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I have a hard time understanding the difference between a service and a function of a layer in the OSI reference model. For instance is multiplexing/demultiplexing a service or a function of the transport layer and why?

sasuke_X220
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  • Which do *you* hypothesize it is? Justify your answer. (If nothing else this will likely make for better responses.) – user2864740 Aug 09 '17 at 00:04
  • I have read the definition of session multiplexing/demultiplexing on wikipedia. I don't have a real thought on that. – sasuke_X220 Aug 09 '17 at 00:08
  • The OSI model is just a model, and nothing in the real world follows it. The IP Services model is what was used for OSes, but it, too, is just a model, albeit much closer to what you find in the real world. – Ron Maupin Jan 08 '20 at 16:17

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A service has a function. A service is functionality available to an outside context; in the OSI case, services may be used by other layers. In the OSI context, services are synonymous with cross-layer function.

With layering, typically cross-layer usage (reference) is one-way ("down") and only with adjacent layers.

See also: https://en.wikipedia.org/wiki/OSI_model#Cross-layer_functions

Peter Ritchie
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