The OSI Reference Model

The Open System Interconnection (OSI) reference model was first developed by the International Organization for Standardization in 1977. (The OSI model is based on an IBM model developed in 1974). The OSI model is not a regulation. It is simply a guide that is commmonly referred to but in practise the OSI model is rarely followed.

The model consists of seven layers and each layer corresponds to a different function, like error contol. The highest layer (the application layer) is closest to the uer; the lowest layer (the physical layer) is closest to the media. The lower two layers are implemented in hardware and software, while the upper five layers are implemented in software. Each layer interacts with the layers adjoining it.

One of the best methods for understanding the OSI model is to remember that each layer adds functionality and this functionality normally corresponds to additional administrative information being glued onto the front of the user data.

The OSI layers defined

OSI Layer Function Example
Application Provide special network
access to applications
HTTP and FTP
Presentation Compression and Encryption Secure Socket Layer (SSL)*
Session** Maintaining Connections Netbios
Transport Insure data delivery TCP, SPX and NetBEUI
Network Direct data to address IP, IPX and UDP
Data Link Transfer data units and error checking*** ATM, Ethernet and Token Ring
Physical Transfer ones and zeros Cables and network devices

* The Secure Socket Layer (SSL) is not really an example of the presentation layer at this time. SSL is currently implemented at the application layer but there are proposals to implement it at the presentation layer.

** Currently the session layer of the OSI model is not implemented on the Internet.

*** The error checking that occurs at the data link layer is related to making sure that the data did not get garbled in tranmission.

The Decline and Fall of the OSI model

As noted above, the OSI model is based on work done at IBM in 1974. The OSI model is becoming very dated and its effectivness will quickly diminish with the rise of the new version of the IP protocol (IPv6). Many protocols have extended beyond a single OSI layer. The original versions of IP and UDP provide services which lie in both the network and transport layers. PPP works at both the Data Link and Network layers. Additionally the TCP protocol resides partly in the transport layer and partly in the session layer. IPv6 will include support for data security, which is done at the presentation layer in the OSI model. Therefore IPv6 will provide services which lie in three different layers, which are not contiguous. As a result of this IPv6 does not fit into the OSI model very well.