What is the open systems interconnection model?
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The OSI Model by International Organization for Standardization (ISO).
Wikipedia link on OSI model: https://en.wikipedia.org/wiki/OSI_model. The different layer explanation is based off this work. The various authors do a very good job of sharing knowledge.
The diagram below is the OSI model maybe additional information added as and when required.
The Layers.
Physical - Layer 1
https://en.wikipedia.org/wiki/Physical_layerData Link - Layer 2
https://en.wikipedia.org/wiki/Data_link_layerNetwork - Layer 3
https://en.wikipedia.org/wiki/Network_layerTransport - Layer 4
https://en.wikipedia.org/wiki/Transport_layerSession - Layer 5
https://en.wikipedia.org/wiki/Session_layerPresentation - Layer 6
https://en.wikipedia.org/wiki/Presentation_layerApplication - Layer 7
https://en.wikipedia.org/wiki/Application_layerThe Unit Of Transmission (numbered on diagram from 1 - 7)
The explanation of the different units of transmission is best explained using the Protocol Data Unit (PDU) The segment (datagram), packet, frame, and bit are explained in this link: https://en.wikipedia.org/wiki/Protocol_data_unit. Depending on the type of protcol, The Layer 4 - Transport layer PDU is either a segment or datagram, e.g. TCP is a segment and UDP is a datagram
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