msgid "" msgstr "" "Project-Id-Version: Czech (cnp3-ebook)\n" "Report-Msgid-Bugs-To: \n" "POT-Creation-Date: 2026-04-19 15:43+0200\n" "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n" "Last-Translator: FULL NAME \n" "Language-Team: Czech \n" "Language: cs\n" "MIME-Version: 1.0\n" "Content-Type: text/plain; charset=UTF-8\n" "Content-Transfer-Encoding: 8bit\n" "Plural-Forms: nplurals=3; plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;\n" "X-Generator: Weblate 5.14.3\n" #: ../../principles/referencemodels.rst:8 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:12 msgid "The reference models" msgstr "" #: ../../principles/referencemodels.rst:14 msgid "" "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]_." msgstr "" #: ../../principles/referencemodels.rst:19 msgid "The five layers reference model" msgstr "" #: ../../principles/referencemodels.rst:21 msgid "" "Our reference model is divided into five layers, as shown in the figure " "below." msgstr "" #: ../../principles/referencemodels.rst:32 #: ../../principles/referencemodels.rst:49 msgid "The Physical layer" msgstr "" #: ../../principles/referencemodels.rst:34 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:38 msgid "" "the Physical layer may change, e.g. due to electromagnetic interference, the " "value of a bit being transmitted" msgstr "" #: ../../principles/referencemodels.rst:39 msgid "" "the Physical layer may deliver `more` bits to the receiver than the bits " "sent by the sender" msgstr "" #: ../../principles/referencemodels.rst:40 msgid "" "the Physical layer may deliver `fewer` bits to the receiver than the bits " "sent by the sender" msgstr "" #: ../../principles/referencemodels.rst:54 #: ../../principles/referencemodels.rst:65 msgid "The Datalink layer" msgstr "" #: ../../principles/referencemodels.rst:56 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:58 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:69 msgid "The Network layer" msgstr "" #: ../../principles/referencemodels.rst:73 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:85 msgid "The Transport layer" msgstr "" #: ../../principles/referencemodels.rst:99 #: ../../principles/referencemodels.rst:109 msgid "The Application layer" msgstr "" #: ../../principles/referencemodels.rst:103 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:114 msgid "" "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. :" msgstr "" #: ../../principles/referencemodels.rst:116 msgid "physical layer entities exchange bits" msgstr "" #: ../../principles/referencemodels.rst:117 msgid "datalink layer entities exchange *frames*" msgstr "" #: ../../principles/referencemodels.rst:118 msgid "network layer entities exchange *packets*" msgstr "" #: ../../principles/referencemodels.rst:119 msgid "transport layer entities exchange *segments*" msgstr "" #: ../../principles/referencemodels.rst:120 msgid "application layer entities exchange *SDUs*" msgstr "" #: ../../principles/referencemodels.rst:125 msgid "Reference models" msgstr "" #: ../../principles/referencemodels.rst:128 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:132 msgid "The TCP/IP reference model" msgstr "" #: ../../principles/referencemodels.rst:136 msgid "the Application layer" msgstr "" #: ../../principles/referencemodels.rst:137 msgid "the Transport layer" msgstr "" #: ../../principles/referencemodels.rst:138 msgid "" "the Internet layer which is equivalent to the network layer of our reference " "model" msgstr "" #: ../../principles/referencemodels.rst:139 msgid "" "the Link layer which combines the functions of the physical and datalink " "layers of our five-layer reference model" msgstr "" #: ../../principles/referencemodels.rst:141 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:146 msgid "The OSI reference model" msgstr "" #: ../../principles/referencemodels.rst:148 msgid "" "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 " ":" msgstr "" #: ../../principles/referencemodels.rst:152 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:162 msgid "Footnotes" msgstr "" #: ../../principles/referencemodels.rst:163 msgid "" "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." msgstr "" #: ../../principles/referencemodels.rst:165 msgid "" "An interesting historical discussion of the OSI-TCP/IP debate may be found " "in [Russel06]_" msgstr ""