Installing A Workstation > OSI Reference Model

OSI Reference Model

This is not strictly part of a workstation installation, but it's a good idea to understand what a network is set up if you are to deal with it in any way.

I'll start off with a memory aid I learned somewhere:

Please Do Not Throw Sausage Pizza Away

It's not a bad idea unless the pizza gets stale and moldy. But it is in fact a help in remembering the OSI reference model used to design networks and the programs that run on them.

The OSI model envisions seven layers in which a network is supposed to work, and the memory aid lists them from the bottom. The first four layers handle how data is grouped together and transmitted.

Physical

This is the hardware level, where data exists as bits. Network cards as electronics, cables and hubs are here.

Data Link

Here the data is grouped first into bytes, and then into frames with the hardware (MAC) address of the network card from which the data comes. Switches and bridges, which are hub-like devices that keep data from colliding with each other, work at this level.

Network

Here data is grouped into packets with addresses to which they are to go. The Internet Protocol (IP) works at this level. Routers, which filter packets and connect networks, are here.

Transport

This level deals with how computers set up connections with each other, and the grouping of packets into segments to be sent from one computer to another. Transport Control Protocol (TCP) works at this level.

The last three layers deal with how data is used. These are generally dealt with by software.

Session

Here data is grouped, with each program's data kept separate from those of other programs. The database query language SQL is the best example of session-layer programs.

Presentation

In this layer data is formatted (RTF and XML for documents, JPG and PNG for images, MP3 and QuickTime for media), encrypted (PGP), compressed (PKWare and WinZip) and otherwise manipulated.

Application

At this highest layer the user works the data. Web browsers, e-mail and FTP clients, and electronic funds transfers are application-layer software.

A historical note: OSI was supposed to be the interconnection standard for all networks. However, as the product of "a heavily-politicized standards committee" with different countries tugging at it from every which way, it became bloated and complex and simply could not compete with TCP/IP, which won in the end. However, the model is still useful, which is why it still with us.