OSI model:-

OSI (Open System Interconnection) model is developed by ISO (International Standards Organization) to describe
the flow of information from one computer to another. It is an ISO standard that covers all the aspects of network
communications.

OSI model is also called OSI Reference model because it deals with connecting open systems,i.e System that are
open for communication with other system.OSI model has seven layers.

Physical, Data Link, Network, Transport, Session, Presentation and Application Layers are the seven layers of OSI
model. Each layer performs well defined functions.

Physical Layer:-

It is the first or bottom most layer of the OSI model. This layer is used to establish or terminate a connection to a
communication medium. It also defines the electrical and mechanical specification like cables, connectors and
signaling option of the medium.
Physical layer receives data from the upper layer called the data link layer. It converts the received data into bit
stream. The data is then transmitted through the medium to the reciever.at receiving end. Physical layer receive the
data in bit format .it forwards the data to data link layer.

Responsibilities of Physical layer:

1).Characteristics of Media:-it defines the characteristics of the interface which is used for connecting devices. It
also defines the type of transmission media such copper wore or fiber optic cables.
2).Encoding:-it defines the encoding type. Encoding means changing bit stream (0s and 1s) into signal. Before
transmission. Physical layer encode the signal into electrical or optical form depending upon the media.
3).Transmission Rate:- it defines transmission rate of bits.TR is Number of bits transferred per second.
4).Transmission Mode:-it defines the direction of flow of data between two connected devices.
Simplex, half duplex and full duplex modes are its types.
5).Topology:-if defines how the devices are connected to form network.
Data Link Layer:-

it is the second layer of OSI model. It handles the data transfer between network and physical layers. The data unit
at data link layer is called as frame.

Data link layer receives data from network layer .it adds header and trailer to the data and passes the data to the
physical layer. At receiving side, data link layer receiver the data from the physical layer. It detaches the header and
trailer from the data and passes the data to network layer.

Responsibilities of Data Link Layer:

1).Framing:-data link layer divides the stream of bits received from network layer into manageable data units called
frames.
2).Physical addressing:- Data link layer adds header to the frame which contains the physical address of sender
/receiver.
3).Access Control: - When two or more devices in network use the same link for communication. At a time only one
device should transmit the data. Data link layer provide the access to use the link at a given time.
4).Error Control:-error control detects and corrects the errors. If a frame is lost or corrupted, the data link layer
retransmits that frame. Error control is accomplished by trailer at the end of the frame.
Network Layer:-

It is the third layer of the OSI model. The data unit at this layer is known as packet. Network layer is essential for
providing the source to destination delivery of packets.

Network layer receive data from the transport layer .it adds header to the data and passes the data to data link
layer. At the receiver side, network layer receives the data from data link layer. It detaches header from the data
and passes data to transport layer
Responsibilities of the Network Layer:-

1).Logical Addressing:-Network Layer adds the header to data that includes the logical address (IP address) of the
source and destination. It is a 32-bit address that uniquely identifies the device connected to network.
2).Routing:- it is a process where in a proper path is defined for packets to reach the destination. Routing is of two
types, Static or dynamic. In static routing, Route to be followed by a packet is already set by Network administrator.
In dynamic routing, route is decided at the time of transmission of packet.
3).Internetworking:- it means connecting two or more computers networks together. Internet is the best example
of internetworking.
Transport Layer:-

It is the fourth layer of OSI model. The data unit at this layer is called segments. The network layer deals with source
to destination delivery of individual packets transport layer ensures that the entire message reaches in order and
handles error control and flow control at source to destination level.

It receives data from the session layer. It divides the data(message) into segments .each segment has its own header
and passes these segments to network layer .At the receiving side, transport layer receive packets from the network
layer. It detaches the header from them and reassembles the segments into original message and passes that
message to session layer.

Responsibilities of Transport layer:-

1).Service-Point Addressing:-Transport layer include s the service point address (Port address) in the header. Using
these port address, transport layer delivers the packets to the correct process.
2).Segmentation and Reassembly:-A message is divided into segments, with each segment having a sequence
number.whe n message arrives at the destination, with the help of sequence number; transport layer reassembles
the message correctly.
3).Flow Control:-Transport layer is responsible for flow control which is performed end to end.
4).Error checking:-Transport layer also performs error checking. Entire message arrives at the receiving transport
layer without error.
Session Layer:-

It is the fifth layer of OSI Model. It establishes, manages, synchronizes and terminates connection between the
computers. It is network dialog controller.

Session layer receives data from the presentation layer. It adds header and synchronization bits to the data and
passes the data to transport layer. At the receiver side, session layer receives data from transport layer .it detaches
header and synchronization bits from the data and passes data to the presentation layer.

Responsibilities of Session Layer:-

1).Dialog Control:-it allows two devices to enter into a dialog. This dialog can take place either in half duplex or full
duplex mode.
2).Synchronization:-At session layer, checkpoints are added into a stream of data to synchronize the session. If a
device is sending a file of 100 pages then u can insert checkpoints after every 10 pages to ensure that 10 pages are
received without any error.
Presentation Layer:-

It is the sixth layer of the OSI model. It deals with syntax and semantics of data exchange between the devices.

Presentation layer receives data from the application layer. It adds header and passes the data to session layer. At
the receiver side, presentation layer receives data from session layer .it detaches header and passes data to the
presentation layer.

Responsibilities of Presentation Layer:-

1).Translation:-process of converting the information from one format to another.

At sending device, presentation layer changes the information from sender dependent format to the common
format that is accepted by other device. At the receiving device, this layer changes the information from common
format to receiver dependent format.
2).Encryption:-it also adds security to the data by encrypting it.enrption is a process in which data is converted into
a form that prevents unauthorized users from reading.
3).Compression:-it refers to a process of encoding the data using less number of bits. This process reduces the
numbers of bits used to represent the information.
Application Layer:-

It is seventh layer of the OSI model. It provides a means for user to access the information on the network using an
application. it provide the user interface and support for the services like emails, remote file access and transfer etc.

Services Provided by Application Layer:-

1).Network Virtual Terminal:-it is a software version of the physical terminal. Using this software, user can log on to
a computer that is remotely connected on the network.
2).File Transfer, Access and Management (FTAM):-User can access the files in a remote host. it also enables the
user to retrieve and manage the files from a remote computer.
3).Mail Service:-Using this application, emails can be forwarded to another device. Message handling System is the
OSI protocol used to transfer mails over the internet.
4).Directory Services:-A directory is a source of information .this information is managed by the directory services.