cnp3-ebook/principles/reliability — Czech @ Weblate: :math:`10^3`

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	Connecting two hosts
	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=1 or help us by providing pull requests to close the existing issues.
	The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts together. This is illustrated in the figure below.
	To enable the two hosts to exchange information, they need to be linked together by some kind of physical media. Computer networks have used various types of physical media to exchange information, notably :
	`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`.

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