The Open Systems Interconnection Reference Model

(OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. It is also called the OSI seven layer model.

Purpose

The OSI model divides the functions of a protocol into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behavior consisting of a series of these layers is known as a 'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or a mixture of both. Typically, only the lower layers are implemented in hardware, with the higher layers being implemented in software.

This OSI model is roughly adhered to in the computing and networking industry. Its main feature is in the interface between layers which dictates the specifications on how one layer interacts with another. This means that a layer written by one manufacturer can operate with a layer from another (assuming that the specification is interpreted correctly.) These specifications are typically known as Request for Comments or "RFC"s in the TCP/IP community. They are ISO standards in the OSI community.

Usually, the implementation of a protocol is layered in a similar way to the protocol design, with the possible exception of a 'fast path' where the most common transaction allowed by the system may be implemented as a single component encompassing aspects of several layers.

This logical separation of layers makes reasoning about the behaviour of protocol stacks much easier, allowing the design of elaborate but highly reliable protocol stacks. Each layer performs services for the next higher layer, and makes requests of the next lower layer. An implementation of several OSI layers is often referred to as a stack (as in TCP/IP stack).

The OSI reference model is a hierarchical structure of seven layers that defines the requirements for communications between two computers. The model was defined by the International Standards Organization. It was conceived to allow interoperability across the various platforms offered by vendors. The model allows all network elements to operate together, regardless of who built them. By the late 1970's, ISO was recommending the implementation of the OSI model as a networking standard; unfortunately, TCP/IP had been in use for years. TCP/IP was fundamental to ARPANET and the other networks that evolved into the Internet. Only a subset of the whole OSI model is used today. It is widely believed that much of the specification is too complicated and its full functionality has taken too long to implement, although there are many people that strongly support the OSI model.

Description of layers

Physical layer Layer 1: The physical layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices.

The major functions and services performed by the physical layer are:

• establishment and termination of a connection to a communications medium.
  
• participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
  
• modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level.

Data link layer Layer 2: The Data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC and Aloha for local area networks. This is the layer at which bridges and switches operate. Connectivity is provided only among locally attached network nodes.

Network layer Layer 3: The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The router operates at this layer -- sending data throughout the extended network and making the Internet possible, although there are layer 3 (or IP) switches. This is a logical addressing scheme - values are chosen by the network engineer. The addressing scheme is hierarchical.

Transport layer Layer 4: The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is TCP.

Session layer Layer 5: The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. This layer is responsible for setting up and tearing down TCP/IP sessions.

Presentation layer Layer 6: The Presentation layer relieves the Application layer of concern regarding syntactical differences in data representation within the end-user systems. MIME encoding, encryption and similar manipulation of the presentation of data is done at this layer. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

Application layer Layer 7, the highest layer: This layer interfaces directly to and performs common application services for the application processes. The common application services provide semantic conversion between associated application processes. Examples of common application services include the virtual file, virtual terminal (for example, Telnet), and "Job transfer and Manipulation protocol" (JTM, standard ISO/IEC 8832).

The OSI model in the real world

Real-world protocol suites often do not strictly match the seven-layer model. There can be some argument as to where the distinctions between layers are drawn; there is no one correct answer. However, most protocol suites share the concept of three general sections: media, covering layers 1 and 2; transport, covering layers 3 and 4, and application, covering layers 5 through 7.

The DoD model, developed in the 1970s for DARPA, is a 4-layer model that maps closely to current common internet protocols. It is based on a more "pragmatic" approach to networking than OSI.

Strict conformance to the OSI model has not been a common goal in real-world networks, in part because of the negative view of the OSI protocol suite. Andrew Tanenbaum argues in his popular textbook Computer Networks ISBN 0130661023 that the failure of the OSI suite to become popular was due to bad timing, bad technology, bad implementations, and bad politics. The timing was bad because the model was finished only after a significant amount of research time and money had been spent on the TCP/IP model. The technology is "bad" because the session and presentation layers are nearly empty, whereas the data link layer is overfilled. Early implementations were notoriously buggy and in the early days, OSI became synonymous with poor quality, whereas early implementations of TCP/IP were more reliable. Finally, the politics were bad because TCP/IP was closely associated with Unix, making it popular in academia, whereas OSI did not have this association.

An early implementation of the TP4 transport protocol was so inefficient that packet transmission took longer than the connection time-out used by TCP/IP. The X.400 email protocols were stupendously complex, and independent implementations of the standard were unable to actually send email to each other. Ultimately, the biggest cost of OSI's failure was the delay it caused in the adoption of internet infrastructure in Europe and Japan, a cost that no one today would dare to calculate. (Finland opted to install TCP/IP instead of investing in OSI).

Having said all that, the model is still the general reference standard for nearly all networking documentation. All networking phrases referring to numbered layers, such as "layer 3 switching", refer to this OSI model.

Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system-specific). For example, Microsoft Windows' Winsock and Unix's Berkeley sockets and System V Streams are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).