OSI model

"[2] In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model.

As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework.

In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO).

[5][6] The Experimental Packet Switched System in the UK c. 1973–1975 identified the need for defining higher-level protocols.

The British Department of Trade and Industry acted as the secretariat, and universities in the United Kingdom developed prototypes of the standards.

[10] The OSI model was first defined in raw form in Washington, D.C., in February 1978 by French software engineer Hubert Zimmermann, and the refined but still draft standard was published by the ISO in 1980.

It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software.

The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Systems.

For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks.

[23] Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach.

Each PDU contains a payload, called the service data unit (SDU), along with protocol-related headers or footers.

Data processing by two communicating OSI-compatible devices proceeds as follows: The OSI model was defined in ISO/IEC 7498 which consists of the following parts: ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200.

This includes the layout of pins, voltages, line impedance, cable specifications, signal timing and frequency for wireless devices.

As a result, common problems occurring at the physical layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol.

A number of layer-management protocols, a function defined in the management annex, ISO 7498/4, belong to the network layer.

This means that the transport layer can keep track of the segments and retransmit those that fail delivery through the acknowledgment hand-shake system.

Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication.

Streaming media, real-time multiplayer games and voice over IP (VoIP) are examples of applications in which loss of packets is not usually a fatal problem.

Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries.

Detailed characteristics of TP0–4 classes are shown in the following table:[29] An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent.

While Generic Routing Encapsulation (GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at the endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete Layer 2 frames or Layer 3 packets to deliver to the endpoint.

The presentation layer establishes data formatting and data translation into a format specified by the application layer during the encapsulation of outgoing messages while being passed down the protocol stack, and possibly reversed during the deencapsulation of incoming messages when being passed up the protocol stack.

[32] For this reason, the presentation layer negotiates the transfer of syntax structure through the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP.

This correspondence is rough: the OSI model contains idiosyncrasies not found in later systems such as the IP stack in modern Internet.

This made implementation difficult and was resisted by many vendors and users with significant investments in other network technologies.

[48][page needed] Although the OSI model is often still referenced, the Internet protocol suite has become the standard for networking.

TCP/IP's pragmatic approach to computer networking and to independent implementations of simplified protocols made it a practical methodology.