OSI (Open Systems Interconnection):

*We are going to spend the next several days learning about various communication network equipment. Before we do that it will be helpful to understand the OSI Reference Model.

OSI (Open Systems Interconnection): A Reference Model developed by the ISO (International Organization for Standardization, as translated into English). The OSI Reference Model is the only internationally accepted framework of standards for communication between different systems made by different vendors. ISO's goal is to create an open systems networking environment where any vendor's computer system, connected to any network, can freely share data with any other computer system on that network or a linked network. Most of the dominant communication protocols used today have a structure based on the OSI model.

Although OSI is a model and not an actively used protocol, and there are still very few pure OSI-based products on the market today, it is still important to understand its structure. The OSI model organizes the communications process into seven different categories and places these categories in a layered sequence based on their relation to the user. Layers 7 through 4 deal with end to end communications between the message source and the message destination, while layers 3 through 1 deal with network access.

Layer 1 - The Physical Layer deals with the physical means of sending data over lines (i.e. the electrical, mechanical and functional control of data circuits). Examples include EIA-232 , T-Carrier and SONET.

Layer 2 - The Data Link Layer is concerned with procedures and protocols for operating the communications lines. It also has a way of detecting and correcting message errors. Examples include Frame Relay, PPP, and SLIP (Serial Line Internet Protocol). ATM runs at Layers 1 & 2, as do LANs.

Layer 3 - The Network Layer determines how data is transferred between computers. It also addresses routing within and between individual networks. The most visible example is IP (Internet Protocol).

Layer 4 - The Transport Layer defines the rules for information exchanges and manages end-to-end delivery of information within and between networks, including error recovery and flow control. TCP (Transmission Control Protocol) is an example, as is the OSI Transport Protocol (TP), which comprises five layers of its own. Layer 4 protocols ensure end-to-end integrity of the data in a session. The X.25 packet-switching protocol operates at Layers 1, 2, 3 and 4.

Layer 5 - The Session Layer is concerned with dialog management. It controls the use of the basic communication facility provided by the Transport layer. If you’ve ever lost your connection while Web surfing, you've likely experienced a session time-out, so you have some sense of the Session Layer.

Layer 6 - The Presentation layer provides transparent communications services by making the differences of varying data formats (character codes, for example) between dissimilar systems. Conversion of coding schemes (e.g., ASCII to EBDCIC to Unicode) and text compression and decompression exemplify Presentation Layer functions.

Layer 7 - The Applications layer contains functions for particular applications services, such as file transfer, remote file access and virtual terminal. TCP/IP application protocols such as FTP (File Transfer Protocol), Simple Mail Transfer Protocol (SMTP), SNMP (Simply Network Management Protocol) and TELNET (TELecommunications Network) take place at Layer 7.

The OSI Model is an important concept to understand, at least at a high level. I have attached one of Novell's Network Tutorials which goes into the OSI Model in more detail and also provide some very useful diagrams, including one which illustrates where different protocols operate in the OSI model. The tutorial can be found at the following link:

http://www.novell.com/info/primer/prim05.html.