1. 1.5 Network topologies, protocols and layers 2

2. Thought for the day –
Networks transfer and receive a lot of data!! The amount of traffic on the internet is incredible!
In May 2011, officers at South Yorkshire Police were informed by colleagues in Hertfordshire that they had identified an IP address from which more than 100 indecent images of children had been shared in April that year.
The IP address passed on corresponded to an internet account held by Nigel's partner. But it had been typed incorrectly, with an extra digit added by mistake.
When Nigel was arrested, all he could do was to repeatedly assure himself the police would discover the truth.
"You say to yourself, 'Well they're going to find nothing and I haven't done anything, so I'll be alright.’ After three weeks, police returned Nigel's computers to him, and he had been found completely innocent. But the events had caused a deep psychological effect.
"Because of what happened I felt unable to go back into the field of work I was working in," he says. His role as a drug recovery worker had involved helping teenagers.
"It was the best job I'd had in my life, and I felt I was really good at it.
"But I became fearful of working with young females in case any of them said I tried any sexual advances. It made me paranoid."
'Shouting my innocence'
Eleven months after his arrest, and still without knowledge of why his home had been raided, Nigel began the search for answers - filing a complaint against South Yorkshire Police on grounds of racism and sexism
Nigel decided to ask his solicitor to look deeper. The lawyer contacted Hertfordshire Police and discovered the truth of the incorrect IP address.
Nigel received an apology, in writing, from Hertfordshire Police in 2014, which accepted responsibility for the error.
'Six years of fighting'
Following Hertfordshire Police's admission, Nigel sought compensation for a breach of the Data Protection Act 1998, false imprisonment, police assault/battery, and trespass by police.
In October 2016, Hertfordshire Police settled out of court. Nigel received damages of £60,000, plus legal costs.

3. The Big Picture: Network topologies, protocols and layers
Star and mesh network topologies
Networks using wifi and ethernet
Wi-fi : frequency and channels
Encryption
The uses of IP addressing, MAC addressing, and commonly used protocols
The concept of layers
Packet switching

4. Learning Objectives Know what layers are and their role
By the end of today’s lesson, you will understand:
Know what layers are and their role
Be able to explain the role of each layer
Understand the concept of Packet Switching

5. Starter: Do you know your Protocols?
TCP/IP
HTTP
HTTPS
FTP
POP
IMAP
SMTP

6. Do you know your Protocols? - Answers
TCP/IP (Transmission Control Protocol/Internet Protocol)
HTTP (Hyper Text Transfer Protocol)
HTTPS (Hyper Text Transfer Protocol Secure)
FTP (File Transfer Protocol)
POP (Post Office Protocol)
IMAP (Internet Message Access Protocol)
SMTP (Simple Mail Transfer Protocol)

7. What is Internet anyway?

8. Quick recap: How DNS works1.
2. The URL is sent to your ISP and they look up the URL in their DNS ‘address book’. They find it and send back to you the website’s URL address or passes it on to another DNS server to see if that can find it.
1. You can’t remember the IP address of the computer which hosts the BBC website, so you type in the URL into the browser. 2. 3.
3. Now you can communicate with the computer which hosts the website on the internet, directly.

9. Packet Switching
Imagine that you have a large LEGO model to post through a letter box but, it does not fit.
What could you do?
What if you could take the model apart and then post it through the letter box and then reassemble it?
This is packet switching

10. Packet Switching
Now image you want to a photo, first it is broken down into small packets of data
Each packet is then given a header which contains the IP address of the network and device that it is being sent to.
It also contains the IP address of the network that it was sent from

11. Packet Switching
The header also contains the packet number as well as the total number of packets, for example packet 4 of 60, packet 45 of 60.
When the packets arrive at the destination this information is used to reassemble the data.
Packets can be lost so sometimes the computer request the packet to be sent again, if a packet never arrives then it is deleted by the router.

12. Data Packets Data Packet Header
When files are sent over the internet they are split into millions of data packets.
Packets get sent by different routes according to availability.
When you send a file online, the parts of the file might travel one way around the world and the other parts may go in the opposite direction!
Packets are reassembled at receiving end.
Typical packet structure:
Data
Packet Header
Error Check
Sequence No.
Return Address
Destination Address

13. Data Packets Data Packet Header Sequence No. Return Address
An error check is an important aspect of a data packet.
This aspect of the packet is a ‘checksum number’. A checksum made up of a calculation and its correct answer. Once the packet has been received by the destination computer, If the calculation is run and still produces the correct answer, then we know the data hasn’t been corrupted on its journey.
The Header contains 3 pieces of information:
Sequence Number
Return Address
Destination Address.
This this the data itself.
Data
Packet Header
Error Check
Sequence No.
Return Address
Destination Address
As data is split into packets, the sequence number allows the file to be rebuilt by putting the packets back together in the correct order.
When data arrives, the computer which sent the data can be notified that it arrived safely. And if a packet arrives corrupted, the computer which sent the data can be asked to send it again.
Obviously a data packet needs a destination address so that it can be routed to the correct location.

14. The amazing way that data is sent across the internet!

15. Packets Header Body Footer
Contains instructions about the data being carried. These instructions may include: the length of the packet; packet number; position of this packet; protocol address; originating IP address.
Body
This is the actual data that the packet is delivering to the destination. Also called the payload
Footer
Contains a parity bit/checksum to error check and ensure that the complete packet has been delivered intact, and couple of bits that tell the receiving device that the end of the packet has been reached.

16. Which protocol …?
An internationally agreed protocol. It starts by establishing a ‘handshake’ to set up a connection between two devices. The data is then transferred as packets between the two devices. It specifies exactly how data must be transmitted.

17. Understanding a Packet
Research one of the parts of the packets that you do not know. Explain it to another students.

18. Video https://www.youtube.com/watch?v=ewrBalT_eBM

19. Play packet switching game
Resources from

20. Layers
When we write an and send it, we have no idea what device it will be read on, what operating system, which software etc.
It is an extremely complex process that is easier to deal with if broken down into layers of smaller more manageable parts.
As a packet is prepared the data travels through layers where protocols manage it, adding or removing extra information as required so the packet can continue its journey. The layers must be able to work together exchanging information.
Each layer performs specific tasks and interacts with adjacent layers – this is essential – that the layers can communicate with each other.
Layering: rules organised into a distinct order in which they need to be applied

21. Why have layers? The benefits are:
Simplifies things by dividing into parts
Different layers can be combined in different ways as required
One layer can be develop or changed without affecting others
Makes it easier t identify networking errors and problems
Is a universal standard for manufacturers to follow so all devices can communicate with each other
For example, software for an Ethernet system can easily be adapted to a fibre-optic network by changing only the network layer – other layers are not affected

22. Layers and Protocols
TCP/IP is an example of a layered protocol – it defines how data is transferred through a network for example over the internet or over a private network.
The TCP/IP stack is a complete set of protocols covering the movement of data across the network. These include how the data should be formatted, addressed, routed and received.
You need to know the protocols to be able to link them to the layers

23. Also called Network Access Layer or Data Link Layer
Also called Network Layer
Also called Network Access Layer or Data Link Layer
TCP/Ip is set of protocols arranged in 4 layers, the names sometimes vary a little. Data to be transmitted or received must pass through the layers where packaging data is added or read.

24. Layers https://www.youtube.com/watch?v=zyL1Fud1Z1c
Craig and Dave 1.30

25. Internet Protocols and Layers
All of the protocols which enable the internet to function correctly can be divided into ‘Layers of the Internet’.
Each layer has a particular function and each layer provides a base for the layer above it – in other words, each layer does all the hard work for the layer above it!
For example, if you are accessing a website (HTTP ), it requires data to be split into packets and sent across the internet (TCP / IP), which in turns requires the physical transport of data via cables (cables / Wi-Fi )
Layer
Example Protocols
Layer Purpose
Application Layer
FTP, HTTP, SMTP
Provides user access to applications websites, files, .
Transport Layer
TCP
Provides transport of data between devices by splitting files into data packets and checking that they have been sent and received correctly.
Network Layer
Internet Layer IP
Provides the routing of data across the network by making use of addressing.
Data Link Layer & Physical Layer
Ethernet
Provides the physical transport of data through electrical signals etc.

26. I want to send a picture to my friend so I take it to the posting people.
Application Layer

27. They cut the picture up into post cards and number each one.
Application Layer
Transport Layer 3 4 1 2

28. They then write the address which each card has to go to.
Application Layer
Transport Layer 3 4 1 2
Internet Layer

29. The post cards are loaded into the post van and the van driver sets off.
Application Layer 1 2 3 4
Transport Layer
Internet Layer
Network Interface
(Link) Layer

30. The van driver uses the addresses on the post cards to drive to the destination specified.
Application Layer
Transport Layer
Internet Layer
Network Interface
(Link) Layer

31. At the destination the post cards are removed and the address used to check they are in the right place. Unfortunately the van driver dropped card 3 which got washed down a drain.
Application Layer
Transport Layer 1 2 3
Internet Layer 4
Network Interface
(Link) Layer

32. The delivered cards are checked and the missing number 3 is redelivered. The cards are passed in order to the delivery office.
Application Layer
Transport Layer 3 4 1 2
Internet Layer
Network Interface
(Link) Layer

33. Application Layer Transport Layer Internet Layer3 4 1 2
Internet Layer
The cards are reassembled in order and given to my friend. They now have the original picture.
Network Interface
(Link) Layer

34. You need to remember the layers!
Example Protocols
Layer Purpose
Application Layer
FTP, HTTP, SMTP
Provides user access to applications websites, files, .
Transport Layer
TCP
Provides transport of data between devices by splitting files into data packets and checking that they have been sent and received correctly.
Network Layer
Internet Layer IP
Provides the routing of data across the network by making use of addressing.
Data Link Layer & Physical Layer
Ethernet
Provides the physical transport of data through electrical signals etc.

35. Show your understanding
Log into Moodle
ICT & Computing > GCSE Computer Science > Computer Systems
Scroll down to 1.5
5. 2 Folder
Download, save and complete Layers, Protocols, Packets.docx

36. Plenary
BBC Online test:

38. https://sites.google.com/site/tcpipanimation/
OCR Resources: the small print OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board, and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. © OCR This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.
OCR acknowledges the use of the following content: n/a
Please get in touch if you want to discuss the accessibility of resources we offer to support delivery of our qualifications: