1 The 7 layer OSI model Sending an

2 The seven layers

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

4 Application Layer

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

6 Presentation layer

7 The presentation layer is concerned with the format of the data It records that the 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 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 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 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

23 The session layer The session layer receives the data stream of the from the transport layer If the whole of the 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 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 application so that the cousin will be able to read the and open the attachment to see the photo

26 All the way The 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 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 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