The TCP/IP Model and Layers

This lesson covers the TCP/IP model (also known as the Internet Protocol Suite) and its four layers. This is part of OCR J277 Section 1.3.2. The TCP/IP model describes how data is structured, transmitted, and received across networks.

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What Is the TCP/IP Model?

The TCP/IP model is a conceptual framework that describes the layers of protocols used for internet communication. It divides the process of sending data across a network into four layers, each with specific responsibilities.

The model is called TCP/IP because TCP (Transmission Control Protocol) and IP (Internet Protocol) are the two most important protocols in the suite, although many other protocols operate within the model.

The following diagram shows the four layers of the TCP/IP model:

graph TD
    L4["Layer 4: Application\nHTTP, FTP, SMTP, DNS"]
    L3["Layer 3: Transport\nTCP, UDP"]
    L2["Layer 2: Internet\nIP, ICMP"]
    L1["Layer 1: Network Interface\nEthernet, WiFi"]
    L4 --> L3
    L3 --> L2
    L2 --> L1

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The Four Layers

Layer	Name	Protocols	Purpose
4	Application layer	HTTP, HTTPS, FTP, SMTP, IMAP, DNS	Provides network services directly to applications; handles user-facing protocols
3	Transport layer	TCP, UDP	Splits data into packets (segments); ensures reliable delivery and reassembly
2	Internet layer	IP, ICMP	Addresses packets with IP addresses; routes packets across networks
1	Network interface layer (Link layer)	Ethernet, WiFi (802.11)	Handles the physical transmission of data over the network medium (cables, wireless)

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Layer 4: Application Layer

The application layer is the topmost layer — it is the layer that the user interacts with (through applications like web browsers and email clients).

Protocol	Function at This Layer
HTTP/HTTPS	Requesting and displaying web pages
FTP	Transferring files
SMTP	Sending emails
IMAP/POP3	Retrieving emails
DNS	Resolving domain names to IP addresses

When you type a URL in your browser, the browser uses HTTP/HTTPS (application layer) to request the web page.

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Layer 3: Transport Layer

The transport layer manages the end-to-end delivery of data between two devices.

TCP (Transmission Control Protocol)

Feature	Detail
Reliable	Guarantees all data arrives correctly and in order
Connection-oriented	Establishes a connection before data transfer (three-way handshake)
Error checking	Detects lost or corrupted packets and requests retransmission
Sequencing	Numbers each packet so they can be reassembled in the correct order
Use cases	Web browsing, email, file transfer — where accuracy matters

UDP (User Datagram Protocol)

Feature	Detail
Unreliable	Does not guarantee delivery or order
Connectionless	No connection is established before sending
Fast	Less overhead than TCP, so data is transmitted faster
Use cases	Live streaming, online gaming, VoIP — where speed matters more than perfect accuracy

OCR Exam Tip: If asked to compare TCP and UDP: TCP is reliable but slower (used for web and email); UDP is faster but unreliable (used for streaming and gaming).

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Layer 2: Internet Layer

The internet layer handles addressing and routing data packets across networks.

Function	Description
IP addressing	Each packet is labelled with the source and destination IP addresses
Routing	Routers at this layer determine the best path for each packet to travel
Packet forwarding	Packets are forwarded from router to router until they reach the destination network

This layer is where the IP protocol operates. Each packet may take a different route to the destination — the transport layer (TCP) reassembles them.

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Layer 1: Network Interface Layer

The network interface layer (also called the link layer) handles the physical transmission of data over the actual network medium.

Function	Description
Physical addressing	Uses MAC addresses to identify devices on the local network
Data framing	Wraps data into frames for transmission over the physical medium
Physical transmission	Converts data into electrical signals (Ethernet), radio waves (WiFi), or light pulses (fibre optic)

This layer deals with the actual hardware: NICs, Ethernet cables, WiFi radios, and switches.

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How the Layers Work Together: Sending a Web Page Request

When you request a web page, data passes down through the layers on the sender's device and up through the layers on the receiver's:

Step	Layer	Action
1	Application	Browser creates an HTTP request for the web page
2	Transport	TCP breaks the request into packets and adds sequence numbers
3	Internet	IP adds source and destination IP addresses to each packet
4	Network Interface	Packet is converted into electrical/radio signals and transmitted
...	(Travel across network via routers and switches)	...
5	Network Interface	Receiving device receives the physical signals
6	Internet	IP reads the addresses and passes the packets up
7	Transport	TCP reassembles the packets in the correct order
8	Application	The web server processes the HTTP request and sends back the web page

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Why Use Layers?

Benefit	Explanation
Modularity	Each layer has a specific job; changes to one layer do not affect others
Standardisation	Protocols at each layer are standardised, allowing interoperability between different vendors
Troubleshooting	Problems can be isolated to a specific layer
Development	Engineers can develop and update one layer without redesigning the entire system

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Summary