The 7 layer OSI model

1. The 7 layer OSI model Sending an e-mail

2. The seven layers

3. Janet’s e-mail • Janet wants to send an e-mail with a photo attached to her cousin in Australia • We will look at the 7 layers of the OSI model to describe how the e-mail is sent and received • Each layer has its own protocols (rules) for handling the data

4. Application Layer

5. Application layer • Janet uses an e-mail application to write her e-mail and attach the photo • The application layer takes the data from the e-mail application • It provides a service to the e-mail application

6. Presentation layer

7. Presentation layer • The presentation layer is concerned with the format of the data • It records that the e-mail is plain text (or rich text) and that the photo is a graphics file (e.g. jpeg) • The data is in the form of a data stream • Any compression or encryption could also be carried out by the presentation layer

8. Session layer

9. Session layer • The session layer is concerned with starting managing and ending the communication between Janet’s computer and her cousin’s computer • The data still exists as a data stream

10. Transport layer

11. Transport layer • The transport layer takes the data from the session layer and splits it up into segments that are the right size for sending. • It adds information to say which protocol is being used at the upper layers – in this case that an e-mail protocol is used • It checks that all the segments reach their destination – the cousin’s computer

12. Network layer

13. Network layer • The network layer takes the segments • It adds a header to each segment, giving the IP address of Janet’s PC and her cousin’s PC • The new data unit is called a packet • As the packet travels to Australia, the routers along the way will look at the IP address and decide where the packet should go next

14. Data link layer

15. Data link layer • The data link layer takes the packet and adds more information, including the physical (MAC) addresses of the source computer and of the computer or router that will handle the packet next • This new data unit is called a frame • Switches can look at the MAC address and pass it on in the right direction

16. Physical layer

17. Physical layer • The physical layer takes the frame • It sees the data as a string of bits, (0s and 1s) • It converts the bits to electrical signals that can be sent along a cable

18. The journey • Janet’s e-mail is now a stream of electrical pulses travelling along a cable • It will pass through many networks and network devices • It may be converted to light signals on optical fibre cables or to radio or microwaves • Routers will strip off the old physical address, look at the IP address and put in the physical address of the next router

19. The physical layer • The electrical signals arrive at the cousin’s computer • The physical layer takes the signals and converts them back to bits (1s and 0s) • It passes them up to the data link layer

20. The data link layer • The data link layer checks that the physical address is the right address for the cousin’s computer • It checks that the frame does not contain any errors • It strips off the physical addresses and other frame information (leaving a packet) • It passes the result up to the network layer

21. The network layer • The network layer takes the packet from the data link layer • It checks that the IP address is the right address for the cousin’s computer • It strips off the IP address and other packet information, leaving a segment • It passes the result up to the transport layer

22. The transport layer • The transport layer takes the segment from the network layer • It fits all the segments back together in the right order to make a data stream again • If any segments are missing or damaged it can arrange for them to be sent again • It checks to see which higher level protocol was used and finds that the data is an e-mail

23. The session layer • The session layer receives the data stream of the e-mail from the transport layer • If the whole of the e-mail has been received correctly it can close the communication session between the computers • It passes the data stream up to the presentation layer

24. The presentation layer • The presentation layer takes the data stream from the session layer • It finds that the data consists of an e-mail in plain text and an image in the form of a jpeg file • If there was any compression or encryption the presentation layer could deal with it • It passes the data up to the application layer

25. The application layer • The application layer receives the data from the presentation layer • It gives the data in the right form to the cousin’s e-mail application so that the cousin will be able to read the e-mail and open the attachment to see the photo

26. All the way • The e-mail has travelled down all 7 layers of the OSI model in Janet’s computer • It then passed as electrical, light or radio signals across many networks and through many devices • In the cousin’s computer it travelled up through the 7 layers of the OSI model to become an e-mail again

27. Peer to peer • At each level it seems to the protocols as if they are talking directly to protocols at the same level • The detail of the lower layers is hidden from them – the lower layers provide a service to them • Communication between protocols at the same level is called peer to peer communication

28. Peer to peer

29. Person to person • It seems to Janet and her cousin that they are communicating directly with each other • They do not need to know what happens to the e-mail message on its journey • That’s the point of having layers. Each layer does its own special job. No layer has a task that is too big or complicated • For Janet the whole thing is easy

30. END The End