cnp3-ebook/principles/reliability

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10 ../../principles/reliability.rst:44 • ../../principles/reliability.rst:49

	English Bit rate 8/10000 · 8
11 ../../principles/reliability.rst:46

	English In computer networks, the bit rate of the physical layer is always expressed in bits per second. One Mbps is one million bits per second and one Gbps is one billion bits per second. This is in contrast with memory specifications that are usually expressed in bytes (8 bits), KiloBytes (1024 bytes) or MegaBytes (1048576 bytes). Transferring one MByte through a 1 Mbps link lasts 8.39 seconds. 392/10000 · 392
12 ../../principles/reliability.rst:49

	English Bits per second 15/10000 · 15
13 ../../principles/reliability.rst:51

	English 1 Kbps 6/10000 · 6
14 ../../principles/reliability.rst:51

	English :math:`10^3` 12/10000 · 12
15 ../../principles/reliability.rst:52

	English 1 Mbps 6/10000 · 6
16 ../../principles/reliability.rst:52

	English :math:`10^6` 12/10000 · 12
17 ../../principles/reliability.rst:53

	English 1 Gbps 6/10000 · 6
18 ../../principles/reliability.rst:53

	English :math:`10^9` 12/10000 · 12
19 ../../principles/reliability.rst:54

	English 1 Tbps 6/10000 · 6
20 ../../principles/reliability.rst:54

	English :math:`10^{12}` 15/10000 · 15
21 ../../principles/reliability.rst:59

	English To understand some of the principles behind the physical transmission of information, let us consider the simple case of an electrical wire that is used to transmit bits. Assume that the two communicating hosts want to transmit one thousand bits per second. To transmit these bits, the two hosts can agree on the following rules : 330/10000 · 330
22 ../../principles/reliability.rst:63

	English On the sender side : 20/10000 · 20
23 ../../principles/reliability.rst:62

	English set the voltage on the electrical wire at ``+5V`` during one millisecond to transmit a bit set to `1` 101/10000 · 101
24 ../../principles/reliability.rst:63

	English set the voltage on the electrical wire at ``-5V`` during one millisecond to transmit a bit set to `0` 101/10000 · 101
25 ../../principles/reliability.rst:66

	English On the receiver side : 22/10000 · 22
26 ../../principles/reliability.rst:66

	English every millisecond, record the voltage applied on the electrical wire. If the voltage is set to ``+5V``, record the reception of bit `1`. Otherwise, record the reception of bit `0` 179/10000 · 179
27 ../../principles/reliability.rst:70

	English This transmission scheme has been used in some early networks. We use it as a basis to understand how hosts communicate. From a Computer Science viewpoint, dealing with voltages is unusual. Computer scientists frequently rely on models that enable them to reason about the issues that they face without having to consider all implementation details. The physical transmission scheme described above can be represented by using a `time-sequence diagram`. 454/10000 · 454
28 ../../principles/reliability.rst:72

	English A `time-sequence diagram` describes the interactions between communicating hosts. By convention, the communicating hosts are represented in the left and right parts of the diagram while the electrical link occupies the middle of the diagram. In such a time-sequence diagram, time flows from the top to the bottom of the diagram. The transmission of one bit of information is represented by three arrows. Starting from the left, the first horizontal arrow represents the request to transmit one bit of information. This request is represented by a `primitive` which can be considered as a kind of procedure call. This primitive has one parameter (the bit being transmitted) and a name (`DATA.request` in this example). By convention, all primitives that are named `something.request` correspond to a request to transmit some information. The dashed arrow indicates the transmission of the corresponding electrical signal on the wire. Electrical and optical signals do not travel instantaneously. The diagonal dashed arrow indicates that it takes some time for the electrical signal to be transmitted from `Host A` to `Host B`. Upon reception of the electrical signal, the electronics on `Host B`'s network interface detects the voltage and converts it into a bit. This bit is delivered as a `DATA.indication` primitive. All primitives that are named `something.indication` correspond to the reception of some information. The dashed lines also represents the relationship between two (or more) primitives. Such a time-sequence diagram provides information about the ordering of the different primitives, but the distance between two primitives does not represent a precise amount of time. 1687/10000 · 1687
29 ../../principles/reliability.rst:88

	English Time-sequence diagrams are useful when trying to understand the characteristics of a given communication scheme. When considering the above transmission scheme, it is useful to evaluate whether this scheme allows the two communicating hosts to reliably exchange information. A digital transmission is considered as reliable when a sequence of bits that is transmitted by a host is received correctly at the other end of the wire. In practice, achieving perfect reliability when transmitting information using the above scheme is difficult. Several problems can occur with such a transmission scheme. 599/10000 · 599

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