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An important point to note about the `Datalink layer` is that although the figure below indicates that two entities of the `Datalink layer` exchange frames directly, in reality this is slightly different. When the `Datalink layer` entity on the left needs to transmit a frame, it issues as many `Data.request` primitives to the underlying `physical layer` as there are bits in the frame. The physical layer will then convert the sequence of bits in an electromagnetic or optical signal that will be sent over the physical medium. The `physical layer` on the right hand side of the figure will decode the received signal, recover the bits and issue the corresponding `Data.indication` primitives to its `Datalink layer` entity. If there are no transmission errors, this entity will receive the frame sent earlier.
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An important point to note about the Physical layer is the service that it provides. This service is usually an unreliable service that allows the users of the Physical layer to exchange bits. The unit of information transfer in the Physical layer is the bit. The Physical layer service is unreliable because :
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An interesting historical discussion of the OSI-TCP/IP debate may be found in [Russel06]_
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application layer entities exchange *SDUs*
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Besides this difference in the lower layers, the TCP/IP reference model is very close to the five layers that we use throughout this document.
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Compared to the five layers reference model explained above, the :term:`OSI` reference model defined in [X200]_ is divided in seven layers. The four lower layers are similar to the four lower layers described above. The OSI reference model refined the application layer by dividing it in three layers :
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datalink layer entities exchange *frames*
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Footnotes
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Given the growing complexity of computer networks, during the 1970s network researchers proposed various reference models to facilitate the description of network protocols and services. Of these, the Open Systems Interconnection (OSI) model [Zimmermann80]_ was probably the most influential. It served as the basis for the standardization work performed within the :term:`ISO` to develop global computer network standards. The reference model that we use in this book can be considered as a simplified version of the OSI reference model [#fiso-tcp]_.
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In contrast with OSI, the TCP/IP community did not spend a lot of effort defining a detailed reference model; in fact, the goals of the Internet architecture were only documented after TCP/IP had been deployed [Clark88]_. :rfc:`1122`, which defines the requirements for Internet hosts, mentions four different layers. Starting from the top, these are :
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In the remaining chapters of this text, we will often refer to the information exchanged between entities located in different layers. To avoid any confusion, we will stick to the terminology defined earlier, i.e. :
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network layer entities exchange *packets*
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Our reference model is divided into five layers, as shown in the figure below.
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physical layer entities exchange bits
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Reference models
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Starting from the bottom, the first layer is the Physical layer. Two communicating devices are linked through a physical medium. This physical medium is used to transfer an electrical or optical signal between two directly connected devices.
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the Application layer
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The Application layer
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the `Application layer` that contains the mechanisms that do not fit in neither the Presentation nor the Session layer. The OSI Application layer was itself further divided in several generic service elements.
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The Datalink layer
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