1. Circuit and Packet Switching
Component 1
Circuit and Packet Switching

2. Starter: Pre-Reading Recap
What is circuit switching?
What are the advantages of circuit switching?
What are the disadvantages?
What is a packet?
What information is in a packet?
How big is a packet?
What does a checksum do?
What is packet switching?
What are the advantages of packet switching?
What is data redundancy?
What is routing?
Can you think of a real world example of routing?

3. Learning Intentions and Outcomes
Learning Intention: To develop an understanding of how data is transferred around a network
Learning Outcomes:
Explain and give advantages and disadvantages of circuit switching and packet switching
Describe the typical contents of a TCP/IP packet
Describe methods of routing traffic on a network and calculate routing costs

4. Circuit Switching
Circuit switching is a networking technology that provides a temporary but dedicated link between two stations or nodes regardless of the number of switching devices through which the data has to travel.
During the connection no other data can be transmitted along the same route.
The landline telephone system is an example of a circuit switched network.

5. Circuit Switching: Landlines
The telephone system in the UK used to use a technique called 'circuit switching' to let two telephones (or computers) connect across the national network.
Circuit switching enables you to send digital messages across a network by setting up a physical path between the sender and receiver. This is done using wires or fibre optic cable and is set up prior to transmission of the message. The path remains open throughout the communication and is only released for use by others once the message is complete.

6. Circuit Switching: Advantages
The main advantage of circuit switching is that it is reliable and once the connection is established it is fast and generally error free.

7. Circuit Switching: Disadvantages
It takes time to establish the connection.
Should anywhere on the route fail then the connection will be broken
Very inefficient - once the connection is made, the line is tied up and no one else can use them, even if no data is passing between them.
Difficult to scale up the network to deal with millions of devices.
Every connection would have to be capable of supporting the maximum bandwidth required. This was no problem for voice calls as a voice call needs a very low bandwidth (3KHz), but not when bandwidths of 100Mbps + are needed

8. Grade 4 Exercises
Complete questions 1-4 of the Grade D exercises on Circuit Switching
Remember to be smart when using the Internet:
Wikipedia is often complicated – sometimes even I don’t understand!
BBC Bitesize, Teach ICT, and Revision World are better places to find your information!

9. Packet Switching
To overcome the problems with circuit switching, packet switching was developed. Rather than relying on a dedicated connection packet switching breaks the data down into small packets that can be sent by more than one route.

10. Packets
A packet is a collection of data that is transmitted over a packet-switched network.
A file has to be broken up into small chunks of data known as data packets in order to be transmitted over a network.
The data is then re-built once it reaches the destination computer.
Networking hardware is required to connect computers and manage how data packets are communicated. 

11. The destination address Other tracking information
Packets: Content
All Packets contain similar data and they are all built the same. They contain:
The source address (Sender information)
The destination address (Destination information)
Information to enable the data to be reassembled (Like a cipher key)
Other Tracking Information (Such as the number of the packet, and the total number e.g. 1/20, 4/20)
The data itself
A Checksum (This checks that the data has not been corrupted)
The source address
The destination address
Information which enables the data to be reassembled into its original form
Other tracking information
The data itself
A checksum that checks that the data has not been corrupted

12. Packet Switching
Packet switching is the process of delivering packets from one computer system to another using a designated device, such as a switch or a router.
Unlike circuit switching where the whole message is sent in one go, with packet switching the message is broken down into smaller parts (data packets) and these parts are sent individually through the network.
The packets are given a 'packet number' to identify the order they are in.
The packets will travel across the network, usually in different directions, taking the shortest route available.
When the individual packets arrive at their destination they will be reassembled back together in the correct order.

13. Packet Switching
Once all of the packets have safely arrived a message will be sent to the original computer as confirmation.
If a packet is missing or corrupted then the message will instead be sent asking for that particular packet to be resent.

14. Packet Switching: Example
A Message is being sent from A to F
It has been split into 3 parts
“Hel”
“lo B”
“ill!”
What does the message say?
What information is required to get from A to F?

15. Packet Switching: Advantages and Disadvantages
Communication – it is easy (and often free) to communicate using , text messages, voice calls and video calls.
Roaming – if information is stored on a network, it means users are not fixed to one place. They can use computers anywhere in the world to access their information.
Sharing information – it is easy to share files and information over a network. Music and video files, for instance, can be stored on one device and shared across many computers, so every computer does not need to fill the hard drive with files.
Sharing resources – it is easy to share resources such as printers. Twenty computers in a room could share one printer over a network.
Sharing software – it is possible to stream software using web applications. This avoids needing to download and store the whole software file.
Dependence – users relying on a network might be stuck without access to it.
Hacking - criminal hackers attempt to break into networks in order to steal personal information and banking details. This wouldn't be possible on a stand-alone computer without physically getting into the room, but with a network it is easier to gain access.
Hardware – routers, network cards and other network hardware is required to set up a network. At home, it is quite easy to set up a wireless network without much technical expertise. However, a complicated network in a school or an office would require professional expertise.
Viruses - networks make it easier to share viruses and other malware. They can quickly spread and damage files on many computers via a network.

16. Graded Exercises: Packet Switching
Complete questions 5-8 of the grade 4 graded exercises on Packet Switching
Remember to be smart when using the Internet:
Wikipedia is often complicated – sometimes even I don’t understand!
BBC Bitesize, Teach ICT, and Revision World are better places to find your information!

17. Routing
Routing is the name given to the method of selecting paths along which packets are sent on a computer network.
Specialist computer systems such as routers, switches, bridges, firewalls and gateways construct in their memory a routing table, which stores a number of paths along which it is best to send packets to reach a specific destination. – Think about commuting. People know their route to work/school, but if something goes wrong on the route they probably know their second best route!
Maintaining accurate routing tables is essential for ensuring that packets are delivered as quickly as possible.

18. Routing: Example
In the example shown, computer system 1 is sending a packet to computer system Clearly, the quickest route for the packet to arrive at its destination is to be sent from router A, on to router B followed by router E for delivery to computer system 126.
Good Route
A > B > E
Bad Route
A > C > D > E

19. Routing: Network Paths – Failure!
Most routers use only one network path at a time, such as the preferred route above (Computer system 1 > Router A > Router B > Router E > Computer system 126).
Some multipath routing techniques enable the same packets to be sent using multiple alternative paths at the same time.
This means that in the event of Router B failing in the transmission above, the same packet would also have been sent via the alternative longer route set out above (Computer system 1 > Router A > Router C > Router D > Router E > Computer system 126), to ensure that the packet arrives at its destination.

20. Graded Exercises: Routing
Complete questions 5-8 of the grade 4 graded exercises on Routing
Remember to be smart when using the Internet:
Wikipedia is often complicated – sometimes even I don’t understand!
BBC Bitesize, Teach ICT, and Revision World are better places to find your information!

21. Graded Exercises Complete Grade 5-7 Graded Exercises
When you have completed these self- assess them using the mark schemes provided
Extension
Move on to the Grade 8 Extension
Remember to be smart when using the Internet:
Wikipedia is often complicated – sometimes even I don’t understand!
BBC Bitesize, Teach ICT, and Revision World are better places to find your information!

22. Crib Sheet – What I must remember about _______
Starting Point: Read the learning outcomes from the exam board
Record the key facts that you need to remember about Circuit and Packet switching.
Think about what you have learnt today, what questions you have been asked, definitions of words, or anything else you think is important!

23. Homework Question Sheets: Pre-reading notes: Q37 (Network Hardware)
29-30 (IP and MAC addresses, DNS Servers)