In this post, we will have an introduction into the world of communication systems, related to computer engineering.
The communication process
The communication process can be summarized in the following diagram:
The source of information delivers what is called the signal message. This is the message we want to transmit. It can be text, computer data, sounds, images or video.
At the transmitter, level, the message signal is converted into symbols, and symbols are then encoded into a form that is understandable by the communication channel.
At the receiver, the received signal is a set of encoded symbols. These are decoded and used to re-generate the original message signal. However, it is only an estimate of the original message signal, and that’s due to the noise in the communication channel. In fact, the communication channel is usually affected by internal or external -or both- noise signals, which alter some characteristics of the traversing signal, thus altering the content of the original message. This is why we can not be 100% that we received an exact copy of the emitted signal.
Principle communication resources: channel bandwidth, signal bandwidth and signal power
Channel bandwidth: refers to the range of frequencies that a communication channel provides for the transmission of a signal.
When defining signal bandwidth, pay attention that there is a difference between signal bandwidth and signal spectrum. The spectrum of a signal is the range of frequencies occupied by a signal. The signal bandwidth, however, is the upper limit over which the signal spectrum is negligible.
In communication system design, we tend to maximize the signal power while leveraging the signal bandwidth and the channel bandwidth. Let’s consider the diagram below. The horizontal axis is the frequencies and the vertical axis is the signal power.
In legacy voice networks for example, we only consider the signal between 300Hz and 3400kHz. It’s true that the spectrum of the signal energy -here the energy with which we speak- can go up to 7kHz. But the signal bandwidth -the bandwidth of the voice signal- is concentrated in the [300-3400]Hz interval, and the voice energy is at its higher level in that same interval. That’s why a communication channel with a bandwidth of 4kHz is enough.
[box] The frequency of doing something is the number of times it is done per a certain observation period. In the case of communication systems, frequency is the number of oscillations of a waveform per second. The frequency is measured in Hertz after its inventor Heinrich Hertz. So a frequency of 3000Hz means 3000 oscillations of the waveform per second, or 3000 waves per second.[/box]
In the upper parts of the post, we learned that the transmitter should adapt the message signal to the communication channel, by converting it into symbols and encoding those symbols. This process is called modulation.
There are two big families of modulations: Continuous Wave modulation and Pulse modulation.
Continuous Wave modulation (CW)
This family of modulation techniques modifies the characteristics of a sine wave signal called the carrier , in accordance to the message signal.
- the street is the message signal,
- the color pencils are the carrier, since the painter uses them to reproduce the shapes and colors of the street on paper
- The variation of lines, colors and shapes in the drawing are in accordance with the details of the street.
The characteristics of a carrier signal -and any other signal- are:
- amplitude (A)
- phase (theta): measured in degrees or in multiples of π
- wavelength (T)
Here is a sample form of the carrier:
And here is another graph that shows the phase:
We can modify the amplitude, the frequency or the phase of the carrier signal over time.
- If we modify the amplitude of the carrier in accordance with the message signal, we get the Amplitude Modulation technique (AM)
- If we modify the frequency of the carrier in accordance with the message signal, we get the Frequency Modulation technique (FM)
- If we modify the phase of the carrier in accordance with the message signal, we get the Phase Modulation technique (PM)
Both FM and PM are also called angle modulations.
Here is a visual example of amplitude modulation:
When modulation occurs, the carrier signal goes in accordance with the message signal:
There are two categories for Pulse Modulation: analog pulse modulation and digital pulse modulation.
Analog pulse modulation techniques are :
- PAM: Pulse Amplitude Modulation
- PDM: Pulse Division Modulation
- PPM: Pulse Position Modulation
Digital modulation techniques are also called Pulse Code Modulation (PCM). I’ll discuss about digital modulation techniques in this post.
This diagram summarizes the different modulation families and techniques:
Multiplexing & Demultiplexing
One interesting benefit of modulation is the ability to transmit many signals on the same communication channel. this process is called Multiplexing. At the receiver side, demultiplexing occurs to separate signals.
The multiplexing techniques you need to know are:
I’ll particylarly discuss about TDM.
TDM -or Time Division Multiplexing- is a technique that allows two functions:
- aggregating multiple input signals onto the same communication media, and this is called Multiplexing
- chunking a transmitted signal into many output signals, and this is called Demultiplexing.
These two functions are performed consecutively by a Multiplexer and a Demultiplexer.
How can we transmit many signals onto the same media, without having some sort of collision? And how can we distinguish the output signals at the receiver side? That’s the power of timeslots.
Over a period iof time T, we divide the time into timeslots. Each timeslot is used to carry a portion of a signal X. T is repeated over time and that means another portion of signal X is transmitted, etc … until all input signals are completely transmitted.
Out in the world, there are TDM links that use this TDM technology. These TDM links are used for example in these scenarios:
- connecting a PABX to a Cisco voice gateway,
- connecting two PABXs together,
- connecting a voice gateway to the PSTN
Baseband and Passband in communication systems
Baseband communication is when the encoded signal is sent at its original low frequencies. Such examples are voice telephony (not voice over IP) and analog video camera output.
Passband communication is when the encoded signal is “modulated” to a higher range of frequencies. And it can be analog modulation or digital modulation. Passband communication allow to multiplex different signals together on the same physical media, given that demultiplexing occurs at the receiver side to distinguish the various signals. Wired and wireless technologies operate on a passband basis.
The cadence of signal transmission is regulated by a regular type of signals called the clock signal. The transmitter uses a flip flop circuit to generate the clock. Both the clock signal and the data signal are put on the medium.
When both the transmitter and the receiver have the same clock signal, then it is easy to regenerate the sent signal at the receiver level. However, in communication systems, receivers do not know which clock transmitters are using. Thus, we need a system that “tries” to guess the clock.
This is the job of the clock recovery circuits. They take as input the received signal and they estimate the clock at their output.