ARQ is a method used in data transmission to ensure reliable delivery of data over a network. It works by requiring the receiver to acknowledge (ACK) the successful receipt of data. If the sender doesn't receive an ACK within a certain timeframe, it assumes the data was lost or corrupted and retransmits it. This process continues until the data is successfully received.

There are different types of ACK methods:

• Stop-and-Wait ARQ: The sender waits for an ACK after transmitting each data packet. If an ACK isn't received, the sender stops transmission and retransmits the packet. This is simple but inefficient as the sender is idle waiting.
• Go-Back-N ARQ: The sender can transmit multiple packets without waiting for ACKs. If a packet is lost or corrupted, the receiver discards all subsequent packets until it receives a correct packet. The sender then retransmits those packets. This is faster than stop-and-wait but can be inefficient if many packets are lost.
• Selective Repeat ARQ: The receiver acknowledges correctly received packets and requests retransmission only for those packets that are lost or corrupted. This is the most efficient method as it avoids unnecessary retransmissions.

The ACK methods contribute to data reliability by providing a mechanism to detect and recover from data loss or corruption. By requiring confirmation of receipt, ARQ ensures that data is delivered accurately and completely.

Two common methods for checking errors after data transmission are parity checking and checksums.

Parity Checking: This method adds an extra bit (the parity bit) to a block of data. The parity bit is set to either 0 or 1 to make the total number of 1s in the data block either even (even parity) or odd (odd parity). The receiver checks the parity of the received data. If the parity is incorrect, it indicates an error has occurred.

Parity checking is most effective at detecting single-bit errors. It can often miss errors caused by multiple bits being corrupted.

Checksums: A checksum is a more sophisticated error detection method. It involves calculating a value (the checksum) from the data being transmitted. This checksum is sent along with the data. The receiver recalculates the checksum from the received data and compares it to the received checksum. If the checksums don't match, an error is detected.

Checksums are effective at detecting burst errors, which are errors that occur in consecutive data bits. They are also more reliable than parity checking for detecting multiple errors.

A sequence number is a numerical value assigned to each data packet sent over the network. It serves a crucial role in ARQ by allowing the receiver to identify and reorder packets that arrive out of sequence.

Here's how it works:

1. The sender assigns a unique sequence number to each packet.
2. The receiver uses the sequence numbers to reconstruct the original order of the packets.
3. If a packet is received out of order, the receiver can store it and request the missing packets from the sender.
4. The sender uses the sequence numbers to ensure that the packets are delivered in the correct order. If a packet is lost, the receiver can detect this by noting the gap in the sequence numbers and requesting a retransmission of the missing packet.

Without sequence numbers, the receiver would have no way of knowing the order in which packets were received, leading to data corruption and incorrect data reconstruction. Sequence numbers are therefore essential for reliable data transmission using ARQ.