Layers

The four-layer TCP/IP model, encapsulation, and why layers are used.

F10.8NetworksGCSE15 min

Covers OCR J277 1.3.2 Wired and wireless networks, protocols and layers · AQA 8525 3.5 Fundamentals of computer networks · Edexcel 1CP2 4.1.7 The 4-layer TCP/IP model

Do this lesson in the simulator

Print it: worksheet (PDF) · answers (PDF) · as a web page

Networking is a big problem, so it is split into layers. Each layer does one job, uses the layer below it, and provides a service to the layer above. The web browser does not need to know whether your data goes by Wi-Fi or cable; the Wi-Fi does not need to know it is carrying a web page. This lesson describes the TCP/IP model and shows data being wrapped and unwrapped as it goes down and up the layers.

The TCP/IP model

Layer What it does Protocols
4. Application the programs people use, and the rules for their data HTTP, HTTPS, FTP, SMTP, POP, IMAP
3. Transport splits data into packets, numbers them, checks they arrive, and deals with the port for each application TCP, UDP
2. Internet adds the source and destination IP addresses, and routes packets between networks IP
1. Link sends the data across the physical network, using MAC addresses Ethernet, Wi-Fi

Down one side, up the other

When you send data, it goes down the layers. Each layer adds its own header to the front, a label for the matching layer at the other end. This wrapping is called encapsulation. At the receiving device, the data goes up the layers, and each one reads and removes its header.

The data GET /battery goes down the sender's application, transport, internet and link layers, gaining a TCP, then an IP, then a Wi-Fi header in front; it crosses the network and goes up the receiver's layers, each removing its own headersenderadds a headerreceiverremoves its headerwhat passes between the layersapplicationapplicationGET /batterytransporttransportTCPGET /batteryinternetinternetIPTCPGET /batterylinklinkWIFIIPTCPGET /batteryacross the network, by Wi-Fi
Going down, each layer puts its own header in front (green is the one just added), so the link layer sends the most. Going up, each layer reads the header meant for it and takes it off, until the application gets GET /battery back exactly as it was sent.
# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

data = "GET /battery"
layers = ["TCP 80", "IP 10.0.0.2>10.0.0.9", "WIFI 3C-02>3C-09"]
for header in layers:
    data = header + " | " + data          # each layer adds its header in front
    print(data)
print()
for header in reversed(layers):
    data = data.split(" | ", 1)[1]        # each layer takes its own header off
    print(data)

Run this in the simulator

Why layers?

  • Each layer can be designed, built and tested on its own: a smaller problem.
  • A layer can be changed without changing the others. Wi-Fi 6 replaced older Wi-Fi without any change to web browsers.
  • Different companies can each make one layer, because the rules between the layers are standards.
  • It is easier to find a fault: the problem is in one layer.

Task: wrap and unwrap

Write wrap(data, headers) and unwrap(frame). wrap adds each header in headers, in order, in front of the data with | between, printing <layer>: <data so far> after each (starting with application: GET /battery before any header). Send the finished frame by radio. Then unwrap removes the three headers, and you print received by application: <data>.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

data = "GET /battery"
names = ["transport", "internet", "link"]
headers = ["TCP 80", "IP 10.0.0.2>10.0.0.9", "WIFI 3C-02>3C-09"]

Challenges

  1. Change the link layer header to ETH 3C-02>3C-09. Which other lines had to change?
  2. A router reads the internet layer header but not the application data. Print just the header a router needs.
  3. Match each device to the highest layer it reads: switch, router, web browser.