Data transmission - The physical connection

What is a transmission channel?

A transmission line is a connection between two machines. The term transmitter generally refers to the machine that sends the data while receiver refers to the one receiving the data. The machines can sometimes be both receivers and transmitters (this is generally the case with computers connected to a network).

A transmission line, also sometimes called a transmission channel, does not necessarily consist of a single physical medium, which is why the end machines (as opposed to the intermediary machines), called DTE, (Data Terminal Equipment) each have equipment for the physical medium to which they are connected called DCTE (Data Circuit Terminating Equipment) or DCE (Data Communication Equipment). The term data circuit refers to the assembly consisting of the DTCE of each machine and the data line.

The data circuit and transmission line

The basics of electromagnetic waves

Data is transmitted on a physical medium by propagation of a vibratory phenomenon. An undulating signal results from this depending on the physical quantity that is being varied:

  • in the case of light, it is a light wave
  • in the case of sound, it is a sound wave
  • in the case of voltage or amperage of an electric current, it is an electrical wave
  • ...

Electromagnetic waves are characterised by their frequency, their amplitude and their phase.

Types of physical media

The physical transmission media are the elements that allow information to flow between transmission devices. These media are generally divided into three categories, according to the type of physical quantity that they allow to circulate, and therefore according to their physical composition:

  • Wire media allow an electrical quantity to circulate on a cable that is generally metallic
  • Aerial media refers to the air or a vacuum which allow the circulation of electromagnetic waves and various types of radio-electric waves
  • Optical media allow information to be sent in the form of light

The speed of the physical quantity will vary depending on the physical medium (for example, sound propagates through the air at a speed of on the order of 300 m/s whereas the speed of light is close to 300,000 km/s).


Data transmission on a line is not lossless. First of all, the transmission time is not immediate, which requires a certain "synchronization" of the data on reception.
In addition, interference or signal degradation can occur.

  • Interference (often called noise) refers to any perturbations that locally modify the signal form. Generally, there are three types of noise:
    • White noise is a uniform perturbation of the signal, in other words, it adds a small amplitude to the signal the average effect of which on the signal is nil. White noise is generally characterised by a ratio called the signal/noise ratio, which translates the percentage of amplitude of the symbol, with respect to the noise (its unit is the decibel). It should be as high as possible.
    • Impulsive noises are small peaks of intensity causing transmission errors.
  • Signal line loss or attenuation represents the loss of signal through energy dissipated in the line. Attenuation results in an output signal that is weaker than the input signal and is characterised by the formula:
    A = 20 log (Output signal level / Input signal level)

    Attenuation is proportional to the length of the transmission channel and the frequency of the signal.

  • Signal distortion characterises the phase difference between the input signal and the output signal.

Bandwidth and capacity

The bandwidth of a transmission channel is the frequency interval over which the signal does not experience line loss greater than a certain value (generally 3 dB, as 3 decibels corresponds to a signal loss of 50%):

A telephone line, for example, has a bandwidth of between 300 and 3400 Hertz approximately for an attenuation rate of 3 dB.

The capacity of a channel is the amount of information (in bits) that can be transmitted on the channel in 1 second.
Capacity is characterised by the following formula:

C = W log2 (1 + S/N)
  • C capacity (en bps)
  • W bandwidth (in Hz)
  • S/N represents the signal-to-noise ratio of the channel.

Upload and download

Download refers to data transfer from the server to your computer and upload refers to data transfer from your computer to the server. It is of interest to know that upload and download occur on separate transmission channels (whether this be on a modem or on a special-purpose line). So, when you are sending (uploading) a document you are not losing any download bandwidth!

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