Show understanding of why a protocol is essential for communication between computers

14.1 Protocols – Why They Are Essential for Computer Communication

1. What is a protocol?

A protocol is a **shared set of rules** that govern how data is formatted, transmitted and interpreted between two or more devices. Every protocol defines three fundamental aspects:

• Syntax – the structure of the message (headers, footers, delimiters, field lengths).
• Semantics – the meaning of each part of the message (commands, responses, error codes).
• Timing – the order of messages, flow‑control, timeout and retransmission requirements.

2. Why a protocol is essential for communication between computers

According to the Cambridge 9618 syllabus, a protocol enables reliable, interpretable data exchange by providing:

• Common language – all devices understand the same format and meaning.
• Synchronization – sender and receiver agree on the sequence and timing of messages.
• Error detection & recovery – checksums, acknowledgements and retransmission mechanisms protect data integrity.
• Interoperability – hardware and software from different manufacturers can communicate.
• Scalability – the rules allow many devices to join the network without conflict.
• Security (where required) – encryption, authentication and integrity checks protect confidentiality and trust.

3. Supporting concepts required by the syllabus

3.1 Information representation

Representation	Typical Use	Example
Binary (base‑2)	All low‑level data	1010 ₂ = 10 ₁₀
BCD (Binary‑Coded Decimal)	Decimal display in embedded systems	0101 0011 ₂ = 53₁₀
Hexadecimal (base‑16)	Memory addresses, colour codes	0x3F = 0011 1111₂
ASCII (7‑bit)	Basic English characters	‘A’ = 0100 0001₂
Unicode (UTF‑8)	World‑wide character set	‘€’ = 1110 0010 1000 0010 1010 1100₂

3.2 Network topologies & addressing

• Common LAN/WAN topologies: bus, star, ring, mesh, hybrid (diagram can be added in slides).
• IP addressing – IPv4 (32‑bit) vs IPv6 (128‑bit). Example IPv4 address = 192.168.1.10/24; IPv6 example = 2001:0db8:85a3::8a2e:0370:7334/64.
• Subnetting basics – a /24 network provides 256 addresses (254 usable).
• Domain Name System (DNS) – translates human‑readable domain names to IP addresses.

3.3 Security & data integrity (brief overview)

• Confidentiality – encryption (e.g., AES, TLS).
• Integrity – checksums, hash functions (MD5, SHA‑256), digital signatures.
• Authentication – passwords, certificates, two‑factor methods.
• Common threats – malware, phishing, denial‑of‑service, man‑in‑the‑middle.

4. Protocol stack – a layered view

Network communication is built as a **stack of protocols**, each layer adding a distinct service. The OSI model (7 layers) is used for teaching; the TCP/IP suite implements a simplified 4‑layer version that maps onto the OSI model.

OSI Layer	TCP/IP Layer	Primary Function	Typical Protocols
Application (7)	Application	Network‑wide services for end‑users	HTTP, HTTPS, SMTP, FTP, DNS
Presentation (6)	— (handled by application protocols)	Data representation, encryption, compression	—
Session (5)	— (handled by application protocols)	Dialogue control, session management	—
Transport (4)	Transport	End‑to‑end reliability and flow control	TCP, UDP
Network (3)	Internet	Routing of packets between networks	IP, ICMP, IPv6
Data‑link (2)	Network Access	Framing, MAC addressing, error detection on a link	Ethernet, PPP, Wi‑Fi (IEEE 802.11)
Physical (1)	Physical	Transmission of raw bits over a medium	Twisted‑pair, fibre, wireless, coaxial

Diagram suggestion: a vertical stack showing the five layers used in the Cambridge syllabus (Application, Transport, Network, Data‑link, Physical) with example protocols (HTTP, TCP, IP, Ethernet, Wi‑Fi) placed on the appropriate layers.

5. Step‑by‑step example – uploading a file to a web server

1. Application layer (HTTP) – the file is wrapped in an HTTP POST request.
2. Transport layer (TCP) – the request is split into segments; each segment gets a sequence number, checksum and acknowledgement rules.
3. Internet layer (IP) – each segment receives source and destination IP addresses; IP determines the route across networks.
4. Network‑access layer (Ethernet / Wi‑Fi) – frames are built with MAC addresses and a CRC for link‑level error detection.
5. Physical layer – the bits travel over the chosen medium (copper, fibre or radio).

At the receiver the process is reversed: each layer strips its own header, checks its own error‑control fields, and passes the payload upward.

5.1 Reliability of TCP (illustrative formula)

\[ P_{\text{success}} = 1 - \bigl(P_{\text{loss}}\bigr)^{\,n} \]

where \(P_{\text{loss}}\) is the probability that a single packet is lost and \(n\) is the maximum number of retransmission attempts permitted by TCP.

6. Interoperability, standards and scalability

• Standards bodies – ISO (OSI), IETF (TCP/IP), IEEE (Ethernet, Wi‑Fi) publish open specifications that all manufacturers can implement.
• Interoperability – a computer running Windows, a Mac, or a Linux server can all exchange data because they all implement the same protocol suite.
• Scalability – hierarchical addressing (IP subnetting) and routing protocols (e.g., OSPF, BGP) let the Internet grow to billions of devices without a single point of failure.

7. Summary

Protocols are the backbone of digital communication. They provide a **structured, reliable, secure and interoperable framework** that lets heterogeneous computers exchange information accurately and efficiently. By organising functionality into a layered stack, each protocol contributes a specific service, making networks manageable, scalable and adaptable to future technologies such as IPv6, cloud services and the Internet of Things.