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The ISO/OSI model
The OSI (Open System Interconnection) model was create in 1978 by ISO (International Organization Standardization) to create a standard for the communications between the computers.
It is constituted from a pile (stack) with 7 levels (5 in the simplify version); at each level corresponds a protocol, for the way two levels from the different systems can communicate; this happen virtually in a direct mode ignoring the others levels.
At the internal of the same system, each level can communicate just with the adjacent levels by an interface.
The simplify model levels
- Level 1 : physical. It is occupied to transmit the dates in the way physical (cable, air, optic fiber, etc.)
- Level 2 : Datalink. Prepares the dates to be send to the physical way.
- Level 3 : Net. It is useful to "
- Level 4 : Transport. Guarantees the wright dates reception, occupies of the retransmissions in case of errors and permits to establish the connexions.
- Level 5 : Application. It is the last level of the pile and like the name suggests is where reside the different services (HTTP, FTP, E-mail, etc.).
Encapsulation
In a net packet , the dates of the different levels are encapsulated one in another, like in a matriosca.
That it means:
- in the transmission phase each level sends the own dates to the inferior level which adds the own encapsulation (and queue) and then at its turn send it to the underneath level until reach the physical way;
- in the reception phase each level examines the own encapsulation and gives the dates to the superior level
To notice that the communication doesn't concern necessarily the last level, but it 's happen always between two equals levels.
This modulation permits, for example, to transport the same dates on the different physicals supports: you just need to think at the nets Ethernet and WiFi which both transport the TCP/IP packets, but on the different ways (cable and air).
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