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.
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:
Electromagnetic waves are characterised by their frequency, their amplitude and their phase.
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:
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.
A = 20 log (Output signal level / Input signal level)
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)
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!