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:math:`10^{12}`
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`electrical cable`. Information can be transmitted over different types of electrical cables. The most common ones are the twisted pairs (that are used in the telephone network, but also in enterprise networks) and the coaxial cables (that are still used in cable TV networks, but are no longer used in enterprise networks). Some networking technologies operate over the classical electrical cable.
`wireless`. In this case, a radio signal is used to encode the information exchanged between the communicating devices. Many types of modulation techniques are used to send information over a wireless channel and there is lot of innovation in this field with new techniques appearing every year. While most wireless networks rely on radio signals, some use a laser that sends light pulses to a remote detector. These optical techniques allow to create point-to-point links while radio-based techniques can be used to build networks containing devices spread over a small geographical area.
The physical layer
Bit rate
Bits per second
1 Kbps
:math:`10^3`
1 Mbps
:math:`10^6`
1 Gbps
:math:`10^9`
1 Tbps
:math:`10^{12}`
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 :
On the sender side :
set the voltage on the electrical wire at ``+5V`` during one millisecond to transmit a bit set to `1`
set the voltage on the electrical wire at ``-5V`` during one millisecond to transmit a bit set to `0`
On the receiver side :
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`
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`.
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.
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.
the `Physical layer service` may change, e.g. due to electromagnetic interference, the value of a bit being transmitted
the `Physical layer service` may deliver `more` bits to the receiver than the bits sent by the sender

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../../principles/reliability.rst:54
String age
5 years ago
Source string age
5 years ago
Translation file
locale/cs/LC_MESSAGES/principles/reliability.po, string 20