Transcript The 7 layer OSI model

The 7 layer OSI model
Sending an e-mail
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The seven layers
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Janet’s e-mail
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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
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Application Layer
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Application layer
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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
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Presentation layer
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Presentation layer
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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
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Session layer
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Session layer
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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
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Transport layer
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Transport layer
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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
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Network layer
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Network layer
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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
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Data link layer
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Data link layer
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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
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Physical layer
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Physical layer
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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
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The journey
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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
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The physical layer
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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
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The data link layer
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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
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The network layer
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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
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The transport layer
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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
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The session layer
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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
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The presentation layer
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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
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The application layer
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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
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All the way
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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
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Peer to peer
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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
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Peer to peer
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Person to person
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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
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END
The End
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