OSI MODEL
OSI model is defined and is used to understand how data is transferred from one computer to
another in a computer network
In the most basic form two computers are connected to each other with LAN cables and connectors
sharing data with help of network interface cards forms network. But if one computer is based on
ms windows and the other one has mac OS installed , then how these computers are going to
communicate with each other.
In order to accomplish sucessful communication between computers or
networks of different architectures, 7 layered OSI model was introduced by
ISO IN 1984 containing 7 layers. Each layer is a package of protocols
1. Application layer :Application layer is used by network applications. Network application
means computer application that use internet like Chrome, Firefox,
Outlook,Skype,etc… Web browser is a network application in your PC, it
does not reside in the application layer but it use its application layer
protocols HTTP or HTTPS to do web browsing.
There are dozens of
application layer protocols that enable various functions at this layer. All
these application layer protocols are collectively known as Application
layer.
These protocols form the basis for various network services
like File transfer,web surfing,email,Virtual terminals,etc…
File transfer ---- FTP
web surfing ------ HTTP/S
Email -------------- SMTP
Virtual terminals --- TELNET
Application layer provides services for network application by
the help of protocols to perform user activities
2. Presentation layer ::It receives data from application layer.This data in the form of
characters and numbers. This layer converts these characters and
numbers machine understandable binary format.This function of
presentation layer is called Translation.
Before data is transmitted this layer reduces the number of
bits that are used to represent original data. This bit reduction
process is called Data compression.It also reduces the amount of
OSI MODEL
place used to stored the original file. As the file is reduced, it can
be received at the destination in a very less time.
To maintain integrity of data, before transmission data is
encrypted. Encryption enhances the security of data. At sender
side data is encrypted and at receiver side data is decrypted.
Secure Sockets layer(SSL) protocol is used for encryption and
decryption.
3. Session layer
Session layer helps in setting up and managing
connections,enabling sending and receiving of data followed by
termination of connections or sessions. Session layer has its
helpers called APIs (Application Programming Interfaces) which
allows applications on different computers to communicate with
each other.
Just before a connection is establish to the server,server
performs a function called Authentication. Authentication is a
process of verifying who you are?. For this server uses its user
name and password,once enterd user name and password are
match a connection is established between your computer and
server. After authenticating the user authorization is checked.
Authorization is the process used by server to determine if you
have permission to access a file. Authentication and authorization
both performed by session layer
Session layer keeps a track on the file being downloaded. It
organizes as the data should come under the same data type (like
image packet should come under image and text packet should
come under text) This process is called Session management.
4. Transport layer::It controls the reliability of communication through
segmentation, flow control, error control.
a) Segmentation: The data received from session layer is divided
into small data units called segments. Each segment contains a
source and destination port number and a sequence number.
OSI MODEL
Port number helps to direct each segment to the correct
application. Sequence number helps to reassemble the
segment in correct order to form correct message at receiver.
b) Flow control::Transport layer controls the amount of data being transmitted.
Consider we connected mobile to the server, server can send
data at speed of 100 MBps but phone can receive only at speed
of 10 MBps. If the server giving data at 50 MBps , the phone by
making use of transport layer sends a message to the server to
slow down the speed to 10 MBps.
c) Error control::If some data does not arrive the destination, transport
layer uses its Automatic Repeat Request scheme to retransmit
the lost or corrupted data. A group of bits called Checksum is
added each segment by the transport layer to find out the
received corrupted segment
Transport layer protocols
Transmission control protocol(TCP) – has feedback, slow
User datagram protocol (UDP) -- no feedback, fast
Transport layer perform services :::
1. Connection- oriented transmission(uses TCP)
2. Connectionless transmission(uses UDP)
5. Network layer::It receives segments from transport layer. This layer works for
the transmission of received segments from one computer to
another located in different networks. Data units in the network
layer are called packets. It is the layer where to decide
OSI MODEL
The function of network layer are Logical addressing, Routing,
Path determination.
a) Logical addressing ::- IP addressing done in network layer is
called Logical addressing. Every computer in a network has a
unique IP address. Network layer assigns sender and receiver IP
address to each segment to form an IP packet. IP address are
assigned to ensure that each data packet can reach the correct
destination.
b) Routing::- It is the method of moving data packet from source
to destination and it is based on the logical address format IPv4
and IPv6.
c) Path determination::- The process of choosing the best path for
delivery of data packets from source to destination is called
path determination. It uses protocols OSPF,BGP,IS-IS.
6. Data link layer::It receives data packets from network layer. Data packets
contains IP addresses of sender and receiver. There are 2 kinds of
addressing 1. Logical addressing 2. Physical addressing
Logical addressing is done at network layer. Physical addressing is
done in data link layer where mac address of sender and receiver
are assigned to each data packet to form a frame.
Data link layer used for 1. Access data 2. Controls how data is
placed and received from the media(media access control and
error detection)
7. Physical layer::Physical layer converts received data from data link layer into
signals and transmit over local media. At the receiver physical
layer receives signal convert into bits and pass into data link layer
as frames and frames again decapsulated and passes to the higher
layers. Finally data is moved to application layer . Application layer
protocols makes the sender message visible in the receiver
computer screen