Ch 1 · 2.1 Types and Methods of Data Transmission · IGCSE Computer Science 0478
Topic 1 of 3 in Data Transmission — IGCSE Computer Science 0478 — 49 lessons.
Introduction
When you send a photo to a friend, it does not travel as one piece. It is chopped into small packets that may take different routes and are put back together at the other end. This lesson covers how data is packaged and the different ways bits travel along a connection. (Syllabus section 2.1.)
Key ideas — Packets
Data is broken down into packets to be transmitted. Each packet has three parts:
🧑🏫 Teacher's explanation
Imagine you are sending a massive 500-page encyclopedia to a friend via the postal service, but the post office only accepts small envelopes. You cannot fit the whole book in one envelope, so you tear out each page, put them in separate envelopes, and number them (Page 1, Page 2, etc.). Each envelope needs a label showing where it is going and where it came from. This is exactly how the internet works.
📖 Definitions
🔍 Worked example — Easy
A file is broken into 4 packets. Identify the three components that must be present in every packet to ensure successful reassembly at the destination.
🔍 Worked example — Standard
A computer is sending a file. The packets arrive in the following order: Packet 3, Packet 1, Packet 4, Packet 2. Explain how the receiving computer handles this.
🔍 Worked example — Exam-style
A network router receives a packet. It must decide the route. List the specific pieces of information in the packet header that the router uses to make this decision.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the three main components of a packet. [3] (b) Explain why the data is broken into packets rather than being sent as one single continuous stream. [2] [5 marks]
Key ideas — Packet switching
Benefits: packets can avoid busy or broken links; no single dedicated path is needed, so the network is shared efficiently.
🧑🏫 Teacher's explanation
Imagine you are sending a massive, 500-page encyclopedia to a friend via the postal service. If you tried to send the whole book in one giant box, it would be too heavy to carry and if the truck crashed, you'd lose everything. Instead, you rip the pages out, put them in individual envelopes, and mail them separately.
📖 Definitions
🔍 Worked example — Easy
A file is broken into 4 packets. Each packet contains a header, a payload, and a trailer. If the payload is 100 bytes, the header is 20 bytes, and the trailer is 10 bytes, calculate the total size of the transmitted data in bytes.
🔍 Worked example — Standard
A data stream is split into packets with the following sequence numbers: 1, 2, 4, 5, 3. Identify the missing packet number and state what must happen for the data to be reconstructed correctly.
🔍 Worked example — Exam-style
A network administrator observes that packets are taking different routes to reach a destination. Explain why this is an advantage of packet switching compared to circuit switching (dedicated paths).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the three components that make up a single packet. [3] (b) Explain why packets might arrive at the destination in a different order than they were sent. [2] [5 marks]
Key ideas — Methods of transmission
Choosing a method for a scenario: long-distance links (e.g. between buildings) use serial; very short, fast internal links may use parallel; anything that needs two-way conversation at once needs full-duplex.
🧑🏫 Teacher's explanation
Imagine you are sending a massive LEGO castle to a friend through the mail. You can't fit the whole castle in one box, so you break it down into smaller, manageable boxes called 'packets'. Each box has a label (the header) telling your friend where it's going and which piece comes next. Because some roads might be blocked, some boxes might take different routes and arrive out of order, but once they all arrive, your friend uses the labels to rebuild the castle perfectly.
📖 Definitions
🔍 Worked example — Easy
A data packet is being prepared for transmission. The payload is 48 bytes. The header is 12 bytes and the trailer is 4 bytes. Calculate the total size of the packet in bits.
🔍 Worked example — Standard
A network administrator must choose between serial and parallel transmission for a connection between two buildings 500 meters apart. Which method is most suitable and why?
🔍 Worked example — Exam-style
A system uses packet switching to send a file. The file is split into 5 packets. Each packet header contains a destination address, an originator address, and a packet number. If the packets arrive in the order: P_3, P_1, P_5, P_2, P_4, explain how the receiving device reconstructs the file.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the three components that make up a data packet. [3] (b) Explain why the data is broken into packets rather than sent as one continuous stream. [2] [5 marks]
Key ideas — USB (universal serial bus)
USB is a standard interface for connecting devices (keyboards, printers, phones, flash drives) to a computer. It transmits data using serial transmission.
🧑🏫 Teacher's explanation
Imagine you are sending a massive LEGO castle to a friend through the mail. You cannot fit the whole castle in one box, so you break it down into several smaller boxes (packets). Each box has a label (header) saying where it's going and which piece of the castle it contains. Once all boxes arrive, your friend uses those labels to put the castle back together.
📖 Definitions
🔍 Worked example — Easy
A computer uses USB to connect a printer. List three distinct benefits of using the USB interface instead of older, specialized ports.
🔍 Worked example — Standard
Compare serial and parallel transmission in the context of a USB connection.
🔍 Worked example — Exam-style
A user is experiencing issues with a USB hard drive connected via a 15-metre cable. Explain why this is a likely problem based on USB characteristics.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one benefit of using the USB interface for these devices. [1] (b) Explain why the computer is able to communicate with the mouse once it has been plugged in. [2] [3 marks]
Worked example
A company wants to connect two buildings 300 m apart, and staff will use video calls. Which transmission methods are most suitable? Justify your answer.
🔍 Worked example — Easy
A single packet is being sent across a network. Identify the three essential components that must be included in the packet structure.
🔍 Worked example — Standard
Compare serial and parallel transmission for a long-distance cable connection.
🔍 Worked example — Exam-style
A network administrator is setting up a VoIP (Voice over IP) system for a call center. The system requires real-time, two-way audio. Which transmission mode is required and why?
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Header, payload, trailer; destination address, packet number, originator's address. (2) The file is split into packets; each packet is sent separately and may take a different route chosen by routers; they may arrive out of order; when the last arrives, they are reordered by packet number. (3) Bits are sent one after another on one wire, so they cannot get out of step (no skewing), and it needs fewer wires, so it is cheaper. (4) Simplex: computer to printer / TV broadcast; half-duplex: walkie-talkie; full-duplex: phone call. (5) Any two: standard connection for many devices; devices detected automatically; supplies power; hard to connect incorrectly.