In the early days of telephony, there were manual telephone exchanges where operators would connect you with the person you wished to speak to. When you picked up the phone to make a call, a warning light automatically lit up in front of the operator assigned to our telephone number; she was the one who answered and requested the number we wanted to connect to; immediately after, she would contact the exchange and operator in charge of that number. Then, the operator in charge of the destination number would contact the person, and thus both would create the communication channel. This process took more time than it now takes us to make a call, not to mention it was very costly, and while we were talking, a special channel was created that could not be occupied; it was like using a special cable just for us.
Now, when we make a phone call, send a WhatsApp message, watch satellite television, play online video games, or talk on the phone with several people simultaneously, the service is fast, and we don't realize how it happens, but all of this is thanks to multiplexing.
Multiplexing is the action of combining two or more information channels into a single transmission medium. Transmission media are guided or unguided; unguided media include free space, and all wireless communication systems such as Bluetooth, WiFi, and antennas that achieve this type of communication fall into this category. On the other hand, there are guided media such as copper cables and fiber optics.
Multiplexing aims to minimize the number of physical lines and maximize the bandwidth of the transmission medium; there are several types of multiplexing, and their scheme is the same regardless of the type, as shown in Figure 1:
Figure 1. Multiplexing and demultiplexing
Satellite communication systems use Time Division Multiplexing (TDM), conventional radio uses Frequency Division Multiplexing (FDM), and Code Division Multiplexing (CDM) is used in cellular communications and GPS; it is also very common to combine several of these techniques in a single system. WDM multiplexing is what we will focus on, as it is used for optical communication systems.
WDM
As we can see in Figure 1, we will have "n" number of channels, or rather, fibers connected to a device known as a multiplexer. Each fiber must have a light beam of a different wavelength within the optical communication window and separated by at least 20nm. Multiplexers (Figure 2) already have defined the wavelengths they work with. The optical multiplexer acts like a prism, "combining" all the wavelengths into a single beam and sending it through the transmission medium as shown in Figure 3.
Figure 2. Multiplexer
Figure 3. WDM Multiplexing
To reverse the effect and separate the wavelengths, we have the demultiplexer, which performs the inverse operation of the multiplexer.
There are two types of WDM:
DWDM (Dense WDM):
Dense Wavelength Division Multiplexing uses the C-band of the third optical communication window, is used for long distances, and for up to 240 wavelengths. One of the disadvantages is that more attenuation may occur.
CWDM (Coarse WDM):
Coarse Wavelength Division Multiplexing is used in the range of 1270 to 1610 nm; it is for a few wavelengths, up to 18. It is used in metropolitan areas and can be implemented with point-to-point topologies or local rings and PON networks with distances not exceeding 80 km. Its speed limit is 2.5 Gbps.
