cnp3-ebook/principles/referencemodels

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7 ../../principles/referencemodels.rst:46

	English 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. 241/10000 · 241
8 ../../principles/referencemodels.rst:48

	English 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 : 310/10000 · 310
9 ../../principles/referencemodels.rst:50

	English the Physical layer may change, e.g. due to electromagnetic interference, the value of a bit being transmitted 109/10000 · 109
10 ../../principles/referencemodels.rst:51

	English the Physical layer may deliver `more` bits to the receiver than the bits sent by the sender 91/10000 · 91
11 ../../principles/referencemodels.rst:52

	English the Physical layer may deliver `fewer` bits to the receiver than the bits sent by the sender 92/10000 · 92
12 ../../principles/referencemodels.rst:72

	English The Datalink layer 18/10000 · 18
13 ../../principles/referencemodels.rst:74

	English 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. 653/10000 · 653
14 ../../principles/referencemodels.rst:76

	English 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. 812/10000 · 812
15 ../../principles/referencemodels.rst:96

	English The Network layer 17/10000 · 17
16 ../../principles/referencemodels.rst:100

	English 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. 637/10000 · 637
17 ../../principles/referencemodels.rst:131

	English The Transport layer 19/10000 · 19
18 ../../principles/referencemodels.rst:133

	English The network layer enables hosts to reach each others. However, different communication flows can take place between the same hosts. These communication flows might have different needs (some require reliable delivery, other not) and need to be distinguished. Ensuring an identification of a communication flow between two given hosts is the task of the `transport layer`. `Transport layer` entities exchange `segments`. A segment is a finite sequence of bytes that are transported inside one or more packets. A transport layer entity issues segments (or sometimes part of segments) as `Data.request` to the underlying network layer entity. 639/10000 · 639
19 ../../principles/referencemodels.rst:135

	English There are different types of transport layers. The most widely used transport layers on the Internet are :term:`TCP`, that provides a reliable connection-oriented bytestream transport service, and :term:`UDP`, that provides an unreliable connection-less transport service. 272/10000 · 272
20 ../../principles/referencemodels.rst:164

	English The Application layer 21/10000 · 21
21 ../../principles/referencemodels.rst:168

	English 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. 325/10000 · 325
22 ../../principles/referencemodels.rst:201

	English 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. : 215/10000 · 215
23 ../../principles/referencemodels.rst:203

	English physical layer entities exchange bits 37/10000 · 37
24 ../../principles/referencemodels.rst:204

	English datalink layer entities exchange frames 41/10000 · 41
25 ../../principles/referencemodels.rst:205

	English network layer entities exchange packets 41/10000 · 41
26 ../../principles/referencemodels.rst:206

	English transport layer entities exchange segments 44/10000 · 44

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