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The five layers reference model
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the Internet layer which is equivalent to the network layer of our reference model
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the Link layer which combines the functions of the physical and datalink layers of our five-layer reference model
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The Network layer
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The OSI reference model
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The Physical layer
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the Physical layer may change, e.g. due to electromagnetic interference, the value of a bit being transmitted
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the Physical layer may deliver `fewer` bits to the receiver than the bits sent by the sender
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the Physical layer may deliver `more` bits to the receiver than the bits sent by the sender
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The reference models
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There is now a rough consensus for the greater use of the Unicode_ character format. Unicode can represent more than 100,000 different characters from the known written languages on Earth. Maybe one day, all computers will only use Unicode to represent all their stored characters and Unicode could become the standard format to exchange characters, but we are not yet at this stage today.
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The TCP/IP reference model
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the Transport layer
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The Transport layer
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The upper layer of our architecture is the `Application layer`. This layer includes all the mechanisms and data structures that are necessary for the applications. We will use Application Data Unit (ADU) or the generic Service Data Unit (SDU) term to indicate the data exchanged between two entities of the Application layer.
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This is an unpolished draft of the third edition of this e-book. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/CNP3/ebook/issues?milestone=3 or help us by providing pull requests to close the existing issues.
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transport layer entities exchange *segments*
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Two reference models have been successful in the networking community : the OSI reference model and the TCP/IP reference model. We discuss them briefly in this section.
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