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.

Advantages of OSI Model

The OSI Model defines the communication of a computing system into 7
different layers. Its advantages include:
 It divides network communication into 7 layers which makes it easier to
understand and troubleshoot.
 It standardizes network communications, as each layer has fixed
functions and protocols.
 Diagnosing network problems is easier with the OSI model.
 It is easier to improve with advancements as each layer can get updates
separately.

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498

Physical Layer – Layer 1
The lowest layer of the OSI reference model is the physical layer. It is
responsible for the actual physical connection between the devices. The
physical layer contains information in the form of bits. It is responsible for
transmitting individual bits from one node to the next. 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.
Note:
1. Hub, Repeater, Modem, and Cables are Physical Layer devices.
2. Network Layer, Data Link Layer, and Physical Layer are also known
as Lower Layers or Hardware Layers.

Data Link Layer (DLL) – Layer 2

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 the DLL to transmit it to the
Host using its MAC address.

 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.
This can be accomplished by attaching special bit patterns to the
beginning and end of the frame.
Note:
 Packet in the Data Link layer is referred to as Frame.
 Data Link layer is handled by the NIC (Network Interface Card) and
device drivers of host machines.
 Switch & Bridge are Data Link Layer devices.

Network Layer – Layer 3

The 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 addresses are placed in the
header by the network layer.
Note:
 Segment in the Network layer is referred to as Packet.
 Network layer is implemented by networking devices such as routers
and switches.

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498

Transport Layer – Layer 4
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.

Services Provided by Transport Layer

 Connection-Oriented Service
 Connectionless Service

Connection-Oriented Service:
It is a three-phase process that includes
1. Connection Establishment
2. Data Transfer
3. Termination/disconnection

Connectionless service: It is a one-phase process and includes Data

Transfer. In this type of transmission, the receiver does not acknowledge
receipt of a packet. This approach allows for much faster communication
between devices. Connection-oriented service is more reliable than
connectionless Service.

 Data in the Transport Layer is called Segments.

 Transport layer is operated by the Operating System. It is a part of
the OS and communicates with the Application Layer by making
system calls.
 The transport layer is called as Heart of the OSI model.
 Device or Protocol Use : TCP, UDP NetBIOS, PPTP

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498

Session Layer – Layer 5
This layer is responsible for the establishment of connection, maintenance
of sessions, and authentication, and also ensures security.
Device or Protocol Use : NetBIOS, PPTP.

Presentation Layer – Layer 6

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.
 Encryption/ Decryption: Data encryption translates the data into
another form or code. The encrypted data is known as the cipher text
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.
Note: Device or Protocol Use: JPEG, MPEG, GIF

Application Layer – Layer 7

At the very top of the OSI Reference Model stack of layers, we find the
Application layer which is implemented by the network applications. These
applications produce the data to be transferred over the network. This layer
also serves as a window for the application services to access the network
and for displaying the received information to the user.
Example: Application – Browsers, zoom, Messenger, etc.
Note: The application Layer is also called Desktop Layer.

Device or Protocol Use : SMTP

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498

How data flows through the OSI Model
In order for human-readable information to be transferred over a network from
one device to another, the data must travel down the seven layers of the OSI Model

on the sending device and then travel up the seven layers on the receiving end.

For example: Mr. Hassan wants to send Ms. Maria an email. Mr. Hassan
composes his message in an email application on his laptop and then hits ‘send’.
His email application will pass his email message over to the application layer,
which will pick a protocol (SMTP) and pass the data along to the presentation
layer. The presentation layer will then compress the data and then it will hit the
session layer, which will initialize the communication session.

The data will then hit the sender’s transportation layer where it will be
segmented, then those segments will be broken up into packets at the network
layer, which will be broken down even further into frames at the data link layer.
The data link layer will then deliver those frames to the physical layer, which will
convert the data into a bit stream of 1s and 0s and send it through a physical
medium, such as a cable.

Once Ms. Maria’s computer receives the bit stream through a physical medium
(such as her Wi-Fi), the data will flow through the same series of layers on her
device, but in the opposite order. First the physical layer will convert the bit
stream from 1s and 0s into frames that get passed to the data link layer. The data
link layer will then reassemble the frames into packets for the network layer. The
network layer will then make segments out of the packets for the transport layer,
which will reassemble the segments into one piece of data.

The data will then flow into the receiver's session layer, which will pass the data
along to the presentation layer and then end the communication session. The
presentation layer will then remove the compression and pass the raw data up to
the application layer. The application layer will then feed the human-readable
data along to Ms. Maria’s email software, which will allow her to read Mr.
Hassan’s email on her laptop screen

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498

 OSI Model – Layer Architecture
Information
Layer Name Responsibility
Layer Form Device or
No (Data Unit) Protocol

Helps in identifying the client

Application
Layer
and synchronizing Message SMTP
7 communication.

Data from the application layer

Presentation is extracted and manipulated in JPEG,
Message
Layer the required format for MPEG, GIF
6 transmission.

Establishes Connection,
Maintenance, Ensures Encryption
Session Layer Gateway
Authentication and Ensures compression
5 security.

end-to-end delivery of the

complete message.
acknowledgment of the
Transport Layer successful data Segment Firewall
transmission and re-
transmits the data if an error
4 is found

Transmission of data from one

Network Layer host to another, located in Packet Router
3 different networks.

Node to Node Delivery of

Data Link Layer Frame Switch, Bridge
2 Message.

Hub, Repeater,
Establishing Physical
Physical Layer Bits Modem,
Connections between Devices.
1 Cables

MUHAMAMD WAQAS Coordinator Computer department APSACS 0312-5506498