New Telecom Networks
New Telecom Networks
In order to be able to communicate with an interlocutor, we need to be able to transmit our data message on an available support, which entails taking account of the transmission parameters of the supports. Then, the message is introduced into the frequency band by modulation and encoding. The messages are channeled along their path by switching systems, with which a signaling protocol is associated. This channeling over worldwide connections only becomes coherent because of a judicious set of data protocols, established from the reference a seven-layer model defined by the ISO. Thus, local area networks (LANs) can communicate remotely through wide-area networks (WANs). As the picture of communication techniques painted here is merely a brief overview of more complex topics, which have evolved rapidly over the course of the past forty years, it only includes those aspects which might be useful for the understanding of the other chapters in this book.
2.1. Transmission parameters
The longer a communication link is, the more degraded the useable signal may become. Maintenance can help reduce occasional faults which occur on the connection by restoring its original performances from when it was first commissioned. The goal is to reproduce the signals sent by the calling terminal on the called terminal. Telecommunications use the properties of electromagnetic waves which, like any vibrating movements, are characterized by their frequency, amplitude and phase.
2.1.1. Frequency ranges in transmission
Electromagnetic (EM) waves are characterized by their frequency "F", expressed in Hertz (Hz), their period "T" in seconds, and their wavelength in a vacuum (notated as ""), expressed in meters. The frequency "F" indicates, for a given unit of time (a second), the number of times that the phenomenon recurs in identical fashion. The period is the inverse of the frequency. With "C" being the speed of light in a vacuum (3 × 108, expressed in m/s), we can derive the relation "wavelength = C × T = C / N". The wavelength is also the distance covered by the wave over the period. The range of an EM wave on a transmission support depends on its frequency, the quality of its support, any obstacles encountered during its propagation, the power transmitted and the performances of the transmitter and the receiver. The electromagnetic spectrum is divided into a number of defined bandwidths, on the basis of wavelength. Appendix A.1 specifies the domain of use of the different types of radio electric wave.
The rare resources which are radio electric frequencies are auctioned off for use for a determinate period, under the control of regulatory authorities. There is lively competition, in this regard, between network operators and radio broadcasters over ranges of frequencies available in the radio spectrum, and also over content distribution networks and access networks. The constant cheap broadcasting of television programs reduces the throughput available for Internet users and slows down the exchanges of their applications. The major principle of universal network access, defined in the context of "net neutrality", is therefore lacking, and no solution has yet been put forward.
NOTE.- It is important not to confuse "bandwidth" and "bit rate". The terminology commonly employed uses rather similar expressions to speak of the width of a transmission channel. The bandwidth quantifies the frequency space, whilst the "throughput" looks at the binary stream of pulses which can be transported in this frequency "corridor". Quite often, a certain lack of rigor in spoken expression leads to confusion between these two associated notions, which are actually as different as the width of a highway and the number of trucks that can pass along it in the course of an hour (see Appendix A.2.1).