Packet Switching

What's in a Packet?

When data is split into packets, it is bundled with extra information which tries to make sure that it will reach its destination safely.

Payload

The payload of a packet is the actual data that was intended to be sent.

Header

A header is also attached to the packet.
A header includes useful information such as:
- The intended recipient's IP address.
- The sender's IP address.
- The sequence number (so we can re-arrange the packets into the correct order at the other end).
- A checksum (to make sure the data was sent correctly).

How do Packets get Routed?

Packet switching is the process of breaking data down into packets, sending them across the Internet to another computer, and then re-assembling the data.

Sender's local area network

The sender will generate data in an application on their computer.
- This happens at the application layer.
The data is split into packets of data.
- This happens at the transport layer.
Internet Protocol adds IP addresses to each packet.
- This happens at the Internet layer.
Packets are grouped into frames and sent to the router on the LAN.
- This happens at the Network layer.

Over the Internet

The sender's router inspects the packet to determine if the destination IP is on the current LAN.
- If it is not then it will send the packet onto the Internet (or other wide area network).
- This happens at the Internet layer.
Each packet is sent from the sender's router to the destination router via connected routers.
- The path of each packet can be different, which maximises efficient use of the network by avoiding bottlenecks.

Recipient's local area network

Because they have taken different paths, the packets at the destination will be in the wrong order.
- These packets are unpackaged and reassembled into the correct order using the sequence number of each packet.

Did my Data Arrive Safely?

Interference across a network can cause data to become corrupted (changed) as it is transmitted to the recipient.

Lost packets

Some packets might get lost within the network.
Each packet transfer will have a certain timeout, which is the maximum amount of time that the packet transfer is expected to take.
If the time taken is more than the timeout, then we send a timeout signal to the sender to ask them to re-send the packet.

Checksums

A checksum is a piece of data which is generated from the payload.
When data is assembled into packets, we calculate the checksum and send this along with the packet.
When the data is received, the recipient can re-calculate the checksum from the data they receive and compare it to the checksum they received.
- If the values are different, then some of the data was corrupted.
- They can then request the data again.

Confirmation message

If all packets have been successfully received, then a confirmation message is sent to the sender.
This lets them know that there were no issues in transmission that they might need to fix.

1Components of a Computer

1.1Structure & Function of the Processor

1.1.1Structure of the CPU

1.1.2Fetch, Decode, Execute Cycle

1.1.3Factors Affecting CPU Performance

1.1.4CPU Architecture

1.2Types of Processors

1.2.1CISC & RISC

1.2.2Multicore & Parallel Cores

1.3Input, Output & Storage

1.3.1Computer Systems

1.3.2Magnetic Storage

1.3.3Optical Storage

1.3.4Types of Optical Storage

1.3.5Types of Memory

1.3.6Flash Memory

1.3.7Virtual Memory

2Software & Software Development

3Exchanging Data

4Data Types, Data Structures & Algorithms

4.1Data Types

4.1.1Data Types

4.1.2Arrays & Strings

4.1.3Representing Numbers

4.1.4Binary Arithmetic

4.1.5Hexadecimal

4.1.6Converting Binary Numbers

4.1.7Floating Points in Binary

4.1.8Floating Point Arithmetic

4.1.9Bitwise Manipulation

4.1.10Character Sets

4.2Data Structures

4.2.1Arrays, Linked Lists & Queues

4.2.2Graphs, Hash Tables & Stacks

4.2.3Trees

4.3Boolean Algebra

4.3.1Computational Logic

4.3.2Boolean Logic

4.3.3Karnaugh Maps

4.3.4Boolean Identities

4.3.5Flip Flops & Adder Circuits

5Legal, Moral, Cultural & Ethical Issues

5.1Computing Related Legislation

5.1.1Data Protection Act (1998)

5.1.2Computer Misuse Act (1990)

5.1.3Copyright Designs and Patents Act (1988)

5.1.4Regulation of Investigatory Powers Act (2000)

5.2Moral & Ethical Issues

5.2.1Online Activity Tracking

5.2.2Censorship

5.2.3Cultural Issues

5.2.4Environmental Issues

5.2.5Layout, Colour Paradigms & Character Sets

6Elements of Computational Thinking

6.1Thinking Abstractly

6.1.1Abstraction

6.2Thinking Procedurally

6.2.1Decomposition

6.3Thinking Logically

6.3.1Pseudocode & Flow Diagrams

7Problem Solving & Programming

7.1Programming Techniques

7.1.1Programming Fundamentals

7.1.2Modularity

7.2Programming Construction

7.2.1Integrated Development Environments

7.2.2Testing

8Algorithms

8.1Algorithms

8.1.1Interpreting, Correcting & Completing Algorithm

8.1.2Sort Algorithms

8.1.3Insertion & Merge Sort

8.1.4Search Algorithms

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