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ARQ (Automatic retransmission request)

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ARQ (Automatic Repeat request) is a reliable data transmission protocol that enables the transmission of data in an error-prone communication channel by detecting and correcting errors that may occur during the transmission. The protocol is implemented at the transport layer of the Open Systems Interconnection (OSI) model, and its main function is to ensure that all transmitted data is received correctly at the receiving end. This protocol is often used in communication systems that use wireless communication, satellite communication, or any other communication medium that is susceptible to transmission errors.

ARQ works by transmitting data in a series of packets. Each packet contains a sequence number that identifies it uniquely. When the receiving end receives a packet, it checks the packet for errors. If the packet is error-free, the receiving end sends an acknowledgement (ACK) to the transmitting end. If the packet contains errors, the receiving end sends a negative acknowledgement (NACK) to the transmitting end, indicating that the packet needs to be retransmitted.

There are three main types of ARQ protocols:

  1. Stop-and-Wait ARQ: In this protocol, the transmitting end sends a packet and waits for an acknowledgement from the receiving end before sending the next packet. The receiving end sends an ACK or NACK as appropriate. If the transmitting end receives a NACK, it retransmits the packet.
  2. Go-Back-N ARQ: In this protocol, the transmitting end sends multiple packets before receiving an acknowledgement from the receiving end. The receiving end sends an ACK or NACK as appropriate. If the receiving end sends a NACK, the transmitting end retransmits all the packets from the point where the error occurred.
  3. Selective Repeat ARQ: In this protocol, the transmitting end sends multiple packets before receiving an acknowledgement from the receiving end. The receiving end sends an ACK or NACK as appropriate. If the receiving end sends a NACK, the transmitting end retransmits only the packets that were received in error.

Each of these protocols has its advantages and disadvantages. Stop-and-Wait ARQ is simple to implement, but it has a low throughput because the transmitting end must wait for an acknowledgement before sending the next packet. Go-Back-N ARQ has a higher throughput because it allows the transmitting end to send multiple packets before receiving an acknowledgement. However, if an error occurs, it requires the retransmission of all the packets sent since the error occurred, which can be inefficient. Selective Repeat ARQ has a higher throughput than Stop-and-Wait ARQ and is more efficient than Go-Back-N ARQ because it only requires the retransmission of the packets that were received in error.

ARQ protocols can also be classified based on the type of error correction mechanism used. There are two types of ARQ protocols:

  1. Forward Error Correction (FEC) ARQ: In this protocol, the transmitting end adds extra bits to the data before transmitting it. The extra bits are used to correct errors that may occur during transmission. The receiving end uses these extra bits to correct errors in the received data. If the errors cannot be corrected, the receiving end sends a NACK to the transmitting end, indicating that the packet needs to be retransmitted.
  2. Retransmission ARQ: In this protocol, the transmitting end sends the data packets without any additional error correction bits. If an error occurs during transmission, the receiving end sends a NACK to the transmitting end, indicating that the packet needs to be retransmitted.

The choice of ARQ protocol and error correction mechanism depends on the specific communication channel being used and the requirements of the communication system. For example, a communication system that requires low latency may prefer to use Stop-and-Wait ARQ because it has a lower delay than Go-Back-N ARQ or Selective Repeat ARQ. On the other hand, a communication system that requires high throughput may prefer to use Go-Back-N ARQ or Selective Repeat ARQ because they allow the transmission of multiple packets before an acknowledgement is received.

ARQ protocols are designed to be robust against transmission errors, but they can also introduce additional delays and overhead to the communication system. For example, if a packet is lost during transmission, the receiving end must wait for the packet to be retransmitted before it can continue processing the data. This can introduce delays in the communication system, which can be unacceptable in certain applications. Additionally, the process of retransmitting packets can introduce additional overhead in the communication system, which can reduce the overall throughput.

To address these issues, ARQ protocols are often used in conjunction with other techniques such as interleaving, error correction coding, and congestion control. Interleaving is a technique that involves rearranging the data packets before transmission so that if some packets are lost, the remaining packets can still be received in the correct order. Error correction coding is a technique that involves adding redundant bits to the data packets to enable the receiving end to detect and correct errors in the received data. Congestion control is a technique that regulates the flow of data in the communication system to prevent congestion and ensure that the system operates at optimal performance.

In conclusion, ARQ is a reliable data transmission protocol that enables the transmission of data in an error-prone communication channel by detecting and correcting errors that may occur during the transmission. The protocol is implemented at the transport layer of the OSI model and can be classified based on the type of ARQ protocol used and the type of error correction mechanism used. The choice of ARQ protocol and error correction mechanism depends on the specific communication channel being used and the requirements of the communication system. ARQ protocols can be used in conjunction with other techniques such as interleaving, error correction coding, and congestion control to improve the performance of the communication system.