Open Systems Interconnection Reference Model

The OSI (Open Systems Interconnection) Reference Model is a formal model of computer networking promulgated in the mid-80s by the Internatonal Standards Organisation. It is widely used in teaching about networks and commonly referred to by people from the telephony industry.

However, it has almost nothing to do with real computer networking today, which is almost entirely based on the Internet Protocol. The Internet antedates the OSI effort and does not conform to the model. There have been an enormous series of rather complex arguments in which the most extreme positions were something like "The Internet is an irrelevant academic toy; real networking must be done with a solid formal model" and "OSI got it wrong by ignoring the Internet (the only proof of concept that what they were standardising was even possible). The best we can hope for is that it fails before doing too much damage."

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.

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)

End-to-end transfer of data is the responsibility of protocols that reside on the Transport layer. There are transport layer protocols that provide both reliable and unreliable end-to-end transfer, and both have legitimate applications.

Newer protocols in this area, however, are being developed in the IETF, not ISO, in such applications as voice over IP and streaming video.

One significant difference between OSI transport and IETF tranport is that since IETF has no transport layer, TCP has certain features (e.g., graceful close) that OSI assigned to the session layer. Further, OSI Transport classes 0-3 have no real IETF equivalent, but are various levels of shims intended to work over a reliable, connection-oriented X.25-based network layer.

Physical devices on this layer include:

Network protocols categorized on this layer include:

In the OSI suite, the network data transfer protocols were the Connectionless Network Protocol (CLNP), and the Connection-oriented Network Service (CONS), the latter being extension of X.25.

In the IETF model, the only data transfer protocol is the Internet Protocol (IP), Internet Protocol version 4 for many years, and now Internet Protocol version 6. There are also layer management protocols such as the Internet Control Message Protocol, Internet Group Management Protocol, and dynamic routing protocols, and IPv6 variants such as Neighbor Discovery.

 

Layer 3 (Network)

Correct routing of packets happens on the Network layer.

Physical devices on this layer include:

• Routers can be standalone devices that must also contain layer 1 and 2 functions, or hosts that also implement layers 1 to 3. Devices that do network protocol encapsulation, for compatibility (e.g., Generic Route Encapsulation) or security (e.g., IPSec Tunnel Mode) also may do "layer 3.5". Stateful firewalls and network address translators often go into the transport layer.

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 bridges

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

Network protocols categorized on this layer include:

• Raw voltages that represent bits
• Repeaters