The open systems interconnection (OSI) model was developed by the International Organization for Standardization (ISO) as a model for a computer communications systems architecture and as a framework for developing protocol standards. It consists of seven layers:
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
* The function of the OSI layer:
Physical:
The physical layer is responsible for movements of individual bits from one hope (node) to the next.
Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristics to access the physical medium.
Fig: Physical layer
The functions of the physical layer protocol are as given below:
1) Physical characteristics of interfaces and medium
2) Representation of bits
3) Data rate
4) Synchronization of bits
5) Line configuration
6) Physical topology
7) Transmission mode.
Data Link:
The data link layer is responsible for moving frames from one hop (node) to the next. Provides for the reliable transfer of information across the physical link; sends blocks of data (frames) with the necessary synchronization.
Fig: Data link layer
The functions of the data link layer protocol are as given below:
1) Framing
2) Physical addressing
3) Flow control
4) Error control
5) Access control
Network:
The network layer is responsible for the delivery of individual packets from the source host to the destination host.
Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections.
Open systems interconnection
The functions of the network layer are as given below:
1) Logical addressing
2) Routing
Transport:
The transport layer protocol is responsible for the delivery of a message from one process to another.
Fig: Network layer
Fig: Transport layer
* The functions of the transport layer are as given below:
1) Provides reliable, transparent transfer of data between end points
2) Provides end-to-end error
3) Recovery
4) Flow control
5) Error control.
Session:
The session layer is responsible for dialog control and synchronization.
Fig: Session layer
* The functions of the session layer are as given below:
1) Provides the control structure for communication between applications
2) Establishes, manages, and terminates connections (sessions) between cooperating applications
3) Dialog
4) Synchronization.
Presentation:
The presentation layer is responsible for translation, compression and encryption.
Fig: Presentation layer
The functions of the presentation layer are as given below:
1) Provides independence to the application processes from differences in data representation (syntax).
2) Translation
3) Compression and
4) Encryption.
Application:
The application layer is responsible for providing services to the user. Provides access to the OSI environment for users and also provides distributed information services
Fig: Application layer
The functions of the application layer are as given below:
1) Network virtual terminal
2) File transfer, access and management
3) Mail services
4) Directory.
