Protocols · TCP/IP Model · Packet Structure · Routing
CSZoneCambridge IGCSE Computer Science 0478
What is a Protocol?
Rules for Communication
A protocol is a set of agreed rules that govern how data is transmitted between devices on a network. Without protocols, devices from different manufacturers couldn't communicate.
Protocols define: data format, transmission speed, error checking methods, and how connections are established and terminated
Common protocols: HTTP/HTTPS (web browsing), FTP (file transfer), SMTP (email sending), DNS (domain name lookup)
TCP/IP is the suite of protocols that underpins the entire internet — it defines how data is packaged, addressed, and routed
TCP/IP Model
Four Layers to Know
Layer 4 — Application: provides network services directly to the user (web, email, file transfer). Protocols: HTTP, HTTPS, FTP, SMTP, DNS
Layer 3 — Transport: breaks data into packets; reassembles on arrival; handles error checking and flow control. Protocols: TCP (reliable), UDP (fast, no guarantee)
Layer 2 — Internet: adds IP addresses for logical addressing; handles routing between networks. Protocol: IP
Layer 1 — Link (Network Access): physical transmission of bits over hardware (cables, Wi-Fi). Protocols: Ethernet, Wi-Fi
Encapsulation
Data Travels Down & Up the Layers
Sending (downward): each layer adds its own header (control information) to the data — called encapsulation. The Transport layer adds port numbers; the Internet layer adds IP addresses; the Link layer adds MAC addresses.
Receiving (upward): each layer strips off its header and passes the data up — called decapsulation. Each layer only handles its own header; it doesn't need to understand the others.
This layered approach means different manufacturers can build hardware/software that works together, as long as they follow the same layer protocols
Packet Switching
How Data Travels the Internet
Data is split into small, fixed-size units called packets. Each packet travels independently across the network — possibly taking different routes to the destination.
Routers read each packet's destination IP address and decide the best path to forward it — this process is called routing
Packets are reassembled in the correct order at the destination using sequence numbers in the packet header
If a packet is lost or corrupted, only that packet is re-sent — not the whole file. TCP handles this automatically
Advantages: network is resilient (packets reroute around failures), bandwidth is shared efficiently across many users simultaneously
Inside a Packet
Packet Structure
[ Header ] + [ Data (payload) ] + [ Trailer ]
Source IP address: where the packet came from
Destination IP address: where the packet is going — used by routers to forward it
Sequence number: packet's position in the original data — allows reassembly in correct order
Checksum: error-checking value — receiver recalculates and compares; mismatch means data was corrupted in transit
The payload (actual data) is typically much larger than the header — headers add a small overhead but are essential for reliable delivery
Exam Practice
Have a go at this question
Cambridge IGCSE 0478 style
Explain what is meant by packet switching and state two advantages of using packet switching to transmit data over the internet.
4 marks
Packet switching: data is broken into small units called packets [1]; each packet travels independently and may take different routes to the destination [1]. Advantage — any two from: if one route is busy or fails, packets can be rerouted automatically [1]; multiple users can share the same network simultaneously (efficient bandwidth use) [1]; if a packet is corrupted, only that packet needs to be re-sent [1].
Key Takeaways
What to Remember
A protocol is a set of rules for data transmission; TCP/IP is the internet's core protocol suite
TCP/IP has 4 layers: Application (HTTP/FTP), Transport (TCP/UDP), Internet (IP), Link (Ethernet/Wi-Fi)
Packets contain: source IP, destination IP, sequence number, checksum, and payload data
Packet switching: independent routing, resilient to failure, only lost packets re-sent, shared bandwidth