Understanding Physical Media and Modulation in Telecommunications
Previous Concepts: Physical Concepts
Dun Signal Spectrum and Bandwidth
We have seen that signals have an associated RMS frequency, called the fundamental parameter. We have also seen that any continuous and differentiable signal can be decomposed into a sum (finite or infinite) of sinusoidal signals. Each of these sine components will therefore also be associated with a maximum amplitude. The spectrum of a signal is nothing other than the two-dimensional graphical representation of amplitude vs. frequency. While electrical signals as a function of time are visualized with an oscilloscope, frequency is monitored using a measuring device called a spectrum analyzer. Therefore, any signal will have two possible representations:
- In the time domain.
- In the frequency domain.
From the concept of a signal’s spectrum, we can make a different definition essential in the world of communication and therefore telematics: bandwidth. Bandwidth is defined as the difference between the maximum and minimum frequencies of a signal’s frequency spectrum. For example, a signal with a spectrum from 8kHz to 9kHz has a bandwidth of 1kHz.
2.1.1. Basic Characteristics of Physical Media
The components of a communication system are physical components that follow the laws of physics:
Ohm’s Law
All electrical signals suffer a reduction in their level when energy is transmitted through any means of transmission. This attenuation is given by Ohm’s Law:
R = V / I
Pollution and Distortion of the Signal
In the transmission process, a series of undesirable physical phenomena can distort or alter the original transmitted signal. This can obviously lead to the loss or misinterpretation of the transmitted information.
Among the most common adverse effects are the following:
- Attenuation
- Distortion
- Interference
- Noise
Attenuation
Attenuation is an effect produced by the weakening of the signal due to electrical resistance in both the channel and the other elements involved in the transmission. This weakness is manifested in the form of a drop in the amplitude (power) of the transmitted signal. There may come a time when the signal amplitude becomes so low that it is unrecognizable by the receiver.
Distortion
Distortion consists of the deformation of the signal because the channel does not behave the same way with various frequencies. Tuning is used to correct these distortion effects.
Interference
Interference is any disturbance in the reception of a signal caused by undesirable signals, either natural or artificial, that appear in the corresponding transmission channel.
Noise
Noise is any disturbance introduced by the transmission channel itself, due to imperfections in the materials or resources, environmental conditions such as temperature, etc.
Concepts Requirements: Encoding
Encoding is the process of expressing information according to a standard or code. In order to have communication, the sender and receiver need to agree on the code used to express their messages. This agreement will involve physical aspects such as the amplitude and duration of the signal used for transmission.
Concepts Requirements: Modulation
By varying the three basic parameters of a linear sinusoidal signal, we obtain three linear continuous modulation possibilities:
- Amplitude Modulation (AM)
- Frequency Modulation (FM)
- Phase Modulation (PM)
Any transmission channel behaves like a filter that allows certain frequencies to pass better than others. Modulation is used to maximize the characteristics of the transmission channel for the signal.
Wavelength (λ), measured in meters, is a physical parameter of periodic signals. When multiplied by the frequency, it gives the velocity of the signal on a channel. In a vacuum:
C = λ · f
We can understand wavelength as the distance a sinusoidal signal travels in a time interval equal to the signal period (T). In antenna engineering, a radio antenna designed for a certain electromagnetic wave must have a length of 0.1λ, where λ is the wavelength of the electromagnetic signal to be transmitted. Modulation allows us to change the original signal frequency to a new one that allows us to build antennas of a reasonable size. This frequency change can also be useful to avoid noisy bands or the presence of strong interference. It also allows us to separate the transmission channels of signals that naturally travel at the same frequency and would otherwise interfere with each other. This technique is called frequency multiplexing, which we will discuss further later.
The Electromagnetic Spectrum
The electromagnetic spectrum is the map of frequencies that have the potential to generate electromagnetic signals.
The Physical Layer
The purpose of the physical layer is to transport a bit stream from one machine to another. Each physical transmission medium will be characterized by its bandwidth, delay, cost, and ease of installation and maintenance.
Classification of Transmission Media
Basically, we can classify transmission media into two groups: guided (copper cable, fiber optics) and unguided (radio, infrared).
Guided media include metal cables (copper, aluminum, etc.) and fiber optics. Cables are typically installed inside buildings or in underground conduits. Metallic cables may have a coaxial or twisted pair structure, and copper is the preferred material for the core elements of transmission networks. Fiber optic cable is available in plastic or glass.