Describe how ARQ is used to confirm data was received correctly

Data Transmission, Error Management and Related Networking Topics – Cambridge IGCSE/A‑Level (0478)

1. Number‑system refresher (Topic 1)

• Binary (base‑2) – digits 0 and 1. 1 byte = 8 bits.
• Hexadecimal (base‑16) – digits 0‑9 and A‑F; useful for representing binary groups of four bits.
• Two’s‑complement – method for representing signed integers; invert all bits and add 1 to get the negative.
• All error‑detecting codes (parity, checksum, CRC) operate on binary data, so a solid grasp of these systems is essential.

2. Data‑transmission fundamentals (Topic 2.1)

2.1 Transmission modes

Mode	Description	Typical use
Serial	Bits sent one after another over a single wire or pair.	USB, Ethernet, long‑distance links.
Parallel	Multiple bits transmitted simultaneously on separate wires.	Computer‑internal buses (e.g., PCIe).
Simplex	Data flows in only one direction.	Keyboard → PC, sensor → controller.
Half‑duplex	Both ends can transmit, but not at the same time.	Walkie‑talkies, early Ethernet (coax).
Full‑duplex	Simultaneous two‑way communication.	Telephone, modern Ethernet, USB 3.0.
USB (Universal Serial Bus)	Serial, full‑duplex, hot‑plugable interface for peripherals.	Keyboards, flash drives, printers.

2.2 Packet structure

A data packet (or frame) is divided into three logical parts:

• Header – control information such as source/destination addresses, protocol identifier, and sequence number.
• Payload (or data field) – the actual user data being transferred.
• Trailer – usually contains an error‑detecting code (e.g., CRC) and sometimes a frame‑end flag.

2.3 Packet switching

• Routers examine the header to decide the next hop.
• Packets are stored temporarily (store‑and‑forward) and then forwarded, allowing many devices to share a single physical link efficiently.
• Switches operate at the data‑link layer, forwarding frames based on MAC addresses.

2.4 IP addressing (required by the syllabus)

• IPv4 – 32‑bit address written as four decimal octets (e.g., 192.168.1.10). Supports unicast, broadcast, and multicast.
• IPv6 – 128‑bit address written in hexadecimal groups (e.g., 2001:0db8:85a3::8a2e:0370:7334). Provides a vastly larger address space and built‑in support for anycast.
• Both address families coexist on modern networks; devices may have a “dual‑stack” configuration.

2.5 Domain Name System (DNS)

DNS translates human‑readable domain names (e.g., www.example.com) into IP addresses. It is a hierarchical, distributed database that operates over UDP/TCP port 53.

3. Methods of error detection (Topic 2.2)

Technique	How it works	Typical use / example
Parity bit (odd / even)	Count the 1‑bits in the data; add one extra bit so the total number of 1‑bits is even (even parity) or odd (odd parity).	Simple serial links, early memory modules.
Checksum	Divide the data into fixed‑size blocks (commonly 16 bits). Add all blocks using one’s‑complement arithmetic; send the complement as the checksum.	IP header, UDP.
Echo‑check (Longitudinal Redundancy Check)	Sum all bytes of the message (often modulo 256) and transmit the sum as an extra byte.	Older modems, some serial standards.
Check‑digit	Calculate a single decimal digit from the other digits using a weighted sum (e.g., ISBN‑10, UPC).	Product codes, library books.
Automatic Repeat reQuest (ARQ)	Combine an error‑detecting code (usually a CRC) with a feedback channel. Receiver sends ACK for a correct frame or NAK / silence for an erroneous one; sender retransmits as needed.	Data‑link protocols (HDLC, PPP), TCP reliability.

4. Automatic Repeat reQuest (ARQ) – confirming correct reception

4.1 Objective

Ensure that every data frame arrives at the receiver without undetected errors; if an error is detected, the frame is automatically retransmitted.

4.2 Basic ARQ cycle

1. Sender transmits a frame together with an error‑detecting code (normally a CRC).
2. Receiver checks the CRC:
   • If the CRC matches → frame is error‑free → send an ACK (acknowledgement).
   • If the CRC does not match → frame is corrupted → send a NAK (negative acknowledgement) or remain silent (depending on the protocol).
3. Sender starts a timer after sending the frame.
   • ACK received before timeout → advance to the next frame.
   • NAK received or timer expires → retransmit the same frame.

4.3 Key concepts

• CRC (Cyclic Redundancy Check) – polynomial‑based code that detects most single‑bit and burst errors.
• ACK / NAK – control messages that confirm success or request a repeat.
• Timeout – the maximum waiting period for an ACK; set slightly longer than the round‑trip time.
• Sequence numbers – identify each frame so that the receiver can recognise duplicates.

4.4 Types of ARQ protocols

Protocol	Operation	Advantages	Disadvantages
Stop‑and‑Wait	Send one frame; wait for its ACK before sending the next.	Very simple; easy to implement.	Low utilisation on high‑delay links (the channel is idle while waiting).
Go‑Back‑N	Sender may have up to N unacknowledged frames in flight. On a NAK, all frames from the erroneous one onward are resent.	Better bandwidth utilisation than Stop‑and‑Wait.	May retransmit frames that were already received correctly.
Selective Repeat	Only the specific erroneous frames are resent; the receiver buffers out‑of‑order frames.	Highest efficiency; minimal unnecessary retransmissions.	More complex buffering and sequence‑number handling.

4.5 Probability of successful transmission

If a single frame is received correctly with probability p:

• Stop‑and‑Wait with at most one retransmission:
  $$P_{\text{success}} = p + (1-p)p = p\,(2-p)$$
• With up to k retransmissions:
  $$P_{\text{success}} = 1 - (1-p)^{k+1}$$

4.6 Worked example

Assume a frame has a 95 % chance of being error‑free (p = 0.95). Using Stop‑and‑Wait with one possible retry:

$$P_{\text{success}} = 0.95 \times (2 - 0.95) = 0.95 \times 1.05 = 0.9975$$

Thus 99.75 % of frames are finally correct after at most one repeat.

5. Encryption – symmetric vs. asymmetric (Topic 2.3)

• Symmetric encryption – the same secret key encrypts and decrypts data (e.g., AES, DES). Very fast, but key distribution must be done securely.
• Asymmetric encryption – a mathematically linked key pair: a public key (shared openly) and a private key (kept secret) (e.g., RSA, ECC). Enables secure key exchange and digital signatures.

HTTPS example: When you visit https://example.com, the server sends its public key. Your browser encrypts a temporary symmetric session key with that public key; the server decrypts it with its private key. The session key then encrypts the actual web traffic (fast symmetric encryption) while the initial exchange guarantees confidentiality and authenticity (asymmetric encryption).

6. Network hardware essentials (Topic 3.4)

• Network Interface Card (NIC) – connects a computer to a network; provides a unique MAC address (e.g., 00:1A:2B:3C:4D:5E).
• Router – forwards packets between different networks using the destination IP address; performs IP routing and may provide NAT.
• Switch – operates at the data‑link layer; forwards frames based on MAC addresses, creating separate collision domains for each port.
• Modem – converts digital signals to analogue (and vice‑versa) for transmission over telephone or cable lines.

7. The Internet and the World Wide Web (Topic 5.1)

7.1 URL anatomy

scheme://host[:port]/path?query#fragment

Example:

https://www.example.com:443/articles/ai.html?lang=en#section2

• Scheme – protocol (http, https, ftp).
• Host – domain name (resolved by DNS).
• Port – optional; defaults are 80 for HTTP, 443 for HTTPS.
• Path – location of the resource on the server.
• Query – name‑value pairs passed to the server.
• Fragment – client‑side reference to a part of the page.

7.2 HTTP vs. HTTPS

• HTTP – transfers data in clear text; vulnerable to eavesdropping and tampering.
• HTTPS – adds TLS/SSL encryption, providing confidentiality, integrity, and server authentication.

7.3 Cookies

• Session cookie – lives only while the browser is open; used for temporary state (e.g., shopping‑cart contents).
• Persistent cookie – stored on disk for a set period; used for “remember me” logins or tracking.

8. Cyber‑security – threats and counter‑measures (Topic 5.3)

• Threats
  • Phishing – deceptive emails/websites that steal credentials.
  • Malware – viruses, worms, ransomware that damage or hijack systems.
  • Denial‑of‑Service (DoS/DDoS) – overwhelming a service to make it unavailable.
  • Sniffing / Man‑in‑the‑Middle – intercepting network traffic.
  • Unauthorised access – weak passwords, lack of authentication.
• Counter‑measures
  • Firewalls – filter traffic based on rules (packet‑filtering, stateful).
  • Encryption – TLS/SSL for data in transit; AES for data at rest.
  • Authentication – passwords, two‑factor authentication (2FA), biometrics.
  • Anti‑virus / anti‑malware software – signature and heuristic scanning.
  • Regular updates & patches – close known vulnerabilities.

9. Digital currency & blockchain (Topic 5.2)

• Blockchain – a distributed ledger where each block contains a list of transactions and the cryptographic hash of the previous block, forming an immutable chain.
• Consensus mechanisms (e.g., Proof‑of‑Work) ensure that the majority of nodes agree on the next block.
• Example – Bitcoin: Alice sends 0.5 BTC to Bob. The transaction is broadcast, verified by miners, placed in a new block, and linked to the chain. Changing the transaction would require re‑computing all subsequent blocks, which is computationally infeasible.

10. Automated and emerging technologies (Topic 6)

• Automation & control systems – sensors gather data, controllers (e.g., PLCs) process it, and actuators perform actions (motors, valves).
• Robotics – combines mechanical components, sensors, and programmable control to perform tasks ranging from manufacturing to exploration.
• Artificial Intelligence (AI) – techniques such as machine learning enable systems to recognise patterns, make decisions, and improve performance over time.
• Internet of Things (IoT) – everyday objects equipped with network connectivity, sensors, and often cloud‑based analytics (e.g., smart thermostats, wearables).

11. Summary checklist for the Cambridge IGCSE 0478 syllabus

• Binary, hexadecimal and two’s‑complement basics.
• Transmission modes (serial/parallel, simplex/half‑/full‑duplex, USB).
• Packet structure (header, payload, trailer) and packet‑switching principles.
• IP addressing – IPv4 vs IPv6, address types, and the role of DNS.
• Five error‑detection techniques (parity, checksum, echo‑check, check‑digit, ARQ) with examples.
• ARQ operation, types (Stop‑and‑Wait, Go‑Back‑N, Selective Repeat) and key concepts (ACK/NAK, timeout, sequence numbers, probability of success).
• Symmetric vs. asymmetric encryption; HTTPS as a real‑world illustration.
• Network hardware – NIC, MAC address, router, switch, modem.
• URL anatomy, HTTP vs. HTTPS, DNS function, and cookie types.
• Common cyber‑security threats and appropriate counter‑measures.
• Fundamentals of digital currency and blockchain.
• Automation, robotics, AI, and IoT – basic definitions and examples.