Computer Science IGCSE 0478 C2

Chapter 2

What is a data packet?

A small part of a message that is transmitted over a network.

Why is data split into packets before transmission?

To allow efficient transmission, especially over long distances.

What are the three components of a data packet?

Packet header, packet payload, and packet trailer.

What does a packet header include?

Sender and receiver IP addresses, packet number, sequence number, and packet size.

What is contained in the packet payload?

The actual data being sent, typically around 64 KiB.

What does the packet trailer include?

End-of-packet identification and an error checking method (e.g., CRC).

What is packet switching?

A method where data is broken into packets that take different routes to the destination.

What controls the route packets take in packet switching?

Routers, using the destination IP address in the header.

What happens if packets arrive out of order?

They are reordered using the packet number in the header.

What happens if a packet is missing or corrupted?

It is requested again.

List two benefits of packet switching.

Doesn't tie up a single line, harder to hack due to small data per packet.

List two drawbacks of packet switching.

Packets can be lost, and reassembly can cause delay.

What is data transmission?

The process of transferring data from one device to another via wired or wireless methods.

What are the three factors in data transmission?

Direction of transmission, method (serial/parallel), and data synchronization.

What is simplex transmission?

One-way transmission only (e.g. computer to printer).

What is half-duplex transmission?

Two-way transmission but not simultaneous (e.g. walkie-talkie).

What is full-duplex transmission?

Two-way simultaneous transmission (e.g. broadband).

One advantage and disadvantage of simplex transmission?

Advantage – cheap (only one wire), Disadvantage – one-way only.

One advantage and disadvantage of half-duplex transmission?

Advantage – cheaper for bidirectional use, Disadvantage – can't send/receive at same time.

One advantage and disadvantage of full-duplex transmission?

Advantage – fast, Disadvantage – expensive and not all networks support it.

What is serial transmission?

Bits are sent one at a time over a single wire.

What is parallel transmission?

Multiple bits are sent simultaneously over multiple wires.

Where is serial transmission used?

Long distances (e.g., telephone lines).

Where is parallel transmission used?

Short distances (e.g., internal computer circuits).

One advantage and disadvantage of serial transmission?

Advantage – low interference, Disadvantage – slower speed.

One advantage and disadvantage of parallel transmission?

Advantage – fast, Disadvantage – prone to interference and skewing.

What is USB?

Universal Serial Bus, a serial data transmission standard.

What types of transmission can USB support?

Half-duplex and full-duplex.

What happens when a device is connected to a USB port?

Computer detects device, checks for driver, and loads or installs it.

Two benefits of USB?

Automatic device detection, industry standard.

Two drawbacks of USB?

Slower than Ethernet, 5m max cable length.

Why is error detection important in data transmission?

To check if data was lost, altered, or gained due to interference.

What is a parity check?

Checks if the number of 1s in a byte matches expected even/odd parity.

What is a checksum?

A value sent with data and recalculated to detect corruption.

What is an echo check?

Data is sent back to sender for comparison.

One disadvantage of echo checks?

Errors can occur during echo transmission too.

What is a check digit?

A digit added to data to validate it (e.g., ISBN, barcodes).

What is ARQ?

A protocol that resends data if an error or timeout occurs.

How does ARQ work?

Receiver sends acknowledgment; if none or a negative one is received, sender resends.

What is encryption?

A method of scrambling data to prevent unauthorized access.

What is plaintext?

The original readable data before encryption.

What is ciphertext?

The unreadable, encrypted version of the data.

What is the purpose of encryption?

To secure data from interception and make it unreadable to unauthorized users.

What is symmetric encryption?

Uses the same key to encrypt and decrypt data.

One benefit and one drawback of symmetric encryption?

Benefit – fast; Drawback – less secure if key is intercepted.

What is asymmetric encryption?

Uses a public key to encrypt and a private key to decrypt.

One benefit and one drawback of asymmetric encryption?

Benefit – more secure; Drawback – slower and resource-intensive.

What is the role of the public key in asymmetric encryption?

It is used by the sender to encrypt the message.

What is the role of the private key in asymmetric encryption?

It is used by the receiver to decrypt the message.

What type of data transmission does USB use?

Serial.

Give two benefits of using USB.

Auto device detection; universal compatibility.

Name two items found in a packet header.

Destination IP; packet number.

What are the three parts of a data packet?

Header, payload, trailer.

Why use parallel half-duplex for video uploads?

Fast transfer; one-way needed only.

How does packet switching work?

File split into packets; each routed; reassembled using sequence; missing ones resent.

What is echo checking?

Receiver returns data; sender compares.

Why isn’t echo checking suitable for vending machines?

Too slow and inefficient.

How does ARQ work?

Resends data until ACK received or timeout.

How does a check digit detect errors?

Calculated from digits; mismatches show error.

Where is a check digit used?

ISBN, barcode