English Czech Actions
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 or help us by providing pull requests to close the existing issues.
The reference models
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]_.
The five layers reference model
Our reference model is divided into five layers, as shown in the figure below.
The Physical layer
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 Physical layer may change, e.g. due to electromagnetic interference, the value of a bit being transmitted
the Physical layer may deliver `more` bits to the receiver than the bits sent by the sender
the Physical layer may deliver `fewer` bits to the receiver than the bits sent by the sender
The Datalink layer
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.
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.
The Network layer
Component Translation Difference to current string
This translation Propagated Empty cnp3-ebook/principles/referencemodels
The following strings have the same context and source.
Propagated Empty cnp3-ebook/exercises/transport
Propagated Empty cnp3-ebook/principles/transport


User avatar None

New source string

cnp3-ebook / principles/referencemodelsCzech

New source string 4 years ago
Browse all component changes


English Czech
No related strings found in the glossary.

String information

Source string location
String age
4 years ago
Source string age
4 years ago
Translation file
locale/cs/LC_MESSAGES/principles/referencemodels.po, string 1