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.

It is crucial to check for errors after data transmission for several reasons, even with seemingly reliable transmission mediums. While mediums like fibre optic cables have low error rates, errors can still occur due to various factors:

• Interference: Electromagnetic interference (EMI) can disrupt the signal during transmission, leading to bit flips.
• Cosmic Rays: High-energy particles from space can occasionally cause errors in digital signals.
• Hardware Failures: Components within the transmitting or receiving devices can malfunction, introducing errors.
• Software Bugs: Errors in the software controlling the transmission process can lead to incorrect data being sent or received.
• Environmental Factors: Temperature fluctuations or other environmental conditions can affect the reliability of the transmission.

The consequences of undetected errors can be significant. For example:

• Data Corruption: Errors can alter the meaning of data, leading to incorrect calculations or decisions.
• System Instability: Corrupted data can cause software or hardware to malfunction, potentially leading to system crashes.
• Financial Losses: In financial transactions, even small errors can have serious financial consequences.
• Safety Issues: In critical systems like aircraft control or medical devices, errors can have life-threatening consequences.

Error checking mechanisms provide a way to detect and potentially correct these errors, ensuring the integrity and reliability of the transmitted data.

The parity check method uses either even or odd parity. We'll assume even parity for this example. With even parity, the parity bit is set so that the total number of 1s in the data word (including the parity bit) is even.

Calculation of the Parity Bit:

1. Count the number of 1s in the data word.
2. If the number of 1s is even, the parity bit is 0.
3. If the number of 1s is odd, the parity bit is 1.

Data Word 1: 1011

1. Number of 1s in 1011: 3
2. Since 3 is odd, the parity bit is 1.
3. Therefore, the transmitted data word with parity is 10111.

Data Word 2: 0101

1. Number of 1s in 0101: 2
2. Since 2 is even, the parity bit is 0.
3. Therefore, the transmitted data word with parity is 01010.