OSI MODEL AND ITS LAYERS FUNCTION

OSI stands for Open Systems Interconnection. It
has been developed by ISO – ‘International
Organization of Standardization‘, in the year 1984.
It is a 7 layer architecture with each layer having
specific functionality to perform. All these 7 layers
work collaboratively to transmit the data from one
person to another across the globe.

1. 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.

The functions of the physical layer are :
Bit synchronization: The physical layer provides
the synchronization of the bits by providing a clock.
This clock controls both sender and receiver thus
providing synchronization at bit level.
Bit rate control: The Physical layer also defines the
transmission rate i.e. the number of bits sent per
second.
Physical topologies: Physical layer specifies the
way in which the different, devices/nodes are
arranged in a network i.e. bus, star or mesh topolgy.

Transmission mode: Physical layer also defines the
way in which the data flows between the two
connected devices. The various transmission modes
possible are: Simplex, half-duplex and full-duplex

2. 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 DLL to transmit it to the Host using
its MAC address.
Data Link Layer is divided into two sub layers :
Logical Link Control (LLC)
Media Access Control (MAC)

The functions of the data Link layer are :
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.
Physical addressing: After creating frames, Data
link layer adds physical addresses (MAC address) of
sender and/or receiver in the header of each frame.

Error control: Data link layer provides the
mechanism of error control in which it detects and
retransmits damaged or lost frames.
Flow Control: The data rate must be constant on
both sides else the data may get corrupted thus , flow
control coordinates that amount of data that can be
sent before receiving acknowledgement.
Access control: When a single communication
channel is shared by multiple devices, MAC sub-layer
of data link layer helps to determine which device has
control over the channel at a given time

3. Network Layer (Layer 3) :
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.

4. Transport Layer (Layer 4) :
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.

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.

5. Session Layer (Layer 5) :
This layer is responsible for establishment of
connection, maintenance of sessions, authentication
and also ensures security.

The functions of the session layer are :
Session establishment, maintenance and
termination: The layer allows the two processes to
establish, use and terminate a connection.
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.

Dialog Controller : The session layer allows two
systems to start communication with each other in
half-duplex or full-duplex

6. Presentation Layer (Layer 6) :
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.

The functions of the presentation layer are :
Translation : For example, ASCII to EBCDIC.
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.

7. Application Layer (Layer 7) :
At the very top of the OSI Reference Model stack of
layers, we find Application layer which is
implemented by the network applications. These
applications produce the data, which has 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.

The functions of the Application layer are :
Network Virtual Terminal
FTAM-File transfer access and management
Mail Services
Directory Services

OSI MODEL AND ITS LAYERS FUNCTION

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OSI MODEL AND ITS LAYERS FUNCTION