Open System Interconnection model is foundation for all communications that take place between computers. The main purpose of this model is to allow communication between different systems.
Example: Apple Macintosh is directly communicate with any other device like IBM computers with the help of OSI model.
There are seven layer in this model.
1. Physical Layer
2. Data link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Seven Layers are organized in subgroups :
Network Support Layers :
 Physical Layer
 Data Link Layer
 Network Layer

User Support Layer

 Session Layer
 Presentation Layer
 Application Layer

Data transmission Layer:

 Transport layer
Layers in OSI Model

Physical Layer: This layer is responsible for actual physical connection between the devices. Such as physical connection may be made by using twisted pair cable, fiber optic, coaxial cable or wireless communication media.

Functions of Physical Layer

 Transmission bits into signals.
 Bit synchronization
 Bit rate control
 Line Configuration

Data Link Layer: Data link layer is responsible for converting data stream into signals and send over underlying hardware. At the receiving end, Data link layer picks up data from physical layer in the form of electrical signals and hands over to network layer.

Functions of Data Link Layer

 Framing
 Addressing
 Error control
 Flow control

Network Layer: This layer is responsible for convert the data into packet and also responsible for routing packets from source to destination within and outside the subnet.

Functions of Network Layer:

 Routing
 Logical Addressing

Transport Layer: Transport layer offers peer-to-peer and end-to-end connection between two remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and then breaks it into smaller size segments.

Main Protocol of Transport Layer:

 Transmission Control Protocol
 User Datagram Protocol

Creating a connection involves three steps:

 Connection establishment
 Data transfer
 Connection Release

Functions of Transport Layer

 Segmentation of message into packet and reassembly of packets into message.
 Connection management
 Service point addressing
 Flow control
 Error control

Session Layer: Session layer is fifth layer of OSI model.
 Session layer has the primary responsibility of beginning, maintaining and ending the communication between two devices, which is called session.

Functions of Session Layer
 Establishment, maintaining and ending a session.
 Dialog control
 Dialog separation
Presentation Layer: Presentation layer is sixth layer of OSI model. This layer is concerned with syntax and semantics of information exchanged between devices.
Functions of Presentation Layer:
 Data presentation and translation
 Data encryption
 Data compression

Application Layer: This layer is topmost layer of OSI model.
It enables the user, whether humans or software, to access the network by providing user interface. This layer provide services that directly support user application such as database access, e-mail and file transfer.
Functions of Application Layer:
 Network Virtual Terminal
 File transfer, access and management
 Mail services

Designing a network in form of layer offers several benefits.
 The various networking functions such as error detection, encryption, flow control, routing, multiplexing can be isolated into modules.
 Separating the various functions into different modules make design process simpler and easier.
 Such modules also enables us to do research and development on each layer independently from the other layers.