Telecommunication, whether it is data, voice, or some other type of transmission, occurs over two types of signals. The earliest transmission signaling is analog. Analog technology takes the data (or audio, video, etc.) and converts it into electronic pulses or radio waves depending on the medium over which the information is travelling. On the contrary, digital technology takes information and converts it into binary data. Binary data consists of just zeroes and ones so much more data can be crammed into a digital signal than an analog signal [1].
In the early years of computing, there was no way to transmit a digital signal directly over an analog medium. Therefore, engineers designed the MODEM which takes a digital signal from a computer and converts it to an analog signal (modulates; “MO”) that can be sent over the phone lines. The modem on the receiving end takes the analog signal and converts it back to a digital signal (demodulates; “DEM”) that the computer can understand [1]. Although analog transmission is still used, many devices are switching to digital signals; telephones and television being two examples. However, analog is still important in modern telecommunications. In fact, anytime frequency is mentioned in relating to data transmission, such as 2.4 GHz wireless signal, analog signals are being used.
The easiest way to illustrate the difference between digital and analog signals is by using sound as an example. When you hold a note with your voice, you produce a sound wave that can be recorded. When you record that sound wave on an analog medium, such as a cassette tape, the wave is duplicated exactly as it is. That is to say that every point in that sound wave is captured and recorded to an infinite degree, so when played back the wave is reproduced exactly as it was created. This creates a richness in sound that has audiophiles pining for the days of vinyl records.
Now if you were to do the same thing and record the sound wave digitally, the wave has to be turned into a series of ones and zeros called bits. In order to do this, the recording device has to take a number of quick snapshots of the wave with each snapshot representing a one or a zero. This is called a sampling rate. Since we are taking a certain number of snapshots at timed intervals, some of the sound wave is lost. The higher your sampling rate, the less space between snapshots and the more of the wave you will capture, therefore, the richer the wave will sound. Compact discs typically have a sampling rate of 44.1 kHz or around 44,000 bits per second. For most humans, this is a high enough threshold that the ear cannot tell the difference, but some folks insist that they can.
One of the biggest advantages of digital signaling is that there is no degradation of the signal due to signal noise. Since a digital signal is just a static series of numbers, as long as those numbers can be read, the signal remains intact regardless of any noise introduced into the transmission medium. Since, each one and each zero only has to be within a certain electronic threshold, noise spikes are very unlikely to affect the signal. However, with analog, noise introduced into the transmission medium can corrupt the signal because each noise spike disrupts the natural wave of the signal. The more distance an analog signal has to cover, the more noise that can get introduced in to the signal. Think about when you travel out of range of a radio station. The further you get from the station's broadcast tower the more static or even other signals you start to hear. The same thing holds true for any other analog signal. For example, many of you probably use WiFi on your computers. You may notice that sometimes your Internet connection is faster than it is at other times. This is often because of signal degradation. Just about every electronic device in your home gives off some sort of electromagnetic frequency that can interfere with your WiFi signal, thus degrading it. While analog may give you a more precise representation of the signal, digital is less prone to degradation. So, if an analog medium is being used to transmit digital data, it is important to protect this data from degradation which is why protocols are so important. I will be talking about protocols more specifically starting in Part 7 (and yes I did miscalculate how many parts this series will be. The new count is 9).
However, analog signals do have one important advantage of digital signals. Since analog signals can are sent as a frequency, we can adjust that frequency so that it can fit within a single medium along with other signals. By dividing the medium into channels, we can allow analog signals to travel in ether direction at the same time. This is called "full-duplex" signaling. Digital signals cannot do this because they have to use the entire bandwidth of the medium to send pulses. So this means that if you send a digital signal it has to be either simplex or half-duplex and it can only travel in one direction at a time. I will be talking about simplex and duplex transmissions in Part 4. Until then I will continue explaining transmission methods.
This concludes the Part 3 of the the Data Transmission and Protocol series. In Part 4 I will go over simplex and duplex transmission. These types of transmissions are not as complicated as they sound, and once explained you will have no trouble telling the difference between simplex, half-duplex and full-duplex, which, for engineers, should increase your understanding of which type should be used in a given situation.
[1] Wotel, P. (2004). Analog. Digital. What’s the difference?. Retrieved from http://bit.ly/p8R7i3
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