Open Systems Interconnection Reference Model
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:
- The Transmission Control Protocol (the 'TCP' in TCP/IP, the protocol in use on the Internet)
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:
- The Internet Protocol (the 'IP' in TCP/IP)
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:
- Ethernet MAC (Media Access Control) addresses
- LLC (Logical Link Control)
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