Open Systems Interconnection (OSI) reference model

Definition of Open Systems Interconnection (OSI) reference model in The Network Encyclopedia.

Open Systems Interconnection (OSI) reference model

OSI - An architectural model for open networking systems that was developed by the International Organization for Standardization (ISO) in Europe in 1974. The Open Systems Interconnection (OSI) reference model was intended as a basis for developing universally accepted networking protocols, but this initiative essentially failed for the following reasons:
  • The standards process was relatively closed compared with the open standards process used by the Internet Engineering Task Force (IETF) to develop the TCP/IP protocol suite.
  • The model was overly complex. Some functions (such as connectionless communication) were neglected, while others (such as error correction and flow control) were repeated at several layers.
  • The growth of the Internet and TCP/IP—a simpler, real-world protocol model—pushed the OSI reference model out.

The U.S government tried to require compliance with the OSI reference model for U.S. government networking solutions in the late 1980s by implementing standards called Government Open Systems Interconnection Profiles (GOSIPs). This effort was abandoned in 1995, however, and now few real-world implementations of OSI networking protocols exist outside of Europe.

The OSI reference model is best seen as an idealized model of the logical connections that must occur in order for network communication to take place. Most protocol suites used in the real world, such as TCP/IP, DECnet, and Systems Network Architecture (SNA), map somewhat loosely to the OSI reference model. The OSI model is a good starting point for understanding how various protocols within a protocol suite function and interact.

The OSI reference model has seven logical layers, as shown in the following table.

The OSI Reference Model


Layer Number Layer Name Description
7
Application layer
Interfaces user applications with network functionality, controls how applications access the network, and generates error messages. Protocols at this level include HTTP, FTP, SMTP, and NFS.
6
Presentation layer
Translates data to be transmitted by applications into a format suitable for transport over the network. Redirector software, such as the Workstation service for Microsoft Windows NT, is located at this level. Network shells are also defined at this layer.
5
Session layer
Defines how connections can be established, maintained, and terminated. Also performs name resolution functions.
4
Transport layer
Sequences packets so that they can be reassembled at the destination in the proper order. Generates acknowledgments and retransmits packets. Assembles packets after they are received.
3
Network layer
Defines logical host addresses such as IP addresses, creates packet headers, and routes packets across an internetwork using routers and Layer 3 switches. Strips the headers from the packets at the receiving end.
2
Data-link layer
Specifies how data bits are grouped into frames, and specifies frame formats. Responsible for error correction, flow control, hardware addressing (such as MAC addresses), and how devices such as hubs, bridges, repeaters, and Layer 2 switches operate. The Project 802 specifications divide this layer into two sublayers, the logical link control (LLC) layer and the media access control (MAC) layer.
1
Physical layer
Defines network transmission media, signaling methods, bit synchronization, architecture (such as Ethernet or Token Ring), and cabling topologies. Defines how network interface cards (NICs) interact with the media (cabling).

You can think of each layer as being logically connected to the same layer on a different computer on the network. For example, the application layer on one machine communicates with the application layer on another machine. But this communication is logical only; physical communication occurs when packets of data are sent down from the application layer of the transmitting computer, encapsulated with header information by each lower layer, and then put on the wire at the physical layer of the transmitting computer. After traveling along the wire, the packets are picked up by the physical layer of the receiving computer, passed up the seven layers while each layer strips off its associated header information, and then passed to the application layer of the receiving computer, where the receiving application can process the data.

OSI

OSI is an acronym of open systems interconnection and it is the reference model to all network architecture.


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