Chapter 4 : Network models.
Lecture 9
The OSI model:
Layers in the OSI model :
7- Application Layer.
6- Presentation Layer.
5- Session Layer.
4- Transport Layer.
3- Network Layer.
2- Data Link Layer.
1- Physical Layer.
7- Application Layer :
- The application layer enables the user, to access the
network.
- It provides user interfaces and support for services
such as electronic mail, remote file access and
transfer…. etc.
7- Application Layer :
The application layer is responsible for providing
services to the user.
6- Presentation Layer :
- The presentation layer is responsible for translation,
compression, and encryption.
- Specific responsibilities of the presentation layer
include the following:
1- Translation :
At the sender it changes the information from its
sender –dependent format into common format. At
receiving, changes the common format into its
receiver-dependent format.
- It can be called network's translators.
6- Presentation Layer :
2- Encryption :
To ensure privacy and security.
3- Compression :
- Data compression reduces the number of bits
contained in the information.
- It is important in the transmission of multimedia such
as audio or video.
6- Presentation Layer :
The presentation layer is responsible for translation,
compression, and encryption.
5- Session Layer :
- The session layer is responsible for dialog control and
synchronization.
- Specific responsibilities of the session layer include the
following:
- Dialog control:
- The session layer allows two systems to enter into a
dialog.
- It allows the communication between two processes
to take place in either half-duplex (one way at a time)
or full-duplex (two ways at a time) mode.
5- Session Layer :
- Synchronization:
The Session layer provides synchronization between
user tasks by placing checkpoints in the data stream.
By this way, if the network fails, only the data after the
last checkpoint has to be retransmitted.
5- Session Layer :
The session layer is responsible for dialog control and
synchronization.
4- Transport Layer :
- Layer 4, The transport layer is responsible for processto-process delivery of the entire message and dividing
this message into segments.
- Specific responsibilities of the transport layer include
the following:
- Segmentation and reassembly:
A message is divided into transmittable segments, with
each segment containing a sequence number.
Numbers enable the transport layer to reassemble
the message correctly upon arriving at the
destination.
4- Transport Layer :
- Process to process delivery:
- Computers often run several programs at the same time.
For this reason, source-to-destination delivery means
delivery not only from one computer to the next but also
from a specific process (running program) on one
computer to a specific process (running program) on the
other.
- The network layer gets each packet to the correct
computer; the transport layer gets the entire message to
the correct process on that computer.
- The transport layer ensures that the messages are
delivered error free.
4- Transport Layer :
The transport layer is responsible for the delivery of a
message from one process to another.
Figure : Reliable process-to-process delivery of
a message.
3- Network Layer :
- Layer 3, the Network layer, Responsible from the
delivery of packets from the original source to the
final destination across multiple networks, to
accomplish this , the network layer defines logical
addresses to identify the endpoint of the network.
- Network layer adds unique ( IP or logical address) to
the packet.
3- Network Layer :
- IP addresses 32-bit address that uniquely define a host
connected to the Internet . No two host address on
the internet can have the same IP address.
- This layer also determines the route from the source
to the destination computer.
3- Network Layer :
The network layer is responsible for the delivery of
individual packets from the source host to the
destination host.
Figure: Source-to-destination delivery
2- Data Link Layer :
- Layer 2, the Data Link layer, sends data frames from
the Network layer to the Physical layer.
- Data Link Layer adds a header to the frame to define
the sender and receiver of the frame.
- Error control is normally achieved through a trailer
added to the end of frame.
2- Data Link Layer :
The data link layer is responsible for moving frames
from one hop (node) to the next.
Figure : Hop-to-hop delivery
Example :
1- Physical Layer:
- Layer 1, the bottom layer of the OSI model, is the
physical layer.
- It specifies how data is processed into bits and
physically transferred over medium, such as cables.
1- Physical Layer:
The physical layer is responsible for the movement of
individual bits from one hop (node) to the next.
The OSI model:
OSI model
Layers
Data unit
7- Application Layer.
6- Presentation Layer.
Data
5- Session Layer.
4- Transport Layer.
Segments
3- Network Layer.
Packet
2- Data Link Layer.
Frame
1- Physical Layer.
Bit
Encapsulation and de-encapsulation
Process:
- The process of placing the header before the data and
the trailer (not added by all layers) after the data is
called Encapsulation of data.
- On the receiving computer the process of removing
these headers and trailers by the corresponding
layers is called de-encapsulation. It’s the Data link
layer, which adds the trailer to the data also.