Open Systems Interconnection LAYERS
The open Systems Interconnection model (OSI) is a conceptual model that standardizes and characterizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open systems Interconnection project at the International Organization for Standardization (ISO).
The OSI reference model has seven layers. These were decided on the basis of following principles:
- A layer should be created where a different level of abstraction is needed.
- Each layer should perform a well-defined function
- The laser boundaries should be chosen to minimize the information flow across the interface.
OSI model itself is not a network architecture. It does not specify the exact services and protocols to be used in each layer. It just tell what each laser should do. It provides a basis for coordinating the development of a standard that would facilitate a flexible interconnection of systems.
The seven layers of OSI model are:
- Physical layer
- Data link layer
- Network layer
- Transport layer
- Session layer
- Presentation layer
- Application layer
| S.No. | Layer | Description | Device | Protocol |
| 1 | Physical | Transmits data over physical medium | Multiplexer and repeater, hub, Amplifier, Oscilloscope | IEEE802, IEEE802.2, ISO2110, ISDN |
| 2 | Data link | Packages raw bits into frames and includes a cyclical redundancy check (CRC) | Switch, bridge and brouter, NIC | LIC |
| 3 | Network | Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management | Router and Brouter | IP, IPX, NW Link, NetBEUL |
| 4 | Transport | Repackages messages into smaller formats, provides error free delivers and error handling functions | Gateway | NetBEUI, TCP, SPX and NW Link, ARP, RARP, NETBios |
| 5 | Session | Allows two applications to communicate over a network by opening a session and synchronizing the involved computers | Gateway | NetBios, RPC |
| 6 | Presentation | Performs protocols conversion, encryption and data compression | Gateway and redirectors | AFP, TDI, SMB, NCP |
| 7 | Application | Provides network access for applications, flow control and error recovery | Gateway | SMTP, ETP, SNMP, Telnet, Appletalk, DNS,TFTP, MINE, LOGIN, NFS, BOOTP |
