Chapter 2: Data Transmission

Connecting the World, One Bit at a Time

2.1 Types and methods of data transmission

2.1.1 Data packets

The Anatomy of a Data Packet (Exam Requirements)

A data packet is split into three parts. You must know exactly what lives in each:

1. The Packet Header

This is the most "testable" part. It contains the instructions for the routers.

Destination IP Address: Where the packet is going.
Sender’s IP Address: Where the packet came from (so the receiver can reply).
Packet ID / Sequence Number: This allows the receiving computer to reassemble the packets in the correct order.
Packet Size: Tells the receiver how much data to expect.

2. The Payload

This is the simplest part to remember.

The Data itself: Usually a chunk of around 64KiB of the actual file being sent (e.g., part of a photo or a piece of an email).

3. The Packet Trailer

This is the "safety check" at the end of the packet.

End-of-Packet Marker: Tells the receiver "this is the end of this specific packet."
Error Check (Checksum): A calculation used to see if the data was corrupted during transmission. If the calculation at the end doesn't match the one in the header, the packet is rejected.

The "Why" (The Step-by-Step Process)

In the exam, they might ask you to describe packet switching. Here are the 5 marks you need to hit:

Data is broken down into packets.
Each packet is sent independently.
Routers direct the packets across the network (often using different routes).
Packets can arrive out of order.
The receiver uses the Sequence Numbers to reassemble them.

⚠️ Exam Alert: If a question asks why packets take different routes, the answer is usually "to avoid congestion" or "to go around a broken node/router."

2.1.2 Data Transmission

When data is sent from one device to another, it can be sent using one of two methods. The choice depends on distance, cost, and speed.

Serial Transmission

Definition: Data is sent one bit at a time over a single wire or channel.
Best For: Long-distance communication.
Examples: USB cables, fiber optic cables, and Internet connections.
Reliability: Very high. Because there is only one wire, bits arrive in the same order they were sent.
Advantages:
- No "Data Skew": Bits cannot arrive out of sync.
- Low Cost: Only requires a single wire, making it cheaper to manufacture.
Disadvantages: * Slower than parallel over very short distances because it only sends one bit at a time.

Parallel Transmission

Definition: Multiple bits (usually 1 byte) are sent simultaneously over multiple wires or channels.
Best For: Very short-distance communication (usually under 5 meters).
Examples: Internal computer buses (connecting the CPU to RAM) or older printer cables.
Reliability: Lower over distance. Bits can arrive out of sync, which is known as Data Skew.
Advantages:
- High Speed: Can send large amounts of data very quickly because multiple bits move at once.
Disadvantages:
- Data Skew: Over long distances, bits on different wires travel at slightly different speeds. If they arrive out of sync, the data becomes corrupted.
- High Cost: Requires many wires, making it expensive and bulky.
- Interference: "Crosstalk" can occur between the wires if they are too long.

Direction of Data Flow

Data transmission is categorized by the direction in which bits can travel and when they are allowed to move.

Simplex Transmission

Direction: Data is sent in one direction only (from sender to receiver).
Analogy: A one-way street.
IGCSE Example: A computer sending data to a monitor, or a keyboard sending data to a CPU.

Half-Duplex Transmission

Direction: Data is sent in both directions, but not at the same time.
Analogy: A one-lane bridge or a walkie-talkie (you must wait for the other person to stop talking before you can send your message).
IGCSE Example: A walkie-talkie system or a printer sending an "out of paper" message back to a computer after receiving a print job.

Full-Duplex Transmission

Direction: Data is sent in both directions simultaneously (at the same time).
Analogy: A two-lane highway or a telephone conversation where both people can speak and hear each other at once.
IGCSE Example: A broadband internet connection or a mobile phone call.

Exam Tip: Why is Serial replacing Parallel?

In many modern systems (like USB), Serial is actually used more than Parallel. Even though Parallel sends more bits at once, the problem of Data Skew and Crosstalk makes it impossible to use at high speeds over long distances. Serial is cheaper, more reliable, and can now be clocked at very high frequencies to match parallel speeds.

2.3. USB (Universal Serial Bus)

USB is the most common type of Asynchronous Serial Data Transmission. It has replaced almost all other types of ports (like serial or parallel ports) on modern computers.

How a USB Connection Works

When you plug a device into a USB port:

The computer automatically detects the device.
The device is automatically recognized, and the appropriate "driver" (software) is loaded so they can communicate.
If a new device is detected, the computer looks for the driver; if it can't find it, the user is prompted to install it.

The Advantages of USB

In the exam, you are often asked why USB is preferred over older cables. Here are the key points:

Standardization: It is a global standard, meaning devices from different manufacturers will work together.
"Plug and Play": Devices are automatically detected and configured by the computer.
Power Supply: USB cables can carry power as well as data, meaning small devices (like mice or keyboards) don't need a separate power cable.
High Speed: Modern USB versions are very fast compared to older serial connections.
Errors are Rare: It includes built-in error-checking to ensure data arrives correctly.
Impossible to Plug in Wrong: (Mostly!) The connectors are designed to be "keyed" so they can't be plugged in upside down (especially with USB-C).

The Disadvantages of USB

Distance: USB cables are limited to short distances (usually less than 5 meters).
Transmission Speed: While fast, it is still slower than some internal connections like PCIe or the latest Ethernet cables.
Older Versions: Older USB ports (USB 2.0) are much slower than newer ones (USB 3.0/USB-C), which can cause "bottlenecks."

2.4 Methods of Error Detection

2.4.1 Error Detection Methods

Why do errors occur?

During data transmission, data can be corrupted (changed), lost, or gained. This is caused by:

Interference: Electrical noise on cables can change bits (0 to 1 or vice versa).
Packet Switching Problems: Data packets can be lost or arrive in the wrong order.
Data Skewing: In parallel transmission, bits can arrive out of sync, leading to corruption.

Exam Tip: Computers cannot "guess" the meaning of corrupted data like humans can. If a bit is wrong, the data becomes unintelligible to the system.

2.4.1.1 Parity Checks

A parity check is a simple way to see if a bit has been "flipped" during transmission. A Parity Bit is added to the end of a block of data to make the total number of 1s either Even or Odd.

How it Works (Even Parity)

The sender counts the number of 1s in the byte.
If the number is already even, the parity bit is set to 0.
If the number is odd, the parity bit is set to 1 to make the total even.
The receiver checks the byte. If they find an odd number of 1s in an "Even Parity" system, they know an error occurred.

Odd Parity Transmission

Odd parity is a simple error-detection method used to verify that a byte of data has not been corrupted during transmission.

The Rule: The total number of 1s in a byte (including the parity bit) must always be an Odd number (1, 3, 5, or 7).
How it Works (Sender):
1. The sender counts the 1s in the 7 bits of data.
2. If the count is Even, the parity bit is set to 1 (to make the total odd).
3. If the count is already Odd, the parity bit is set to 0 (to keep the total odd).
How it Works (Receiver):
- The receiver counts the 1s in the received byte.
- If the total count is Even, the receiver knows a bit must have flipped and requests the data be sent again.

2.4.1.2 Checksums: The "Data Fingerprint"

A checksum is used to verify that a large block of data (like a file or an image) hasn't been corrupted during its journey over a network.

How it works in 3 steps:

Calculate: Before sending, the sender runs a mathematical formula on the data to get a unique number (the Checksum).
Send: The data and the Checksum are sent together.
Compare: The receiver runs the same formula on the data. If their result matches the sender's Checksum, the data is perfect.

Important: If the values don't match, the data is rejected, and the receiver asks the sender to "try again" (Re-transmission).

2.4.1.3 Echo Checks

An Echo Check is like asking a friend to repeat a secret back to you to make sure they heard it correctly. It is the simplest but least efficient form of error detection.

How it Works

The Transmission: The sender transmits a block of data to the receiver.
The Echo: The receiver immediately sends an exact copy of that data back to the sender.
The Comparison: The sender compares the "echoed" copy with the original data.
The Action: If they match, the data is safe. If not, the sender re-transmits the data.

The "Why" (Pros & Cons)

Pro: Very simple to program and requires no complex math (unlike Checksums).
Con (Traffic): It doubles the network traffic because every piece of data has to travel twice.
Con (Reliability): If the echo doesn't match, you don't know if the error happened on the way to the receiver or on the way back to the sender.

2.4.1.4 Automatic Repeat Request (ARQ)

An Automatic Repeat Request (ARQ) is a protocol used to ensure data is transmitted correctly. It relies on acknowledgments and timeouts to manage the retransmission of corrupted or lost data.

How ARQ Works

Error Detection: The receiving device uses an error-detection method (like a Checksum) to see if the data arrived intact.
Positive Acknowledgment: If the data is correct, the receiver sends a Positive Acknowledgment back to the sender, and the next packet is sent.
Negative Acknowledgment: If the receiver detects an error, it sends a Negative Acknowledgment to the sender, requesting that the data be sent again.
Timeouts: If the sender does not receive any acknowledgment within a pre-defined period (the timeout), it assumes the data was lost and automatically retransmits the packet.

2.5 Encryption

Encryption is used to protect data when it is transmitted over a public network.

2.4.1 The Purpose of Encryption

The Goal: To make data unreadable to anyone who intercepts it (often called a hacker or eavesdropper).
Important Distinction: Encryption cannot prevent data from being intercepted; it simply ensures that if it is stolen, it makes no sense to the thief.
Plaintext: The original, readable data before it is scrambled.
Ciphertext: The scrambled data produced after an encryption algorithm is applied.

2.5.2 Symmetric vs. Asymmetric Encryption

Symmetric Encryption

The Key: Uses a single, identical key to both encrypt and decrypt the data.
The Process: Both the sender and the receiver must have a copy of the same secret key.
The Risk: The main drawback is the security of the key itself—if the key is intercepted while being sent to the receiver, the encryption is useless.

Asymmetric Encryption

To solve the security issues of symmetric encryption, asymmetric encryption uses a matching pair of keys:

Public Key: Available to everyone; used to encrypt the data.
Private Key: Kept secret by the receiver; used to decrypt the data.

How it works (The Tom & Jane Example):

Jane generates a matching pair of keys (Public and Private).
Jane sends her Public Key to Tom.
Tom uses that Public Key to encrypt a document and sends the Ciphertext to Jane.
Jane uses her Private Key to decrypt it. Even if a hacker has the Public Key, they cannot decrypt the message.

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