OSI REFERENCE LAYERS
Snehal S. Naghate
CONTENTS
❖ INTRODUCTION
❖ WHAT IS OSI?
❖ OSI MODEL
❖ TYPES OF LAYERS
❖ PHYSICAL LAYER
❖ DATA LINK LAYER
❖ NETWORK LAYER
❖ TRANSPORT LAYER
❖ SESSION LAYER
❖ PRESENTATION LAYER
❖ APPLICATION LAYER
❖ CONCLUSION
WHAT IS OSI?
• Layered Task - international standard organization (ISO)
proposed an open system interconnection (OSI) model –
allows two system to communicate regardless of their
architecture.
• An open system is a set of protocol that allows any two
different systems to communicate regardless of their
underlying architectures.
• The purpose of OSI model is to facilitate communication
between different systems without requiring changes to the
logic of the underlying hardware and software.
WHAT IS OSI?
• OSI-OPEN SYSTEMS INTERCONNECTION
• It was designed by ISO-International Organization for
Standardization in late1970s.
• It is a seven-layer model.
• It is a theoretical model designed to show how a protocol
stack should be implemented.
OSI MODEL
Definition
◼ The OSI model is not a protocol; it is a model for understanding and
designing a network architecture that is flexible, robust and
interoperable.
Layered Architecture
The OSI model is composed of seven ordered layers: physical (layer 1),
data link (layer 2), network (layer 3), transport (layer 4), session (layer
5), presentation (layer 6), and application (layer 7).
Within a single machine, each layer calls upon the service of the layer
just below it. Layer 3 for example, uses the services provided by layer 2
and provides services for layer 4.
Between machines, layer x on one machine communicates with layer x
on another machine.
This communication is governed by an agreed-upon series of rules and
conventions called protocols.
The processes on each machine that communicate at a given layer are
called peer-to-peer processes.
Layered Architecture of OSI Model
PHYSICAL LAYER
• The physical layer is responsible for transmitting
individual bits from one node to the next.
• It is responsible for the actual physical connection
between the devices.
• The physical layer contains information in the form
of bits.
• When receiving data, this layer will get the signal received
and convert it into 0s and 1s and send them to the Data
Link layer, which will put the frame back together.
PHYSICAL LAYER
Functions of physical layer:
• Line Configuration: It defines the way how two or more devices
can be connected physically.
• Data Transmission: It defines the transmission mode whether it
is simplex, half-duplex or full-duplex mode between the two
devices on the network.
• Topology: It defines the way how network devices are arranged.
• Signals: It determines the type of the signal used for transmitting
the information.
PHYSICAL LAYER
From data To data
link layer link layer
101010000000101111001 101010000000101111001
Physical Physical
layer layer
Transmission
medium
DATA LINK LAYER
• The data link layer is responsible for the node-to-node delivery
of the message.
• The main function of this layer is to make sure data transfer is
error-free from one node to another, over the physical layer.
• FUNCTIONS OF DATA LINK LAYER:
▪ Framing
▪ Physical addressing
▪ Flow control
▪ Error control
▪ Access control
DATA LINK LAYER
FUNCTIONS OF DATA LINK LAYER:
▪ Framing: Framing is a function of the data link layer. It provides a way for a
sender to transmit a set of bits that are meaningful to the receiver.
▪ Physical addressing: After creating frames, the Data link layer adds physical
addresses (MAC addresses) of the sender and/or receiver in the header of
each frame.
▪ Flow control: The data rate must be constant on both sides else the data may
get corrupted thus, flow control coordinates the amount of data that can be
sent before receiving an acknowledgment.
▪ Error control: The data link layer provides the mechanism of error control in
which it detects and retransmits damaged or lost frames.
▪ Access control: When a single communication channel is shared by multiple
devices, the MAC sub-layer of the data link layer helps to determine which
device has control over the channel at a given time.
DATA LINK LAYER
From network layer To network layer
frame frame
T2 Data H2 T2 Data H2
Data link Data link
layer To physical layer layer
From physical layer
NETWORK LAYER
• The network layer is responsible for the delivery of packets from
the original source to the final destination.
• It also takes care of packet routing i.e. selection of the shortest path
to transmit the packet, from the number of routes available.
• The sender & receiver’s IP addresses are placed in the header by the
network layer.
NETWORK LAYER
FUNCTIONS OF NETWORK LAYER:
▪ Logical addressing: To identify each device on Internetwork
uniquely, the network layer defines an addressing scheme. The
sender & receiver’s IP addresses are placed in the header by the
network layer. Such an address distinguishes each device uniquely
and universally.
▪ Routing: The network layer protocols determine which route is
suitable from source to destination. This function of the network
layer is known as routing.
NETWORK LAYER
NETWOR NETWOR
K LAYER K LAYER
From Transport Layer To Transport Layer
packet packet
Data H3 Data H3
Network
layer To data link From data link layer
layer
TRANSPORT LAYER
• The transport layer is responsible for delivery of a message from
one process to another(process-to-process delivery).
• The transport layer provides services to the application layer and
takes services from the network layer.
• The data in the transport layer is referred to as Segments.
• It is responsible for the End to End Delivery of the complete
message.
• The transport layer also provides the acknowledgment of the
successful data transmission and re-transmits the data if an error
is found.
TRANSPORT LAYER
FUNCTIONS OF TRANSPORT LAYER:
• Port addressing
• Segmentation and reassembly: When the transport layer receives
the message from the upper layer, it divides the message into
multiple segments, and each segment is assigned with a sequence
number that uniquely identifies each segment.
• When the message has arrived at the destination, then the transport
layer reassembles the message based on their sequence numbers.
TRANSPORT LAYER
FUNCTIONS OF TRANSPORT LAYER:
• Port addressing
• Connection control: Transport layer provides two services Connection-oriented
service and connectionless service. A connectionless service treats each segment as
an individual packet, and they all travel in different routes to reach the destination. A
connection-oriented service makes a connection with the transport layer at the
destination machine before delivering the packets. In connection-oriented service,
all the packets travel in the single route.
• Flow control: The transport layer also responsible for flow control but it is
performed end-to-end rather than across a single link.
• Error control: The transport layer is also responsible for Error control. Error control
is performed end-to-end rather than across the single link. The sender transport layer
ensures that message reach at the destination without any error.
TRANSPORT LAYER
From session layer To session layer
segments segments
Data H4 Data H4 Data H4 Data H4 Data H4 Data H4
Transport Transport
layer layer
To network From network
layer layer
SESSION LAYER
• This layer is responsible for the establishment of connection,
maintenance of sessions, and authentication, and also ensures
security.
FUNCTIONS OF SESSION LAYER:
• Dialog control: The session layer allows two systems to start
communication with each other in half-duplex or full-duplex.
• Synchronization: This layer allows a process to add checkpoints
that are considered synchronization points in the data. These
synchronization points help to identify the error so that the data
is re-synchronized properly, and ends of the messages are not cut
prematurely and data loss is avoided.
SESSION LAYER
To presentation layer
From presentation layer
L6 data L6 data
Session layer Session layer
H5 H5
syn syn syn syn syn syn
L5 data L5 data
To transport layer From transport layer
PRESENTATION LAYER
• The presentation layer is concerned with the syntax and semantics of the
information exchanged between two systems.
• The presentation layer is also called the Translation layer. The data from the
application layer is extracted here and manipulated as per the required format to
transmit over the network.
FUNCTONS OF PRESENTATION LAYER:
• Translation: For example, ASCII(American Standard Code for Information
Interchange )to EBCDIC(Extended binary coded decimal interchange code).
• Encryption: Data encryption translates the data into another form or code. The
encrypted data is known as the ciphertext and the decrypted data is known as
plain text. A key value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the
network.
PRESENTATION LAYER
From application layer To application layer
L7 data L7 data
Presentation layer Presentation layer
Encoded ,encrypted H6 Decoded ,decrypted H6
& compressed data & decompressed data
L6 data L6 data
To session layer From session layer
APPLICATION LAYER
The application layer enables the user, whether human or software, to
access the network.
It provides user interfaces and support for services.
Application – Browsers, Skype Messenger, etc.
FUNCTIONS OF APPLICATION LAYER:
Network virtual terminal: It allows a user to log on to a remote host.
File transfer, access, and management(FTAM): This application allows a
user to access file in a remote host, retrieve files in remote host and manage
or
control files from a remote computer.
Mail services: Provide email service.
Directory services: This application provides distributed database
sources
and access for global information about various objects and services.
APPLICATION LAYER
User
User
Application
X.500 FTAM X.400 layer X.500 FTAM X.400
L7 data L7 data
To presentation layer From presentation layer
