cnp3-ebook/principles/reliability

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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
30 ../../principles/reliability.rst:91

	English The first problem is that electrical transmission can be affected by electromagnetic interference. Interference can have various sources including natural phenomenons (like thunderstorms, variations of the magnetic field,...) but also other electrical signals (such as interference from neighboring cables, interference from neighboring antennas,...). Due to these various types of interference, there is unfortunately no guarantee that when a host transmit one bit on a wire, the same bit is received at the other end. This is illustrated in the figure below where a `DATA.request(0)` on the left host leads to a `Data.indication(1)` on the right host. 653/10000 · 653
31 ../../principles/reliability.rst:106

	English With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interference, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors. However, this is not the only type of problem that can occur. Besides defining the voltages for bits `0` and `1`, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However, when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can "disappear" during their transmission on an electrical cable. This is illustrated in the figure below. 1749/10000 · 1749
32 ../../principles/reliability.rst:128

	English A similar reasoning applies when the clock of the sending host is slower than the clock of the receiving host. In this case, the receiver will sense more bits than the bits that have been transmitted by the sender. This is illustrated in the figure below where the second bit received on the right was not transmitted by the left host. 335/10000 · 335
33 ../../principles/reliability.rst:150

	English From a Computer Science viewpoint, the physical transmission of information through a wire is often considered as a black box that allows transmitting bits. This black box is commonly referred to as the `physical layer service` and is represented by using the `DATA.request` and `DATA.indication` primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits, by abstracting the technological details that are involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the `physical layer service` is imperfect and has the following characteristics : 654/10000 · 654
34 ../../principles/reliability.rst:152

	English the `Physical layer service` may change, e.g. due to electromagnetic interference, the value of a bit being transmitted 119/10000 · 119
35 ../../principles/reliability.rst:153

	English the `Physical layer service` may deliver `more` bits to the receiver than the bits sent by the sender 101/10000 · 101

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