Automatic Repeat Request (ARQ) is a protocol used alongside error detection methods. (a) Describe how ARQ works in data transmission. [3] (b) Explain the ro...

(a) How ARQ works in data transmission: [3]

The sender transmits a block of data along with error detection information (such as a checksum or parity bits). [1] The receiver checks the received data for errors using the same error detection method. [1] If no error is found, the receiver sends an ACK (acknowledgement). If an error is detected, the receiver sends a NAK (negative acknowledgement), and the sender retransmits the data block. [1]

(b) Role of ACK and NAK: [2]

ACK (acknowledgement): Sent by the receiver to confirm that the data was received correctly. This signals the sender to proceed with transmitting the next data block. [1]

NAK (negative acknowledgement): Sent by the receiver when an error is detected in the received data. This tells the sender to retransmit the same data block. [1]

(c) What happens when no response is received within the timeout period: [2]

The sender assumes that either the data or the response (ACK/NAK) was lost during transmission. [1] The sender automatically retransmits the data block. This timeout mechanism prevents the system from waiting indefinitely for a response that will never arrive. [1]

(d) Error detection method commonly used with ARQ: [1]

CRC (Cyclic Redundancy Check), checksum, or parity check. CRC is the most robust option for detecting transmission errors.

(e) Difference between error detection and error correction: [2]

Error detection identifies that an error has occurred in the data but cannot fix it. The corrupted data must be retransmitted. Example: a parity check detects that a bit has changed but cannot determine which bit is wrong. [1]

Error correction not only detects an error but also determines the correct data and fixes it without requiring retransmission. Example: Hamming code uses multiple parity bits positioned at specific locations to pinpoint and correct single-bit errors. [1]

(f) Packet switching vs circuit switching: [4]

Advantages of packet switching:

• More efficient use of bandwidth because network links are shared by multiple communications simultaneously, rather than being dedicated to a single connection. [1]
• More resilient: if one route becomes unavailable, packets can be rerouted via alternative paths, so the communication continues. [1]

Disadvantages of packet switching:

• Packets may arrive out of order because they take different routes, requiring reassembly at the destination, which adds processing overhead. [1]
• Variable delays (latency jitter) occur as packets travel different paths, making packet switching less suitable for real-time applications like voice or video calls. [1]

(g) Network protocol layers: [3]

(i) Application layer: Provides the interface for applications to access network services, handling protocols such as HTTP (web), FTP (file transfer), and SMTP (email). [1]

(ii) Transport layer: Ensures reliable delivery of data between source and destination, handling segmentation of data into packets, flow control, and error recovery. [1]

(iii) Network layer: Handles routing of data packets across networks, determining the best path from source to destination using IP addresses. [1]

(h) Network device roles: [3]

(i) Router: Forwards data packets between different networks, using routing tables to determine the best path for each packet to reach its destination network. [1]

(ii) Switch: Connects devices within the same local area network (LAN) and forwards data frames to the correct device based on its MAC address, rather than broadcasting to all devices. [1]

(iii) Firewall: Monitors incoming and outgoing network traffic and blocks or allows data packets based on predefined security rules, protecting the network from unauthorised access and threats. [1]