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🌐 1. Packet Transmission

1(a) Why data is broken into packets​

When data is sent over a network (e.g. the internet), it is split into smaller chunks called packets.

✅ Reasons:​

  • Efficiency → smaller pieces are easier to send
  • Error handling → only corrupted packets are resent, not the whole file
  • Routing flexibility → packets can take different routes

📌 Example:​

Sending a video file:

  • Instead of sending one huge file, it is split into many packets
  • Each packet travels separately across the network

1(b) Structure of a Packet​

Each packet has three main parts:

1. Header​

Contains important routing information:

  • Destination address → where the packet is going
  • Originator’s address → sender’s address
  • Packet number → position in the sequence

2. Payload​

  • The actual data being transmitted

3. Trailer​

  • Contains error-checking data (e.g. checksum)
  • Used to detect corruption during transmission

📦 Packet Structure Summary:​

PartPurpose
HeaderRouting & identification
PayloadActual data
TrailerError detection

1(c) Packet Switching Process​

🔁 Steps:​

  1. Data is split into packets
  2. Packets are sent independently across the network
  3. Each packet may take a different route
  4. Routers decide the best path
  5. Packets may arrive:
    • Out of order
    • At different times
  6. Once all packets arrive:
    • They are reassembled in the correct order
    • Errors are checked and corrected if needed

📌 Key Concepts:​

  • Router → directs packets to destination
  • Packet switching → dynamic routing system

✅ Advantages:​

  • Efficient use of network
  • Fault tolerant (can reroute if path fails)
  • Faster overall transmission

❌ Disadvantages:​

  • Packets may be lost or delayed
  • Requires reordering at destination
  • Can cause latency

📝 Exam Tip:​

If asked “Explain packet switching”, include:

  • splitting into packets
  • different routes
  • routers
  • reordering at destination

🔄 2. Methods of Data Transmission

2(a) Types of Transmission​

1. Serial Transmission​

📖 Definition:​

Data is sent one bit at a time, in a single stream.

✅ Advantages:​

  • Less interference (crosstalk)
  • Reliable over long distances
  • Cheaper cables

❌ Disadvantages:​

  • Slower than parallel (per unit time)

📌 Example:​

  • USB cables

2. Parallel Transmission​

📖 Definition:​

Multiple bits are sent simultaneously along multiple wires.

✅ Advantages:​

  • Faster over short distances

❌ Disadvantages:​

  • Signal interference (crosstalk)
  • Expensive cables
  • Not suitable for long distances

📌 Example:​

  • Old printer connections

🆚 Serial vs Parallel​

FeatureSerialParallel
SpeedSlowerFaster (short distance)
DistanceLongShort
CostCheaperMore expensive
InterferenceLowHigh

3. Simplex Transmission​

📖 Definition:​

Data flows in one direction only.

📌 Example:​

  • Keyboard → Computer
  • TV broadcast

✅ Advantage:​

  • Simple and efficient for one-way communication

❌ Disadvantage:​

  • No feedback possible

4. Half-Duplex Transmission​

📖 Definition:​

Data flows in both directions, but not at the same time.

📌 Example:​

  • Walkie-talkies

✅ Advantages:​

  • Allows two-way communication
  • Uses single channel

❌ Disadvantages:​

  • Delays (must wait to send)

5. Full-Duplex Transmission​

📖 Definition:​

Data flows in both directions at the same time.

📌 Example:​

  • Phone calls
  • Video calls

✅ Advantages:​

  • Fast communication
  • No waiting

❌ Disadvantages:​

  • More complex
  • Higher cost

2(b) Choosing the Right Method​

📊 Scenario-Based Suitability:​

ScenarioBest MethodReason
Long-distance communicationSerialLess interference
Short-distance high speedParallelFaster transfer
TV broadcastSimplexOne-way only
Walkie-talkieHalf-duplexTwo-way but not simultaneous
Phone/video callFull-duplexSimultaneous communication

📝 Exam Tip:​

If asked “Suggest a suitable method”:

  • ALWAYS justify your answer
    (e.g. “Serial because it reduces interference over long distances”)

🔌 3. Universal Serial Bus (USB)

What is USB?​

USB (Universal Serial Bus) is a standard interface used to connect devices and transfer data.

How USB Transmits Data​

  • Uses serial transmission (one bit at a time)
  • Data is sent in packets
  • Devices communicate with a host controller

Key Features​

🔑 1. Plug and Play​

  • Devices are automatically detected

🔑 2. Hot Swapping​

  • Devices can be connected/removed without restarting

🔑 3. Provides Power​

  • Can charge devices (e.g. phones)

Advantages of USB​

  • Easy to use
  • No need to restart (hot swapping)
  • Universal standard
  • Provides power + data transfer
  • Reliable

Disadvantages of USB​

  • Limited cable length
  • Slower than some modern alternatives (e.g. Ethernet for networks)
  • Host-controlled (devices cannot communicate directly without host)

📌 Examples of USB Devices:​

  • Flash drives
  • Keyboards
  • Printers
  • External hard drives

📝 Exam Tip:​

If asked about USB:
Include:

  • serial transmission
  • plug and play
  • hot swapping
  • power + data

🔥 Common Exam Questions

1. Describe packet switching (4–6 marks)​

✔ Mention:

  • packets
  • routes
  • routers
  • reordering

2. Compare serial and parallel transmission​

✔ Include:

  • speed
  • distance
  • interference

3. Choose a transmission method for a scenario​

✔ Always:

  • Pick correct type
  • Justify clearly

🧠 Key Definitions (Memorise!)

  • Packet → A small unit of data sent across a network
  • Packet switching → Sending data in packets via different routes
  • Serial transmission → One bit at a time
  • Parallel transmission → Multiple bits at once
  • Simplex → One-way communication
  • Half-duplex → Two-way, not simultaneous
  • Full-duplex → Two-way, simultaneous
  • USB → Standard interface for data and power transfer

🚀 Final Revision Tips

  • Always use technical terms (router, packet, payload, etc.)
  • Include examples in longer answers
  • Practice comparison questions
  • Don’t forget advantages + disadvantages