Chapter 2: Data Transmission – IGCSE/GCE CS Notes (0478/2210)

Based on Cambridge Syllabus 2026-2028

What is Data Transmission?

Data transmission is the process of sending digital data from one device to another using a medium such as copper wire, fibre optic cable, or wireless signals.

Why Data is Broken into Packets

When data is transmitted over a network, it is broken down into smaller pieces called packets. This is done because:

Networks can only handle a certain amount of data at once
If errors occur, only the affected packet needs to be resent
Multiple packets can take different routes, making transmission more efficient
Large files would block the network for other users

Packet Structure

Each packet contains three main parts:

┌─────────────────────────────────────┐
│           PACKET HEADER             │
├─────────────────────────────────────┤
│          PAYLOAD (DATA)              │
├─────────────────────────────────────┤
│              TRAILER                 │
└─────────────────────────────────────┘

1. Packet Header

The header contains control information needed for delivery:

Component	Purpose
Destination Address	IP address of where the packet is going
Source Address	IP address of where the packet came from
Packet Number	The packet’s position in the sequence (e.g., packet 3 of 10)
Protocol Information	How the packet should be handled

2. Payload

The actual data being transmitted
Could be part of a file, email, image, etc.
Size varies depending on network protocol

3. Trailer

Marks the end of the packet
Often contains error checking information (like a checksum)
Helps verify the packet arrived correctly

Packet Switching

Packet switching is the method used to transmit packets across a network.

The Process of Packet Switching

Data is divided into packets at the source device
Each packet is labelled with:

Destination address
Source address
Packet number

Packets are sent into the network
Routers direct packets – Each router examines the destination address and decides the best path
Different routes possible – Packets from the same message may take different paths
Packets may arrive out of order – Due to different routes or network congestion
Packets are reordered at the destination using packet numbers
Message is reassembled – The original data is reconstructed

Visual Representation

Source Device
      │
      ▼
[Packet 1] ──────► Router A ──────► Router C ──────┐
[Packet 2] ──────► Router B ──────► Router D ──────► Destination
[Packet 3] ──────► Router A ──────► Router E ──────┘
      │                                            │
      └─────────── Different routes ───────────────┘

Packets arrive: [Packet 2] [Packet 1] [Packet 3]
Reordered:      [Packet 1] [Packet 2] [Packet 3]

Role of Routers

Routers are devices that direct packets through a network
They examine each packet’s destination address
They use routing tables to determine the best path
They can adapt to network congestion or failures

Advantages of Packet Switching

Efficient use of network resources
Fault tolerant – if one route fails, packets can use another
Allows multiple devices to share the network
Can prioritise different types of data

Methods of Data Transmission

1. Serial vs Parallel Transmission

Serial Transmission

How it works: Data is sent one bit at a time over a single wire
Use cases: USB, Ethernet, long-distance communication

Advantages	Disadvantages
Reliable over long distances	Slower than parallel for short distances
Less interference	Requires conversion at each end
Cheaper cabling
No synchronisation issues

Parallel Transmission

How it works: Multiple bits are sent simultaneously over multiple wires
Use cases: Internal computer buses (CPU to RAM), printer cables (older)

Advantages	Disadvantages
Faster over short distances	Signal skew over longer distances
Simple to implement	More expensive cabling
	More interference between wires
	Limited to short distances

2. Direction of Data Flow

Mode	Description	Example
Simplex	One-way data flow only	Keyboard to computer, TV broadcast
Half-Duplex	Two-way, but only one direction at a time	Walkie-talkies, CB radio
Full-Duplex	Simultaneous two-way data flow	Phone calls, Zoom meetings

Simplex

Data flows in only one direction
The receiving device cannot send data back
Simple and cheap to implement
Examples: TV/radio broadcasting, keyboard input

Half-Duplex

Both devices can transmit and receive
But only one can transmit at a time
Requires a turn-taking protocol
Examples: Walkie-talkies, some network protocols

Full-Duplex

Both devices can transmit simultaneously
Requires separate channels or sophisticated timing
Most efficient for interactive communication
Examples: Telephone calls, most modern network connections

Choosing the Right Method

When selecting a transmission method, consider:

Distance – Serial for long distances, parallel for short
Speed required – Parallel for high speed over short distances
Cost – Serial is cheaper for cabling
Direction needed – Simplex, half-duplex, or full-duplex
Interference – Serial is less susceptible to interference

USB Interface

What is USB?

Universal Serial Bus (USB) is an industry standard for connecting peripherals to computers. It uses serial transmission to communicate.

How USB Transmits Data

Connection: Device plugs into USB port
Enumeration: Computer identifies the device
Data transfer: Data is sent in packets using serial transmission
Power delivery: USB also provides power to devices

USB Features

Feature	Description
Hot-swappable	Devices can be connected/disconnected while powered on
Plug and Play	Automatic driver installation
Power delivery	Can charge devices (up to 100W with USB-C)
Daisy-chaining	Some versions allow multiple devices
Backward compatible	Newer versions work with older devices

Benefits of USB

Universal – One connector for many devices
Easy to use – No need to restart computer
Fast – High data transfer rates
Power – Charges devices while transferring data
Cost-effective – Inexpensive to manufacture

Drawbacks of USB

Cable length limits – Maximum 5 metres without powered hub
Version confusion – Different speeds (USB 2.0, 3.0, 3.1, etc.)
Power limitations – Some devices need external power
Single point of failure – If USB controller fails, all ports may fail

USB Versions

Version	Speed	Year
USB 1.1	12 Mbps	1998
USB 2.0	480 Mbps	2000
USB 3.0	5 Gbps	2008
USB 3.1	10 Gbps	2013
USB 3.2	20 Gbps	2017
USB4	40 Gbps	2019

Error Detection Methods

Why Errors Occur During Transmission

Errors can happen during data transmission due to:

Cause	Description
Interference	Electromagnetic interference from other devices
Noise	Electrical noise in the transmission medium
Signal degradation	Weakening of signal over long distances
Data loss	Some bits never arrive
Data gain	Extra bits are added
Data change	Bits are flipped (1 becomes 0 or vice versa)

The Need for Error Detection

Data integrity must be maintained
Corrupted data could cause incorrect results
Financial transactions must be accurate
Medical data must be precise
Communications must be reliable

1. Parity Check

How Parity Works

A parity bit is an extra bit added to a byte of data to make the total number of 1s either even or odd.

Even Parity

Count the number of 1s in the data
Add a parity bit to make the total number of 1s even

Odd Parity

Count the number of 1s in the data
Add a parity bit to make the total number of 1s odd

Example: Even Parity

Data: 1011001 (four 1s - already even)
Parity bit: 0
Transmitted: 0 1011001

Data: 1011011 (five 1s - odd)
Parity bit: 1 (to make it even)
Transmitted: 1 1011011

Parity Byte and Parity Block

For multiple bytes, a parity byte can be created by applying parity to each bit position across all bytes.

Limitations of Parity

Can only detect single-bit errors
Cannot detect which bit is wrong
If two bits flip, parity may appear correct

2. Checksum

How Checksum Works

All data bytes are added together
The sum is stored (often as a 16-bit value)
This checksum is transmitted with the data
Receiver recalculates the sum and compares

Example

Data: 45, 32, 78, 91
Sum = 45 + 32 + 78 + 91 = 246
Checksum = 246 (or a modified version)

Receiver recalculates: 45 + 32 + 78 + 91 = 246
If matches → data likely correct
If different → error detected

Advantages

Can detect multiple errors
Simple to implement
Used in TCP/IP protocols

3. Echo Check

How Echo Check Works

Sender transmits data to receiver
Receiver sends the data back to sender
Sender compares original with returned data
If they match, transmission was successful
If not, sender retransmits

Example

Sender sends: "HELLO"
Receiver echoes: "HELLO"
Sender compares: MATCH ✓

Sender sends: "WORLD"
Receiver echoes: "WORLD"
Sender compares: MATCH ✓

Advantages

Very simple to understand
Confirms both transmission and reception

Disadvantages

Doubles transmission time
Only detects errors on the forward path
Cannot detect errors on the return path

Check Digits

What is a Check Digit?

A check digit is an extra digit added to a number to verify its accuracy during data entry. It is calculated from the other digits using a specific algorithm.

How Check Digits Work

The original number is processed using a formula
A check digit is calculated
The check digit is added to the end
When entered, the formula is reapplied
If the calculated check digit matches, the number is likely correct

Common Uses of Check Digits

Application	Example
ISBN (International Standard Book Number)	978-0-13-110362-7
Barcodes	Product codes on items
Credit Cards	16-digit numbers
Bank Account Numbers	Sort codes and account numbers

ISBN-13 Check Digit Example

ISBN-13 uses a modulus 10 algorithm:

Multiply each digit alternately by 1 and 3
Add all results
Find the remainder when divided by 10
Subtract from 10 to get check digit

Example: 978-0-13-149505-?

Position	1	2	3	4	5	6	7	8	9	10	11	12
Digit	9	7	8	0	1	3	1	4	9	5	0	5
Weight	1	3	1	3	1	3	1	3	1	3	1	3
Result	9	21	8	0	1	9	1	12	9	15	0	15

Sum = 9 + 21 + 8 + 0 + 1 + 9 + 1 + 12 + 9 + 15 + 0 + 15 = 100
100 ÷ 10 = 10 remainder 0
Check digit = 0 (since remainder is 0)

Complete ISBN: 978-0-13-149505-0

Benefits of Check Digits

Detects common data entry errors:
Single digit errors (e.g., 1234 → 1235)
Transposition errors (e.g., 1234 → 1243)
Omitted digits
Extra digits
Simple to implement
Fast to verify

Automatic Repeat Query (ARQ)

What is ARQ?

Automatic Repeat Query (ARQ) is an error control method that automatically requests retransmission of data when errors are detected.

How ARQ Works

Sender transmits data with error detection information
Receiver checks for errors using parity, checksum, etc.
Receiver sends acknowledgement:

ACK (Positive Acknowledgement) if data is correct
NAK (Negative Acknowledgement) if errors detected

Sender acts on response:

If ACK received, sends next data
If NAK received, retransmits the data

Timeout mechanism: If no acknowledgement received within a set time, sender automatically retransmits

Visual Representation

Sender                    Receiver
  │                          │
  ├────── Data Packet ──────►│
  │                          │ (checks for errors)
  │                          │
  │◄────── ACK/NAK ──────────┤
  │                          │
  │ (if NAK or timeout)      │
  ├────── Retransmit ───────►│
  │                          │

Components of ARQ

Component	Purpose
ACK	Positive acknowledgement – “Data received correctly”
NAK	Negative acknowledgement – “Error detected, send again”
Timeout	If no response after set time, assume data lost and retransmit

Advantages of ARQ

Reliable – ensures data is received correctly
Adaptive – only retransmits when needed
Widely used – in TCP, email, file transfers

Disadvantages of ARQ

Requires two-way communication
Adds overhead (ACK/NAK packets)
Can be slow if many errors occur
Not suitable for real-time applications (streaming)

Encryption

Why Encryption is Needed

When data is transmitted over networks, it can be intercepted by unauthorised people. Encryption protects data by making it unreadable to anyone without the correct key.

What is Encryption?

Encryption is the process of converting readable data (plaintext) into an unreadable format (ciphertext) using an algorithm and a key.

Plaintext:  "Hello World"
      │
      ▼
[Encryption Algorithm + Key]
      │
      ▼
Ciphertext: "Uryyb Jbeyq"
      │
      ▼
[Decryption Algorithm + Key]
      │
      ▼
Plaintext:  "Hello World"

Types of Encryption

There are two main types of encryption: symmetric and asymmetric.

1. Symmetric Encryption

How It Works

The same key is used for both encryption and decryption
Both sender and receiver must have the same key
The key must be kept secret

Sender                          Receiver
  │                                │
  │   [Secret Key K]               │   [Secret Key K]
  │        │                        │        │
  │   Encrypt with K                │   Decrypt with K
  │        │                        │        │
  ├──────Ciphertext────────────────►│

Advantages

Faster than asymmetric encryption
Simpler to implement
Good for large amounts of data

Disadvantages

Key distribution problem – how to share the key securely?
If key is intercepted, all data is compromised
Many keys needed for many users

Examples

AES (Advanced Encryption Standard)
DES (Data Encryption Standard)
Blowfish

2. Asymmetric Encryption

How It Works

Uses two different keys: a public key and a private key
Public key can be shared with anyone
Private key is kept secret by the owner
Data encrypted with public key can only be decrypted with private key
Data encrypted with private key can only be decrypted with public key

Sender                          Receiver
  │                                │
  │   [Receiver's Public Key]      │   [Receiver's Private Key]
  │        │                        │        │
  │   Encrypt with Public Key       │   Decrypt with Private Key
  │        │                        │        │
  ├──────Ciphertext────────────────►│

Advantages

Solves key distribution problem
More secure for communication
Provides authentication (digital signatures)

Disadvantages

Much slower than symmetric encryption
More computationally intensive
Larger key sizes needed

Examples

RSA (Rivest-Shamir-Adleman)
ECC (Elliptic Curve Cryptography)
Diffie-Hellman key exchange

Comparison: Symmetric vs Asymmetric

Feature	Symmetric	Asymmetric
Keys	One shared key	Public + Private keys
Speed	Fast	Slow
Security	Good	Excellent
Key distribution	Problem	No problem
Use case	Bulk data encryption	Key exchange, digital signatures
Examples	AES, DES	RSA, ECC

Real-World Encryption

Most secure systems use both types:

Asymmetric encryption is used to securely exchange a session key
Symmetric encryption is used for the actual data transmission

This combines the security of asymmetric with the speed of symmetric.

Summary & Key Takeaways

Packet Switching

Data is broken into packets
Each packet has header, payload, trailer
Packets can take different routes
Routers direct packets
Packets are reordered at destination

Transmission Methods

Method	Description
Serial	One bit at a time – good for long distances
Parallel	Multiple bits at once – fast but short distance
Simplex	One-way only
Half-Duplex	Two-way, but one at a time
Full-Duplex	Two-way simultaneously

Error Detection

Method	How It Works
Parity Check	Adds bit to make number of 1s even/odd
Checksum	Adds sum of all data bytes
Echo Check	Receiver sends data back to sender
Check Digit	Extra digit calculated from data
ARQ	Automatic retransmission on error

Encryption

Type	Description
Symmetric	Same key for encrypt/decrypt
Asymmetric	Public and private keys

Practice Questions

Section A: Packets and Packet Switching

Explain why data is broken into packets before transmission.
Draw and label the structure of a data packet.
Describe three items found in a packet header.
Explain the role of routers in packet switching.
What happens if packets arrive at the destination out of order?

Section B: Transmission Methods

Compare serial and parallel transmission. Give one advantage and one disadvantage of each.
Explain the difference between simplex, half-duplex, and full-duplex transmission. Give an example of each.
A company needs to connect two buildings 500 metres apart. Would you recommend serial or parallel transmission? Justify your answer.
Why is full-duplex transmission preferred for telephone calls?

Section C: USB Interface

List three benefits of using USB for connecting peripherals.
Explain what “hot-swappable” means in the context of USB.
Give two drawbacks of USB.

Section D: Error Detection

Explain three different causes of errors during data transmission.
Using even parity, what parity bit would be added to the following data:
a) 1011001
b) 1101011
c) 0000000
Describe how a checksum is used to detect errors.
Explain the process of echo checking. What is its main disadvantage?

Section E: Check Digits

What is a check digit and why is it used?
Give three examples of where check digits are commonly used.
Explain what types of errors check digits can detect.

Section F: ARQ

Describe how Automatic Repeat Query (ARQ) works.
Explain the purpose of:
a) ACK
b) NAK
c) Timeout
Give one advantage and one disadvantage of ARQ.

Section G: Encryption

Why is encryption necessary when transmitting data?
Explain the difference between symmetric and asymmetric encryption.
In asymmetric encryption, what is the difference between a public key and a private key?
Give one example of when asymmetric encryption would be used.
Why do many secure systems use both symmetric and asymmetric encryption?

Quick Reference Card

Concept	Key Point
Packet	Header + Payload + Trailer
Packet switching	Different routes, reorder at end
Serial	One bit at a time, long distance
Parallel	Multiple bits, short distance
Simplex	One-way
Half-duplex	Two-way, one at a time
Full-duplex	Two-way, simultaneous
USB	Serial, hot-swappable, plug and play
Parity	Even/odd count of 1s
Checksum	Sum of data
Echo check	Send back for verification
Check digit	Extra calculated digit
ARQ	ACK, NAK, timeout
Symmetric	Same key
Asymmetric	Public + Private keys

End of Chapter 2: Data Transmission

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