Open Systems Interconnection Reference Model
The OSI 7-layer model, also known as the ISO OSI model, is a communication protocol standard (for computer networks) 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
The early days of 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.
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.
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.
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