Computer Science (0478) | Types and methods of data transmission

Welcome to the Data Transmission Toolkit!

Hi everyone! This chapter is all about how information—from your latest streaming video to a simple text message—moves from one place to another.

Imagine trying to send a huge library of books across the world. You wouldn't send them one by one, and you'd need to make sure they all arrived correctly! Data transmission is similar: we break down massive amounts of digital information, send it efficiently, check for mistakes, and make sure it stays secret.

Don't worry if some terms look new; we will break them down with simple, relatable examples. Let's get started!

Section 1: Breaking Data Down – Packets and Packet Switching (Syllabus 2.1)

1.1 Why We Use Packets

When you send a large file (like a photo or a document), the computer doesn't send it as one enormous blob. It breaks the data down into small, manageable chunks called packets.

Analogy: Think of a packet like a small, standardized shipping box containing a piece of your overall message.

Structure of a Data Packet

Every packet needs specific information so it knows where it’s going and how to be put back together. A packet contains three main parts:

• Packet Header: This is like the address label on the box. It holds routing information.
• Payload: This is the actual data—the useful part of the message.
• Trailer: This is usually placed at the end and is used for error checking (we’ll cover this in Section 3!).

The Packet Header includes:

• Destination Address: Where the packet is going (IP address).
• Originator's Address: Where the packet came from.
• Packet Number: The sequence number (e.g., 1 of 50, 2 of 50, etc.). This is vital for reordering later!

1.2 Packet Switching: The Process

The system used to send these individual packets across a network (like the Internet) is called packet switching.

Step-by-Step Packet Switching

1. The original data is broken down into packets.
2. Each packet is given the addresses and sequence number (in the header).
3. Each packet is sent out onto the network. Crucially, each packet can take a different route to the destination.
4. Routers control the route each packet takes, choosing the fastest or least congested path at that moment.
5. Because they take different routes, the packets may arrive out of order.
6. Once the last packet has arrived, the destination device uses the packet numbers (from the header) to reorder the packets back into the original message.

Why is this good? If one route is busy or broken, only a few packets are affected, not the whole transmission. It makes the network efficient and reliable.

Section 2: How Data is Sent – Types of Transmission (Syllabus 2.1)

Data can be transmitted using several different physical and logical methods. We need to know the suitability, advantages, and disadvantages of each.

2.1 Serial vs. Parallel Data Transmission

Parallel Data Transmission

In parallel transmission, multiple bits are sent simultaneously over separate wires or channels.

Analogy: A highway with eight lanes, where 8 cars (bits) can travel side-by-side at the same time.

Advantages:

• Very fast over short distances.

Disadvantages:

• The wires must be perfectly synchronised. Over longer distances, data skew occurs (bits arrive at slightly different times), causing errors.
• More expensive due to needing multiple wires.

Suitability: Used for very short distances, like internally within a computer (e.g., between the CPU and internal components).

Serial Data Transmission

In serial transmission, bits are sent one after the other (sequentially) down a single wire or channel.

Analogy: A single-lane road where cars (bits) must line up one behind the other.

Advantages:

• Reliable over long distances because there is no data skew.
• Cheaper and simpler (fewer wires needed).

Disadvantages:

• Slower than parallel over short distances.

Suitability: Used for long distances and connecting external devices (e.g., USB, Ethernet, connecting a PC to a printer).

Common Mistake Alert! Even though serial transmission sends data one bit at a time, modern serial communication (like USB) can achieve very high speeds because the data rate is so fast. For IGCSE, remember the primary benefit of serial is reliability over distance.

2.2 Directional Transmission Modes

This describes the direction data can flow between two devices.

Simplex

Data flows in one direction only.
Example: Broadcasting radio or TV, or data sent from a computer to a traditional printer.

Half-Duplex

Data can flow in both directions, but not at the same time. You must wait for the other device to finish transmitting before you can reply.
Example: A walkie-talkie. Only one person can talk at once.

Full-Duplex

Data can flow in both directions simultaneously.
Example: A telephone call or broadband internet connection, where both sides can talk/send data at the same time.

2.3 The Universal Serial Bus (USB) Interface

The Universal Serial Bus (USB) is a standard interface used to connect peripheral devices (like mice, keyboards, and hard drives) to a computer.

As the name suggests, USB uses serial data transmission.

Benefits of the USB Interface:

• It is an international standard, making it compatible with many different devices and manufacturers.
• New devices are automatically detected (Plug and Play).
• It provides power to devices (so small devices don't need a separate power source).
• Connectors are small and robust.

Drawbacks of the USB Interface:

• The maximum cable length is short (usually around 5 meters) before signal boosting is needed.
• Data transfer rates can be limited by the version of USB used.
• It uses a master-slave system; the computer (master) manages the connection, which can sometimes limit simultaneous connections.

Section 3: Checking for Errors (Syllabus 2.2)

When data travels, especially over long distances or noisy channels (like Wi-Fi), it can be corrupted due to interference. This can lead to data loss (bits disappear), data gain (extra bits appear), or data change (bits flip, e.g., 1 becomes 0).

We must use methods to detect these errors after transmission.

3.1 Error Detection Methods

Parity Check (Parity Byte and Parity Block)

A simple error detection method where an extra bit, called the parity bit, is added to a byte of data.

The system agrees beforehand whether to use Even Parity or Odd Parity.

• Even Parity: The total number of 1s in the byte (including the parity bit) must be an even number.
• Odd Parity: The total number of 1s in the byte (including the parity bit) must be an odd number.

Example: If the data is 10110010 (four 1s), and we use EVEN parity:
We add a 0 as the parity bit (10110010 0) to keep the total count of 1s (four) even.

Limitation: Parity checking can only detect errors where an odd number of bits have been flipped. If two bits flip (1 becomes 0, and 0 becomes 1), the parity remains correct, and the error goes undetected.

Parity Block/Parity Byte:
To overcome the limitation above, we can apply parity checking across a block of data (a grid of bytes) using a Parity Byte.

• Parity bits are calculated for each row (checking row integrity).
• An extra row (the parity byte) is calculated for each column (checking column integrity).
• If an error occurs, the system can usually pinpoint the exact bit using the intersection of the failed row and column check.

Checksum

The checksum is calculated by performing a mathematical sum on all the data bytes in a block of data.

Process:

1. The sender calculates the Checksum (the sum of the data).
2. The checksum is sent alongside the data block.
3. The receiver recalculates the checksum based on the received data.
4. The receiver compares their calculated checksum with the received checksum.
5. If they match, the data is likely correct. If they don't match, an error occurred.

Did you know? Checksum is highly effective because it makes it very unlikely that random data errors would result in the exact same sum.

Echo Check

This is a very simple method where the receiving device sends the received data back (echoes it) to the sender.

The sender then compares the echoed data with the original data sent. If they match, the transmission was successful.

Disadvantage: If the communication line is noisy, the error might occur on the way to the receiver *and* on the way back (the echo), meaning the echoed data might be accidentally "corrected" back to the original, leading to a false sense of security.

3.2 Check Digits (Used for Data Entry)

A check digit is a single digit added to the end of a long number (like a code) to detect errors made during manual data entry.

This method uses a specific calculation (algorithm) on the preceding digits to create the check digit.

Examples of Check Digit Use:

• ISBNs (International Standard Book Numbers)
• Bar codes and EAN numbers

If you type the rest of the code incorrectly, the check digit calculation will fail, alerting you to an error immediately.

3.3 Automatic Repeat Query (ARQ)

ARQ is a system used to ensure that data is received without error, typically used in conjunction with one of the detection methods above (like checksum).

How ARQ Works (Step-by-Step):

1. The Sender transmits the data packet.
2. The Receiver performs an error check (e.g., parity or checksum).
3. If the check passes, the Receiver sends a Positive Acknowledgement (ACK) back to the Sender.
4. If the check fails, the Receiver sends a Negative Acknowledgement (NACK) back.
5. If the Sender receives a NACK, or if a specific timeout period elapses without any response (the packet might have been lost entirely), the Sender automatically retransmits the packet.

Section 4: Keeping Data Secret – Encryption (Syllabus 2.3)

4.1 The Purpose of Encryption

The need for encryption arises because data transmitted across networks can be intercepted (data interception/hacking). If sensitive data (like passwords or financial information) is sent as plain text, anyone can read it.

Encryption is the process of scrambling data (plaintext) using an algorithm and a key, turning it into unreadable data (ciphertext). The purpose is to ensure that even if data is intercepted, it remains meaningless to unauthorised third parties.

4.2 Symmetric Encryption

This is the simplest form of encryption.

• It uses only one secret key for both encrypting the plaintext and decrypting the ciphertext.
• Both the sender and the receiver must have this identical secret key.

Analogy: Symmetric encryption is like locking a box with a single key that you then have to safely share with the person receiving the box.

Problem: The challenge is securely sharing the key with the other party without it being intercepted.

4.3 Asymmetric Encryption

Also known as public-key encryption, this method uses two mathematically linked keys: a Public Key and a Private Key.

How it works:

• Public Key: This key can be freely shared with anyone. It is used to encrypt the message.
• Private Key: This key must be kept absolutely secret by the owner. It is the only key that can decrypt the message encrypted by its corresponding Public Key.

Example: If Alice wants to send a secret message to Bob:

1. Bob publishes his Public Key (everyone can see it).
2. Alice uses Bob's Public Key to encrypt her message.
3. Alice sends the encrypted message (ciphertext).
4. Only Bob, who holds the matching Private Key, can decrypt and read the message.

Advantage: This solves the key-sharing problem because the key used for encryption (the Public Key) is not the one needed for decryption. The secret key (Private Key) never leaves the owner's possession.