiers that play an essential role in maintaining the order and integrity of packet flow during data transmission. In the context of the ARQ protocol, each frame sent by the sender is assigned a unique sequence number. This sequence number is crucial in helping both the sender and receiver keep track of which frames have been sent and acknowledged.

When a receiver accepts a frame, it checks if the sequence number follows the anticipated order. If a frame with a matching sequence number has already been received and acknowledged, the receiver can identify it as a duplicate and discard it accordingly. This process prevents duplicate data receipt and ensures the integrity of the data stream, maintaining a smooth and error-free transmission. Therefore, by using unique sequence numbers for each data frame, the ARQ protocol enhances both the order and accuracy of data delivery.

Data transmission reliability

Reliability in data transmission refers to the ability to deliver data accurately and consistently over a communication channel. ARQ protocols contribute significantly to achieving high levels of reliability by employing mechanisms like error control and sequence numbering.

The concept of reliability in data communication embraces the idea that each piece of data must reach its intended recipient without alteration or loss. The ARQ protocol ensures this by requiring explicit confirmations of receipt. The sender knows that only once an acknowledgment is received is the data considered correctly delivered. If acknowledgments do not arrive, the sender can be prompted to resend the data frame in question.

Thus, ARQ protocols help minimize data errors and losses, enhancing the overall reliability of data transmissions over networks. By employing these systematic procedures, communication systems can maintain consistent and accurate transfer of information, which is critical for applications ranging from simple file transfers to complex real-time communication.