TCP/IP Model · 4 Layers · Packet Switching · Encapsulation
CSZoneEdexcel GCSE Computer Science 1CP2
Why Use Layers?
Dividing the Problem
Layered models break network communication into independent layers. Each layer handles a specific task and only communicates with the layers directly above and below it. This means each layer can be developed or replaced independently — essential for interoperability between different manufacturers' equipment.
Edexcel uses the TCP/IP model with 4 layers: Application, Transport, Internet, Link
The TCP/IP Four-Layer Model
Each Layer, Its Job and Protocols
Application layer: user-facing protocols — HTTP, HTTPS, FTP, SMTP, DNS. Defines how data is formatted for the application.
Transport layer: TCP or UDP — breaks data into segments, handles error checking and sequencing.
Internet layer: IP — addresses packets and routes them across networks.
Link layer: physical transmission — Ethernet, Wi-Fi. Handles frames and MAC addresses.
Packet Switching
Breaking Data into Packets
Data is split into small packets. Each packet contains a header (source IP, destination IP, sequence number) and a payload (data). Packets may travel different routes and arrive out of order — TCP reassembles them at the destination.
Advantage: if one route fails, packets take another route — network is resilient
Encapsulation: each layer adds its own header to the packet as data travels down; each layer removes its header on the way up at the receiver
Exam Practice
Have a go at this question
Edexcel-style question
Explain one advantage of using a layered model for network protocols.
2 marks
Each layer can be developed and updated independently [1], so changes at one layer (e.g. swapping from copper to fibre at the link layer) do not affect protocols at other layers [1].
Key Takeaways
What to Remember
TCP/IP: 4 layers — Application, Transport, Internet, Link