The Model

​In late '80s, Eurocontrol did an inventory of the zoo of air traffic control automation used by European countries. A total of 35 systems were running hardware from 18 computer manufacturers, 22 operating systems and applications written in 33 programming languages. There was limited flight data exchange between neighboring systems, using ad-hoc agreements, mostly established at the political level, rather than technical.

They say: all models are wrong, but some are useful! Among them, the Open Systems Interconnection (OSI) Model deserves a particular recognition because it was a conceptual model of system interconnection built at a time when the "as-is" situation was almost out of control. Surprisingly, the OSI Model is still relevant!

People Do Not Throw Salami Pizza Away! (use spinach instead of salami, if you are a vegetarian) is a funny mnemonic for those seven layers, bottom to top.

I dare to list a few examples, below, but don't quote me, please! The border line is blurred for particular implementations. As always, the devil is in the details.

Nowadays, the bottom layers are mostly hardware devices.

• Physical: hubs, repeaters, cables, fibers, wireless

• Data: bridges, modems, network cards, layer-2 switches

• Network: routers, layer-3 switches, X25

The top layers are usually software, or a mix of both.

• Transport: TCP, UDP

• Session: XWindow, DNS, SQL (I guess!)

• Presentation: encryption, PNG, MIDI

• Application: HTTP, FTP, SNMP

The concept is simple: each layer invokes services from the one below and talks to its remote peer(s), as ruled by a standard communication protocol.

Back then, in '84, X.25 was very cool. Not that new, but ISO was close to publish their standard based on CCITT recommendation and everyone was coming onboard. My team was implementing the network layer as a software service, on top of a network card. X.25 is a synchronous communication protocol, which means that data can be sent when the clocks of the two modems are synchronized and the line stays hot as long as the Layer 3 connection was in place. Exchanging empty frames when the transport layer has no data units to be sent is rather expensive. X.25 is still used in real-time applications, such as distributing radar surveillance data.