-Hello. Welcome to week 3 of your MOOC. This week starts with a sequence to introduce the different elements of a basic digital communication chain knowing that they will be detailed in week 4. During this sequence, I will also define two key elements of a digital transmission chain that you probably already know or have heard of, the bit error rate and the bit rate. Let us start with the definition of a digital communication chain. It is a communication chain for which the information to be transmitted is made of a sequence of 0 and 1 binary elements. This binary element sequence can come from a computer file. But it can also come from a physical signal such as a voice or an image that has been digitized beforehand. This sequence does not aim at explaining the notion of signal digitization or the benefits of working in digital. But if you are interested in this issue, I invite you to watch the "Going Further" episode which mentions it. We will have to transmit this binary information. It will cross a communication chain made of three key elements, a transmitter, a propagation or transmission channel, and a receiver. The role of the transmitter is to prepare the binary information to cross the propagation channel. It will start by protecting it against potential errors that could occur during transmission. This is the channel coding operation. From the protected binary information, the transmitter builds a signal that can be propagated through the transmission channel. This is done via the modulation operation. The transmission channel is simply the physical link between the transmitter and the receiver. There are several sorts of propagation channels. We can mention optical fiber at the heart of telephony networks, coaxial cables in an Ethernet transmission, or even the twisted copper pairs of your ADSL link. Of course, when talking about satellite transmission, the propagation channel is the free space between the antenna and the satellite. In any case, no matter the transmission channel, the transmitted signal will be damaged. It delivers a damaged signal to the receiver. The role of the receiver, from this damaged signal, is to retrieve the original binary information. For this purpose, there are, in the receiver, two opposite operations to what is found in the transmitter, a demodulation operation followed by a channel decoding operation.