Layers of OSI Model | OSI Model layers | Network layer | Transport layer

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Layers of OSI Model 

The seven Layer architecture of ISO-OSI reference model.it define seven levels or layers in a coplete communication system,the lowest kayer is physical layer and the highest one is called as the application layer.In the OSI model all the application needs not to use all the seven layers,the lower three layer enough for most of the applicationi, OSI framewok  show how the computer networking function,and the how data transfer flow from server to the client. there are following layers of the OSI model -

  • Application Layer
  • Presentation Layer
  • Session Layer
  • Transport Layer
  • Network Layer
  • Data Link Layer
  • Physical Layer
OSI Model diagram.jpg

Physical Layer ( layer 1)

The Physical layer,the lowest layer of the OSI model, The Physical layer provides a way for the data to be transmitted in a network. Data is converted into a signal which is passed to an endpoint over a physical connection. The Physical layer is responsible for the procedures, mechanics, and the electricity required for operating.It describes the electrical/optical,mechanical,and functional interface to the physical medium,and carries the signals for all the higher layer.

The Physical Layer translates logical communications requests from the Data Link Layer into hardware-specific operations to affect transmission or reception of electronic signals.
osi model physical layer.jpg
physical layer

Data Link Layer ( DLL Layer 2 )
The Data Link Layer is the protocol layer which transfers data between adjacent
network nodes in a wide area network or between nodes on the same local area network segment .The data link layer provides error-free transfer of data frames from one node to another over the physical layer,allowing layers above it to assume virtually error-free transmission over the link.To do this,the data link layer provides:

  • Establishment and termination the logical link between two nodes.
  • Tells the termination node to "back-off" when no frame buffers are available.
  • Transmit/receives frames sequentially.
  • Provides/expects frame acknowledgments. Detect and recovers from errors that occurs in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Create and recognize frame boundries.
  • Check received frame for integrity.
  • Determines when the node "has the right" to use the physical medium.
osi model data link layer.jpg
data link layer

Network Layer ( Layer 3 )

The network layer control the operation of the subnet,deciding which physical path the data should take based on network condition,priority of service and other factor. It provides :

  • Routes frames among networks
  • Routers can isntruct a sending to "throttle back" its frame transmission when the router buffers fills up.
  • It is determines that downstream routers maximum transmission unit size is less then the frame size,a router can fragment a frame for transmission and re-assembly at the destination station.
  • Translate logical addresses,or names,into physical addesses.
  • Accounting functions to keep track of frames forwarded by subnet intermdiate system,to produce billing information.
osi model network layer.jpg
network layer


Transport Layer ( Layer 4 )

The Transport layer ensures that the message are delivered error-free in sequence,and with no losses or duplications.It relieves the higher layer protocols from any concern with transfer of data between then and their peers. The size and complexity of a transport protocol depends on the types of service i can get from the network layer.The transport layer provides:

  • Accept message from the layer about it,splits the message into smaller units,and passes the smaller units down to the network layer.The trasport layer at the destination station reasemble the message.
  • Provides reliable end-to-end messgae delivery with acknowledgements.
  • It tell the transmitting station to "back-off" when no messge buffer are available
  • Multiplexes several messages streams,or session onto one logical links and keeps tracks of which messages belongs to which session.

Typically transport layer can accept relatively large messgae,but there are strict messgae size limits imposed  by the network layer.

osi model transport layer.jpg
transport layer


Session Layer ( Layer 5 )

The Session layer is responsible for setting up communication between nodes. The Session layer responds to service requests from the Presentation layer  as well as sending service requests to the Transport layer. The Session layer may also provide access control services, authentication, data synchronization, and other services.The session layer allow session establishment between processes running of different stations. It provides:

  • Session establishment,maintain and termination:allow two application processes on different machies to establish,use and terminate a connection, called  a session.
  • Session support: perfrom the fnction that allow these processes to communicate over the network,performing security,name recognition and so on.
osi model session layer.jpg
session layer

Presentation Layer ( Layer 6 )

The presentation layer formats the data to be presented to the application layer.It can be viewed as the translator for the network.This layer may translate data from a format use by the application layer into common format at the sending station,then translate the common format to a known to the application layer at the receiving station.
The presentation layer provides:

  • Character code translation : for example,ASCII to EBCDIC
  • Data conversion bit order,CR-CR/LF,integer-floating point and so on.
  • Data compression : reduce the number of bits that needs to be transmitted on the network.
  • Data encryption : encrypt data for security purpose.
osi model presentation layer.jpg
presentation layer

Application Layer ( Layer 7 )

The Application layer contains the operating systems that enable application programs to interface with the network. This layer serves application processes that the network uses. The application layer serves as the window for users and application processes to access network service.This layer contains a variety of commanly needed functions.

Example 1 : Sending an e-mail — The Application layer defines the protocols used in an e-mail transmission, but not the interface that the end user has to initiate in order to send the e-mail.

  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging(such as mail)
  • Network virtual terminals.
osi model application layers.jpg
application layer


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