Open Systems Interconnection Reference Model: Difference between revisions

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The '''OSI 7-layer model''', also known as the ISO OSI model, is a [[communication]] [[Protocol (computer)|protocol]] standard (for [[computer network]]s) produced by the International Standards Organization in the early days of networking (need exact reference).  Its ''Open System Interconnect'' model attempts to define all the aspects of a [[Computer network]] [[architecture]] that are likely ever to be needed.


Throughout the 1980's, the OSI 7-layer model competed with the 3-layer TCP/IP protocol, which originated from an RFC of the IETF (in the United States).  The telecommunications industry adopted the OSI 7-layer model as the standard of choice, but compliance by companies was slow due to the added expense of the 7-layer model over the simpler (and well tried) TCP/IP protocol.


Eventually, industry moved almost completely to the simpler TCP/IP model despite many theoretical objections.  Nowadays, the OSI 7-layer model is considered to be mainly an "abstract" definition, in that there are few if any "live" implementations.  However, the model is widely taught in networking classes as an ideal to strive for.


==Purpose==
In each of these case, the software they use is the same, and it is (ideally) unimportant ''how'' one is  connected to the Internet. The level of abstraction introduced by the layered model described here is one reason that this is even possible.
The early days of [[Computer network|computer networks]] were dominated by a few large companies such as [[IBM]] and [[DEC]]. In order promote interoperability and avoid a situation where a small number of vendors predominated, each with their own proprietary technology, it was necessary to introduce a set of [[open standards]] defining network protocols.
==The abstractions of layers and relationships among them==
 
==Extensions to the basic model==
Another issue addressed by this model is maintaining the level of flexibility needed to adapt when new innovations are introduced. The earliest [[wide area networks]] (or WANs) ran over telephone lines and were used to link a small number of facilities.
While it may seem odd to recognize extensions before discussing the model itself, it is a practical necessity to understand the historical role of the OSI model, including the orientation of the networking specialists that first defined it. It is a necessity to understand that the basic 7-layer model, as typically taught, is an oversimplification of real networking, and this oversimplification was realized very early. The model extensions that dealt with correcting the oversimplifications, but these extensions are very rarely presented in teaching the model, a deficiency leading to unfortunate misunderstanding.
 
Today, we rarely think about why Internet access has become so ubiquitous. Still, this is quite a technical achievement: a user may be connected to an [[Ethernet]] network, FDDI (Fiber optic) ring or a wireless network in a coffee house. They may also use such diverse methods as DSL, cable, or dialup lines to "get online."


In each of these case, the software they use is the same, and it is (ideally) unimportant ''how'' one is connected to the Internet. The level of abstraction introduced by the layered model described here is one reason that this is even possible.
In the late 1970s, some of the first work done on this body was in a committee called Distributed Systems (DISY), under the auspices of the [[American National Standards Institute]]. The work soon become an international project, under a worldwide standards body now called the [[International Telecommunications Union]] (ITU). At the time the DISY work first went global, however, the ITU was called the [[International Consultative Committee for Telephony and Telegraphy]], with the French abbreviation [{CCITT]].  


CCITT had existed for many years, at a time where worldwide telephone systems were largely run by governments, or at least by approved national monopolies such as [[American Telephone and Telegraph]] (AT&T).  The CCITT view of the world was very centralized, and assumed that [[wide area network]]s would be of a form dictated by the national ministries of telephony and other communications called PTTs, or by authorized monopolies called [[Registered Private Operating Agencies]] (RPOA).
==Descriptions of the layers==
==Descriptions of the layers==




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In each of these case, the software they use is the same, and it is (ideally) unimportant how one is connected to the Internet. The level of abstraction introduced by the layered model described here is one reason that this is even possible.

The abstractions of layers and relationships among them

Extensions to the basic model

While it may seem odd to recognize extensions before discussing the model itself, it is a practical necessity to understand the historical role of the OSI model, including the orientation of the networking specialists that first defined it. It is a necessity to understand that the basic 7-layer model, as typically taught, is an oversimplification of real networking, and this oversimplification was realized very early. The model extensions that dealt with correcting the oversimplifications, but these extensions are very rarely presented in teaching the model, a deficiency leading to unfortunate misunderstanding.

In the late 1970s, some of the first work done on this body was in a committee called Distributed Systems (DISY), under the auspices of the American National Standards Institute. The work soon become an international project, under a worldwide standards body now called the International Telecommunications Union (ITU). At the time the DISY work first went global, however, the ITU was called the International Consultative Committee for Telephony and Telegraphy, with the French abbreviation [{CCITT]].

CCITT had existed for many years, at a time where worldwide telephone systems were largely run by governments, or at least by approved national monopolies such as American Telephone and Telegraph (AT&T). The CCITT view of the world was very centralized, and assumed that wide area networks would be of a form dictated by the national ministries of telephony and other communications called PTTs, or by authorized monopolies called Registered Private Operating Agencies (RPOA).

Descriptions of the layers

These are the Seven layers in the OSI Model (from highest to lowest):

Layer 7 (Application)

The Application Layer defines the services provided to user-visible programs, but neither the internal design of such programs nor the interface the programs provide to the end user. The programs seen by the end user reside on top of the Application layer, but are not in it.

Web browsers, such as Firefox or Internet Explorer, for example, use the Hypertext Transfer Protocol (HTTP), which is in the Application layer. Especially on UNIX and LINUX systems, there are command-line client programs for the telnet and ftp protocols, which, confusingly, are named "telnet" and "ftp" commands. The client programs, nevertheless, mediate between the user interface and the program interface to the application layer service.

Layer 6 (Presentation)

Such functions as encoding and decoding, encryption, converting to/from various character sets, and the transformation of data structures into and out of XML are examples of functions performed at the Presentation layer.

Layer 5 (Session)

Establishing and managing "sessions" between computers happens at the Session layer. An example is whether full or half duplex is used between two machines.

note: layers 5 through 7 traditionally consist of only protocols, not devices

Layer 4 (Transport)

Reliable transfer of data is the responsibility of protocols that reside on the Transport layer.

Physical devices on this layer include:

Network protocols categorized on this layer include:

 

Layer 3 (Network)

Correct routing of packets happens on the Network layer.

Physical devices on this layer include:

Network protocols categorized on this layer include:

 

Layer 2 (Data Link)

The Data Link layer has the "lowest" protocols (the next layer, the Physical layer, deals with actual bits and voltages).

Physical devices on this layer include:

Network protocols categorized on this layer include:

 

Layer 1 (Physical)

The Physical layer deals with the raw "bits" or voltages "on the wire."

Physical devices on this layer include:

  • Cabling such as Category 5 (CAT 5) cable
  • Network cards (NIC cards)
  • Hubs
  • Repeaters

Network protocols categorized on this layer include:

 

Related Topics

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