ARQ (automatic repeat request)
Introduction:
ARQ (Automatic Repeat Request) is a technique used in communication protocols for reliable transmission of data. It is a feedback mechanism that detects errors and retransmits data that has been lost or corrupted during transmission. ARQ is widely used in many communication protocols, such as TCP (Transmission Control Protocol), Bluetooth, Wi-Fi, and cellular networks. In this article, we will explore the basics of ARQ, its types, and its working mechanism.
Basics of ARQ:
ARQ is a technique used to detect and correct errors in data transmission. It is a feedback mechanism that ensures that the data transmitted between the sender and the receiver is reliable. The basic principle of ARQ is to send a block of data and wait for the receiver to confirm its successful receipt. If the receiver detects an error in the data, it sends a negative acknowledgment (NAK) message to the sender requesting retransmission of the data. If the sender does not receive an acknowledgment (ACK) message from the receiver within a specified time, it assumes that the data has been lost and retransmits it.
ARQ Types:
There are three types of ARQ techniques:
- Stop-and-wait ARQ
- Go-Back-N ARQ
- Selective Repeat ARQ
Stop-and-wait ARQ:
In stop-and-wait ARQ, the sender sends a block of data and waits for an acknowledgment (ACK) message from the receiver. If the receiver detects an error in the data, it sends a negative acknowledgment (NAK) message requesting retransmission of the data. The sender waits for the ACK or NAK message before sending the next block of data. This process is repeated until all the data has been successfully transmitted.
Stop-and-wait ARQ is a simple ARQ technique that is suitable for low-speed channels with low error rates. However, it is not very efficient for high-speed channels because the sender has to wait for the receiver to acknowledge each block of data before sending the next block.
Go-Back-N ARQ:
In Go-Back-N ARQ, the sender sends a window of data frames and waits for an acknowledgment (ACK) message from the receiver. The receiver sends an acknowledgment message for each correctly received frame. If the receiver detects an error in a frame, it sends a negative acknowledgment (NAK) message requesting retransmission of all frames starting from the one that contained the error. The sender retransmits all the frames in the window starting from the frame that was requested in the NAK message.
Go-Back-N ARQ is more efficient than stop-and-wait ARQ because it allows the sender to transmit multiple frames before waiting for an acknowledgment from the receiver. However, if a frame is lost or corrupted, the sender has to retransmit all the frames in the window, which can cause unnecessary delays and reduce efficiency.
Selective Repeat ARQ:
In Selective Repeat ARQ, the sender sends a window of data frames and waits for an acknowledgment (ACK) message from the receiver. The receiver sends an acknowledgment message for each correctly received frame. If the receiver detects an error in a frame, it sends a negative acknowledgment (NAK) message requesting retransmission of only the frame that contained the error. The sender retransmits only the requested frame.
Selective Repeat ARQ is the most efficient ARQ technique because it allows the sender to transmit multiple frames before waiting for an acknowledgment from the receiver. It also reduces unnecessary retransmissions, which improves efficiency.
Working Mechanism of ARQ: ARQ works by using a feedback mechanism to detect and correct errors in data transmission. The sender sends a block of data and waits for an acknowledgment (ACK) message from the receiver. If the receiver detects an error in the data, it sends a negative acknowledgment (NAK) message requesting retransmission of the data. If the sender does not receive an acknowledgment message from the receiver within a specified time, it assumes that the data has been lost and retransmits it.
ARQ uses sequence numbers to identify each block of data that is transmitted. The receiver uses these sequence numbers to detect missing or out-of-order packets. If a packet is missing, the receiver sends a NAK message requesting retransmission of the missing packet. If a packet is received out of order, the receiver stores it in a buffer until all the missing packets are received.
ARQ also uses a sliding window mechanism to control the amount of data that is transmitted at any given time. The sender maintains a window of frames that can be transmitted without waiting for an acknowledgment from the receiver. The size of the window depends on the capacity of the channel and the round-trip time (RTT) of the transmission. The receiver maintains a window of frames that it expects to receive. If a frame falls outside of the receiver's window, it is ignored and not acknowledged until the next frame within the window is received.
ARQ also uses a timeout mechanism to detect lost or corrupted packets. If the sender does not receive an acknowledgment message within a specified time, it assumes that the packet has been lost or corrupted and retransmits it. The timeout value is dynamically adjusted based on the current network conditions to optimize performance.
Conclusion:
ARQ is a reliable communication protocol used to ensure that data is transmitted accurately and efficiently. It is a feedback mechanism that detects errors and retransmits lost or corrupted data. ARQ has three main types: stop-and-wait ARQ, go-back-n ARQ, and selective repeat ARQ. Each type has its own advantages and disadvantages, and the choice of which type to use depends on the specific application and network conditions. ARQ uses a sequence number and sliding window mechanism to control the amount of data that is transmitted at any given time. It also uses a timeout mechanism to detect lost or corrupted packets. Overall, ARQ is an essential technique in modern communication protocols that ensures reliable data transmission.