Selective Repeat ARQ

Selective Repeat ARQ (Automatic Repeat Request) is a protocol used in communication networks to provide reliable data transmission. It is a type of sliding window protocol that allows a sender to transmit multiple packets before receiving an acknowledgment (ACK) from the receiver.

The Selective Repeat ARQ protocol operates as follows:

1. The sender divides the data into packets and sends them one by one to the receiver.
2. The sender maintains a window of size N that indicates the maximum number of packets that can be sent without receiving an ACK from the receiver.
3. The receiver sends an ACK for each received packet. The ACK contains the sequence number of the next expected packet.
4. If the sender receives an ACK for a packet, it moves the window forward and sends the next packet. If the sender does not receive an ACK for a packet within a certain timeout period, it retransmits only that packet and waits for the ACK again.
5. If the receiver receives a packet with an out-of-order sequence number, it buffers the packet and sends an ACK for the last correctly received packet.
6. The sender can receive multiple ACKs for different packets within the window, indicating that the receiver has received those packets successfully.

The main advantage of Selective Repeat ARQ over Go-Back-N ARQ is that it can retransmit only the lost or damaged packets, rather than retransmitting the entire window of packets. This reduces the delay and bandwidth utilization in case of packet loss. However, Selective Repeat ARQ requires the receiver to have a larger buffer to store out-of-order packets.

Advantages of Selective Repeat ARQ:

1. Efficient use of bandwidth: It only retransmits the lost or damaged packets, which reduces the overall number of transmissions required.
2. Reduced delay: It reduces the delay in case of packet loss, as only the lost or damaged packets need to be retransmitted.
3. Efficient in a network with a high error rate: Selective Repeat ARQ is more efficient than Go-Back-N ARQ in a network with a high error rate, as it reduces the number of unnecessary retransmissions.
4. Receiver buffering: The receiver can buffer out-of-order packets to avoid discarding packets and requesting retransmission.

Disadvantages of Selective Repeat ARQ:

1. Complexity: Selective Repeat ARQ is more complex than Go-Back-N ARQ and requires additional hardware and software to implement.
2. Large buffer requirements: The receiver must maintain a larger buffer to store out-of-order packets, which can be a disadvantage in low-memory devices.

In summary, Selective Repeat ARQ is a more efficient and effective error control protocol than Go-Back-N ARQ in a network with a high error rate. However, it requires additional hardware and software and a larger buffer to store out-of-order packets.