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1. OSI
MODEL
A brief presentation by Mahendra Subedi

2. OSI MODEL
● OSI stands for Open System Interconnection is a reference model that describes how information from a
software application in one computer moves through a physical medium to the software application in another
computer.
● OSI consists of seven layers, and each layer performs a particular network function.
● OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now
considered as an architectural model for the inter-computer communications.
● OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular
task.
● Each layer is self-contained, so that task assigned to each layer can be performed independently.

3. LAYER OF OSI
MODEL
There are the seven OSI layers.
Each layer has different function.
A list of seven layer are given
below.
1. Application Layer
2. Presentation
Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

4. How to Memories 7
Layers
“All People Seems To Need Data Progress”
Just took the first alphabet in the word in given
sentence
All - Application Layer
People - Presentation
Layer Seems - Session
Layer
To - Transport Layer
Need - Network Layer
Data - Data Link Layer

5. Application
Layer
An application layer serves as a window for users and
application processes to access network service.
It handles issues such as network transparency,
resource allocation, etc.
An application layer is not an application, but it
performs the application layer functions.
This layer provides the network services to the
end-users.
Functions of Application
layer:
1. Network Virtual Terminal
2. FTAM-File transfer access and
management
3. Mail Services
4. Directory Services

6. Presentation
Layer
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.
Functions of Presentation layer:
●
●
● Translation: The processes in two systems exchange the information in the form of character
strings, numbers and so on. Different computers use different encoding methods, the
presentation layer handles the interoperability between the different encoding methods. It
converts the data from sender-dependent format into a common format and changes the
common format into receiver-dependent format at the receiving end.
Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting
the sender-transmitted information into another form and sends the resulting message over
the network.
Compression: Data compression is a process of compressing the data, i.e., it reduces the
number of bits to be transmitted. Data compression is very important in multimedia such as
text, audio, video.

7. Session
Layer
This layer is responsible for establishment of connection, maintenance of
sessions, authentication and also ensures security.
The functions of the session layer are :
1. Session establishment, maintenance and termination: The layer allows
the two processes to establish, use and terminate a connection.
2. Synchronization : This layer allows a process to add checkpoints which
are considered as synchronization points into the data. These
synchronization point 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.
3. Dialog Controller : The session layer allows two systems to
start communication with each other in half-duplex or full-
duplex.

8. Transport
Layer
Transport layer provides services to application layer and takes services from 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 acknowledgement of the successful data transmission and re-transmits the data if an error is found.
• At sender’s side:
Transport layer receives the formatted data from the upper layers, performs Segmentation and also implements Flow
& Error control to ensure proper data transmission. It also adds Source and Destination port number in its header and
forwards the segmented data to the Network Layer.
• At receiver’s side:
Transport Layer reads the port number from its header and forwards the Data which it has received to the respective
application. It also performs sequencing and reassembling of the segmented data.

9. Transport
Layer
The functions of the transport layer are :
Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the message
into smaller units . Each of the segment produced has a header associated with it. The transport layer at the
destination station reassembles the message
Service Point Addressing: In order to deliver the message to correct process, transport layer header includes a
type of address called service point address or port address. Thus by specifying this address, transport layer
makes sure that the message is delivered to the correct process

10. Network
Layer
Network layer works for the transmission of data from one host to the other
located in different networks. 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 address are placed in the header
by the network layer.
The functions of the Network layer are :
Routing: The network layer protocols determine which route is suitable
from source to destination. This function of network layer is known as
routing.
Logical Addressing: In order to identify each device on
internetwork uniquely, network layer defines an addressing scheme.
The sender & receiver’s IP address are placed in the header by
network layer. Such an address distinguishes each device uniquely
and universally.

11. 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. When a packet arrives in a network, it is the
responsibility of DLL to transmit it to the Host using its MAC
address.
Data Link Layer is divided into two sub layers :
1. Logical Link Control (LLC)
2. Media Access Control (MAC)
○
○
○
● Logical Link Control Layer
○ It is responsible for transferring the packets to the Network layer
of the receiver that is receiving.
It identifies the address of the network layer protocol from the
header.
It also provides flow control.
● Media Access Control Layer
○ A Media access control layer is a link between the Logical Link
Control layer and the network's physical layer.
It is used for transferring the packets over the network.

12. Data Link Layer
●
●
●
●
● Framing: The data link layer translates the physical raw bit stream into packets known as Frames. The Data
link layer adds the header and trailer to the frame. The header which is added to the frame contains the
hardware destination and source address.
Physical Addressing: The Data link layer adds a header to the frame that contains a destination address. The
frame is transmitted to the destination address mentioned in the header.
Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through which
the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that the
transmitting station such as a server with higher processing speed does not exceed the receiving station, with
lower processing speed.
Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is
placed to the Data link layer's trailer which is added to the message frame before it is sent to the physical
layer. If any error seems to occurr, then the receiver sends the acknowledgment for the retransmission of the
corrupted frames.
Access Control: When two or more devices are connected to the same communication channel, then the data
link layer protocols are used to determine which device has control over the link at a given time.

13. Physical Layer
The main functionality of the physical layer is to transmit the individual bits from one node to another node.
It is the lowest layer of the OSI model.
It establishes, maintains and deactivates the physical connection.
It specifies the mechanical, electrical and procedural network interface specifications.
Functions of a 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.

14. THANK
YOU
Mahendra Subedi