Computer Notes: OSI Model Functions and Layers

OSI Model - With Detail Explanation

Introduction of OSI Models

OSI model stands for Open System Interconnection model. It is a seven layered structure which specifies the requirements for the communication between two computers. This model is defined by the ISO (International Standard Organization). This model allows all of the network elements to function together. 

Features of OSI Model

• It takes into account the data flow from physical connections up to the end user application.
• The OSI model defines the “layered Architecture” as a protocol stack. Each layer is assigned to perform some particular works
• When two systems communicate, the information is sent through the protocol stack in the network.

Layers of OSI Model
The OSI model has 7 layers which are divided into two types on the basis of their features. The layers are –

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

• Host Layer- It provides the accurate data delivery between the computers. The upper four layers i.e. Application, Presentation, Session and Transport layers are considered to be host layer. The functions of the upper four layers must interoperate with the upper four layers of the system with whom it is communicating.

• Media Layers- It controls the physical delivery of messages over the network. The lower three layers i.e. Network, Data Link and Physical Layers are considered as Media Layer.

Functions of the Layers of OSI Model

Application Layer-

• It is the highest layer of OSI model
• It provides the services to the user of OSI environment
• It serves as an interface between the user and the network as it provides the services which supports the user applications directly.
• It supports for file transfer.
• It provides the network services like electronic mail, electronic messaging, terminal emulation etc.
• It is used to browse the World Wide Web.
• Application Layer Protocols- SMTP, FTP, TELNET etc.

Presentation Layer- 

• This layer helps in data translation. In this process the data are interpreted and converted from various formats.
• The layer enables the recipient computer to understand the data sent from the source.
• It reduces the volume of data so that the transfer can be completed in less time. The destination computer returns the information to its original format before passing to the Application layer.
• It allows the data to be converted be the data encryption process to make it secure
• Presentation Layer Protocols- SMB, NCP, NFS.

Session Layer-

• This layer establishes, maintains and terminates a connection named as Session.
• The Session is the exchange of messages between computers.
• The synchronization of data flow is controlled by this layer.
• It partitions the services into small functional groups, send the acknowledgements of data received and re transmit data if it is not received
• Session Layer Protocols- SQL, ZIP, RPC, NFS.

Transport Layer-

• It helps in flow control to eliminate the problem of overrun of memory.
• It performs the error handling of the messages.
• It establishes and maintains both ends of the virtual circuits. 
• Transport Layer Protocols- TCP, UDP.

Network Layer-

• It helps in addressing the best paths for the movement of data avoiding the bottleneck condition.
• It helps in routing of the data packets. 
• Network Layer Protocols- IP, IPX.

Data Link Layer-

• This layer provides physical addressing, network topology, error notification, and flow control of the data.
• It provides the reliable transfer of information across the media.

Physical Layer-

• It is the lowest layer of the OSI model.
• It helps to transmit data between two machines communicating through the physical mediums.