The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model. It divides network communication into seven layers. ... Layers 5-7, called the the upper layers, contain application-level data #imannjeet #mannjeet mn

1. Open Systems Interconnection (OSI) Model
• Open System Interconnection Model (OSI) was developed
by ISO (International Standard Organization) in 1983, for
sending and receiving of data, between two computers.
• OSI represents a concept of intel process
communication so that any open system may be
able to communicate with another open system.
• The OSI architecture decomposes the communication
process into functional layers. Each layer is responsible for
performing special functions.
• The OSI is a theoretical blueprint that helps us
understand how data gets from one user’s computer to
another.
• It is also a model that helps develop standards so that all
of our hardware and software talks nicely to each other

2. Destination
Application Layer
OSI Layers in Arranged
in a Hierarchy
Network Process to applications
Data Representation
Interhost Communication
End to End Communication
Address and Best Path
Access to Media
Binary Transmission
Session Layer
Presentation Layer
Transport Layer
Network Layer
Physical Layer
Data Link Layer
Application Layer
Transport Layer
Presentation Layer
Session Layer
Network Layer
Data Link Layer
Physical Layer
Source
Transmission
Medium
Application Layer
Transport Layer
Presentation Layer
Session Layer
Network Layer
Data Link Layer
Each Layer is package of protocol

3. Application Layer
• The application Layer enables the user, either humun or software access the
network. It provides end user for processing of data and supports for services
such ase-mail, file transfer, shared data base management, network software
services and other types of distributed information service.
This layer acts as
an interface end
user and
network. This
layer mainly
allows access to
network
resources.
Application Layer

4. Presentation Layer
• Presentation layer is concerned with syntax and semantics
of the information
exchanged between two systems
• It translates the application into network format and vice
versa. It provides format and encrypt/ decrypt data to be
send across a network
Presentation Layer

5. Session Layer
Session Layer
 Allows establishment of sessions between machines, e.g. to
 allow remote logins
 provide file transfer service.
 Responsible for:
 dialogue control
 which entity sends when with half-duplex communications.
 token management
 E.g. control which entity can perform an operation on shared data.
 synchronisation
 E.g. insertion of checkpoints in large data transfers.

6. Transport Layer
The transport layer is responsible for source to destination delivery
of theentire message.
This layer concerts data into smaller “segments” for sending and at the
receiving end the segments are converted into original data. This layer is
also responsible for error control and flow control.
Transport Layer

7. Network Layer
This layer converts data segments into packets and at the
receiving end, the packets are converted into data segments.
Network Layer Perform 3 Functions
• Logical Addressing
• Routing
• Path Determination
Network Layer
Border Getaway Protocol
Open Shortage Path first
Intermediate system to intermediate
system

8. Data Link Layer
This layer converts data packets from network Layer into
frames at the receiving end, the packets are converted into
data segments.
The data link layer divides the stream of bits received from
the network layer into manageable data units called frames.
If frames are to be distributed to different system on the
network. The data link layer adds a header to the frame to
define the physical address of the sender and receiver of the
frame.
Data link layer receives the data to be sent from the network
layer, adds header and trailer to it which is now known as a
frame. This frame is then transmitted to the physical layer.
The data link layer can be devided into two
sub-layers. They are :
1. LLC(Logicsal Link Control)
2. MAC (Medium Access Control)
It is used for error control &
flow control
In LAN, it decides who can
send data , when and how
much

9. Data
(Packets) Trailer Data Header
T Data
DestinationSource
Fig: Role of Data Link Layer
Data link layer
Frame
Network layer
To physical layer
Source address : AA Destination address : ZZTrailer Header
error detection Source address
Trailer conains extra bits needed for Destination address
Fig: Frame format of a Datalink layer
9

10. Physical Layer
The physical layer is responsible for transmitting raw bits over a
communication channel.
It converts frames from data link layer into bits and at the
receiving end, bits from the physical layer is given to the
data link layer.
For sending raw bits from source to destination, to do this, the
source anddestination
nodes have to agree on a number of factors such as what
voltage constitute a bit value ‘0’ what voltage constitute a bit
value’1’, what is the bit interval, whether the communication is
in only one direction or both the directions simultaneously i.e.,
simplex, half duplex or full duplex and so on. It also deals with
the electrical specifications of the cables, connectors and
interfaces such as RS232.
Functions of Physical Layer:
1. Signal encoding
2. Medium
3. Bit Synchronization
4. Transmission Byte
5. Transmission Mode
6. Multiplexing
Physical Layer

11. Data in frames Data in frames
101010
(bits)
101010
(bits)
Sender Receiver
Data link
layer
Physical
Layer
Communication channel
Fig: Relation of Physical Layer with Communication channel
Data link
layer
Physical
Layer

12. Short Notes
Supports application and end – user processes.
Provides application service for the file transfers, e-mail and other network
softwareservices
Creates logical paths for transmitting data from node to node. Includes
switching, routing and forwarding.
Packets are encoded and decoded into bits. Teo sub layers. Media Access
Control (MCA) Logical Link Control (LLC).
Conveys the bit stream – electrical light or radio signal through the network
at the electrical and mechanical level.
Establishes, manages and terminates connections
between applications. Data with session and connection coordination
Responsible for end-to-end error recovery and flow control. Ensures
complete data transfer.
Translates from application to network format and vice versa. Format and
encrypts/decrypts data to be send across a network.
Physical Layer
Application Layer
Transport Layer
Presentation Layer
Session Layer
Network Layer
Data Link Layer
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