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OSI is a theoretical model, so why are its concepts used to describe real-world networks?

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  • does this answer your question? networkengineering.stackexchange.com/questions/6380/…
    – This
    Aug 22 '14 at 12:38
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 9 '17 at 2:35
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Because the concepts defined by OSI model are useful for understanding real-world networks.

OSI defines 7 layers, these layers (especially layer 5/Session and 6/Presentation) don't match well with the now dominant well-known telecom protocols (above and around IP) - the model should not be blamed for this. After all we can't say that the IP protocol stack is particularly well designed.

The model defines a lot of very useful concepts, and distinctions to be made, in the analysis/understanding/design of any complex telecom system.
For example: the notions of service and protocol; the notion of primitive (request, confirm, indication, response, signal); the notion of protocol entity and Service Access Point; the notion of connection vs. association; the notions of SDU (Service Data Unit), PDU (Protocol Data Unit), IDU (Interface Data Unit); etc. These concepts are extremely useful.

Also, OSI model is still used heavily to define current protocols.
For example ISO 11073-20601 and friends (Point-of-care medical device communication), used in medical devices (typically over Bluetooth) heavily use OSI modelling of Application Layer. Or the IEEE 802.xxx lower layer protocols (e.g. 802.15.4, radio protocol) - these use the ISO modelling of the lower layers. These protocols are recently designed and in use today.

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