Protocols and Standards

This lesson covers why network protocols are needed, the TCP/IP protocol stack, its four layers, and how it compares to the OSI model. Understanding protocols is essential for the OCR H446 specification section 1.3.

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What is a Protocol?

A protocol is a set of rules that governs how data is transmitted and received over a network. Protocols define the format, timing, sequencing, and error handling of data communication.

Why Are Protocols Needed?

Reason	Explanation
Standardisation	Devices from different manufacturers can communicate using agreed rules
Interoperability	Different operating systems and hardware can work together
Error handling	Protocols define how to detect and handle errors
Data formatting	Protocols specify how data should be structured (headers, payloads)
Routing	Protocols determine how data finds its way across networks
Security	Protocols can include encryption and authentication mechanisms

Without protocols, devices would not be able to understand each other's data, much like people speaking different languages.

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The TCP/IP Protocol Stack

The TCP/IP model (also called the Internet Protocol Suite) is the set of protocols used on the Internet and most modern networks. It has four layers, each responsible for a different aspect of communication.

The Four Layers

Layer	Name	Purpose	Key Protocols
4	Application	Provides network services to applications (user interaction)	HTTP, HTTPS, FTP, SMTP, POP3, IMAP, DNS, SSH, Telnet
3	Transport	Ensures reliable (or fast) delivery of data between applications	TCP, UDP
2	Internet	Addresses and routes packets across networks	IP (IPv4, IPv6), ICMP, ARP
1	Link (Network Interface)	Handles the physical transmission of data on the local network	Ethernet, WiFi (802.11), PPP

How Data Flows Through the Layers

Sending:

1. Application layer creates the data (e.g., an HTTP request).
2. Transport layer segments the data, adds port numbers (TCP/UDP header).
3. Internet layer adds source and destination IP addresses (IP header) — creating a packet.
4. Link layer adds source and destination MAC addresses (frame header/trailer) — creating a frame.
5. The frame is transmitted as bits over the physical medium.

Receiving: The process is reversed — each layer strips its header and passes the data up to the next layer.

Encapsulation

Each layer wraps (encapsulates) the data from the layer above with its own header (and sometimes trailer). This is called encapsulation.

Application layer:  [DATA]
Transport layer:    [TCP Header][DATA]
Internet layer:     [IP Header][TCP Header][DATA]
Link layer:         [Frame Header][IP Header][TCP Header][DATA][Frame Trailer]

At each layer, the data unit has a different name:

Layer	Data Unit Name
Application	Data / Message
Transport	Segment (TCP) / Datagram (UDP)
Internet	Packet
Link	Frame

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Key Application Layer Protocols