OSI MODEL

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 OSI MODEL AND ITS LAYERS

OSI stands for Open System Interconnection Model which was developed by ISO which stands International Organization of Standardization in 1984.

This model has seven layers which have their individual roles which serve whole as interconnection model.

It has layer from the physical layer from bits to the higher level data representation over a distributed application.                  




     OSI model has seven levels which are as follows  :-
  1. Application layer.
  2. Presentation layer.
  3. Session layer.
  4. Transport layer.
  5. Network layer.
  6. Data layer.
  7. Physical layer. 

Following are the defining of the OSI layer:-

1. Physical Layer :- It is the lowest layer of OSI layer .The main work of this layer is to establish the physical connection of the devices .It transmit the data  in form of bits , from one  node to the next in the 1's and 0's . It send the data in bits to Data Link Layer .                                                               

Following are the Functions of Physical layer :-

1.Bit Synchronisation.
2.Bit Rate Control.
3.Physical Topologies .
4.Transmission Mode.

for example:- Hub,Repeater,Modem ,Cables.

2. Data Link Layer :-This is the second last layer of the model .This level has a  function, to made data to be error free and also to storing the MAC address of the Sender or the receiver in the frame (data is in the form in this layer).
This layer also control the flow of Data on both the sides which make it as efficient data transfers System.
Data Link Layer is handled by NIC (Network Interface Card).

For Example:-Switch and Bridge are commonly used.

3. Network Layer :- This layer has a work to receive the data from the Data Link Layer and to transmit it at the intended address which is stored in the frame (which had received from Data Link Layer) .It find address by the local address such as IP(Internet Protocol). 
Data in this layer termed as packets. Routers play a crucial role in this level.

4. Transport Layer :-In this layer data is in the form of segments and their is end to end delivery of  the complete message. It provide services to the Application layer and take services from the Network layer.

This layer is called as Heart of OSI model.

5. Session Layer :-This layer has a responsibility of establishment of connection , maintenance of sessions , authentication, and also ensures security.

Following are the Functions of Session layer :-

1.Session maintenance ,establishment ,termination.
2.Synchronisation.
3.Dialog controller.

6. Presentation Layer :- This layer is also called translation Layer. Encryption and Decryption of the message is done here . It also reduce the size of the message .

Following are the Functions of Presentation layer :- 

1. Translation :- ASCII TO EBCDIC.
2.Encryption/Decryption .
3.compression .

7. Application Layer :- This layer produces the Data which has to transferred over the network.This also acts as a window for accessing the application services to access the Network and displayed the information to the user.

Application Layer is also  called Desktop layer.

Following are the Functions of Application layer :-
 
1.Network Virtual Terminal.
2.FTAM-File Transfer Access and Management.
3.Mail Services .
4. Directory services. 



Physical Layer, Data Link Layer,Network Layer are all come under as Hardware layers or lower layers.
Transport layer coined as Heart of OSI model.
Session Layer, Presentation layer,Application Layer,  are all come as Software layers or upper layers.



PROTOCOLS USED IN NETWORK

There are various type of protocols used in networking some of them are as follows :

1.Transmission Control Protocol(TCP) used in Transport layer.
2.Internet Protocol(IP) used in Network layer.
3.User Datagram Protocol(UDP) used in Transport layer. 
4.Post Office protocol(POP) used in Application layer.
5.Simple Mail Transfer Protocol(SMTP) used in Application layer.
6.File Transfer Protocol(FTP) used in Application layer.
7.HyperText Transfer Protocol(HTTP) used in Application layer.
8.HyperText Transfer Protocol Secure(HTTPS) used in Application layer.
9.Dynamic Host Configuration Protocol(DHCP) used in Application layer.
10.Address Resolution Protocol(ARP) used in Data Link layer.
11.Reverse Address Resolution Protocol(RARP) used in Data Link layer.


These protocols are defining below:

1. Transmission Control Protocol(TCP) :-This protocol is a three way communication with acknowledge of the acceptance of data .It also break the data into packets before transmission.
It has some advantage of using ,it check of error in data, data reach at correct time and destination. It also has a feature of sending those packets which got lost during transmission.
It also arranged the packets while receiving in an ordered way(to formed data as same as before breaking).

It has a disadvantage of not using with the broadcast or multicast connections.

2. Internet Protocol(IP) :-This protocol is used to addressing the packets which are formed by TCP, to reach the correct destination.IP has a work of holding address and control information.It has two major parts :
  • Network Number.
  • Host Number.
Network Number identified a network and assigned by network while Host Number Search for its host and it is assigned by the network admin.

3. User Datagram Protocol(UDP) :-This protocol does not have a system of acknowledgement of acceptance and also for transmission. There is nothing in this to identified error, reliability, flow control.
There is no way to resending the lost packets in UDP.

It is used in broadcast and multicast connections and also fast from TCP.

4. Post Office protocol(POP) :-This is an email protocol. By this protocol ,user can download mails from the mailserver to their email client.  Once an email is downloaded they can be read without internet connection.Once the email is moved to locally the server free up its space .POP3 is the latest version of it.

If the local emails get lost or stolen , they are completely lost, as server removed them as soon as they moved locally for making free space .

5. Simple Mail Transfer Protocol(SMTP) :-This is protocol is used to send the mails from one system to another .POP and IMAP are used to retrieve them from end user side. It notify the user with the incoming emails.
It has ease of installation  and doesn't need anything from the user side.

Certain firewall block ports of SMTP .

6. File Transfer Protocol(FTP) :-This protocol is used to transfer files from host , locally and remote . It need two TCP connection :
  • control.
  • Data connection. 
Control connection is used to control information like username ,passwords while Data connection are used to transfer the files and these two process run parallely during the entire file transfer process.

7.HyperText Transfer Protocol(HTTP) :-This protocol is used in client-server model . Web browser act as client. Client requests the server and server give response the and after giving response server and client both forget about each other.

It is a stateless protocol means, the server and client know each other until the connection is intact.

8.HyperText Transfer Protocol Secure(HTTPS) :- This protocol is an extension of HTTP and this provides security and no one can take advantage between the server and the client as the message from server is going in an encrypted way and only the client know the decryption method. so third man cannot make its advantage .
It encryption is done by using TLS(Transport layer security) or SSL(secure socket layer).

9.Dynamic Host Configuration Protocol(DHCP) :-This protocol work on client-server model. It enables network administrators to assigned IP address to every device which is connecting to the network.It let them to distribute IP address and automatically send new IP address when device plugged in  from different place. 

It also reuse the IP address and reduced the number of IP address.

10.Address Resolution Protocol(ARP) :-This protocol help map IP address with the MAC address recognised in local network.This protocol is used in Correlation of IP and MAC address and helps convert addresses in both direction.

11.Reverse Address Resolution Protocol(RARP) :-This protocol is used to requests its IP address.It send the device physical address to RARP server on the same LAN and actively listening to the server requests.

               

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