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