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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.
An interesting historical discussion of the OSI-TCP/IP debate may be found in [Russel06]_
application layer entities exchange *SDUs*
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.
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 :
datalink layer entities exchange *frames*
Footnotes
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]_.
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. :
network layer entities exchange *packets*
Our reference model is divided into five layers, as shown in the figure below.
physical layer entities exchange bits
Reference models
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.
the Application layer
The Application layer
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.
The Datalink layer
The `Datalink layer` allows directly connected hosts to exchange information, but it is often necessary to exchange information between hosts that are not attached to the same physical medium. This is the task of the `network layer`. The `network layer` is built above the `datalink layer`. Network layer entities exchange `packets`. A `packet` is a finite sequence of bytes that is transported by the datalink layer inside one or more frames. A packet usually contains information about its origin and its destination, and usually passes through several intermediate devices called routers on its way from its origin to its destination.
The `Datalink layer` builds on the service provided by the underlying physical layer. The `Datalink layer` allows two hosts that are directly connected through the physical layer to exchange information. The unit of information exchanged between two entities in the `Datalink layer` is a frame. A frame is a finite sequence of bits. Some `Datalink layers` use variable-length frames while others only use fixed-length frames. Some `Datalink layers` provide a connection-oriented service while others provide a connectionless service. Some `Datalink layers` provide reliable delivery while others do not guarantee the correct delivery of the information.