So, how do wireless technologies work? Wireless technologies, just like the Internet, is a digital technology. That is, it requires the data to be presented as bitstreams, that is; zero and one. So, how does our voice signal get converted into data packets, that is this bitstreams that will be transmitted over the wireless medium? That's what we're going to learn in this video. We use our cell phones every day to communicate, but how does our voice get converted into data packets? Let's look at that process. So, when we speak into a receiver, we are generating sound waves. These sound waves are picked up by our phone. Inside our phones, we have sensors and electrical systems that pick up the vibrations from our voice. They convert these vibrations into electrical signals. These signals would vary over time as we speak depending on the vibrations that are picked up. These are continuous time signals. Now we need to convert this continuous signals that are varying over time depending on what we're speaking into digital format. So, suppose we have a signal like this. Based on what we are speaking, this is how the vibrations have led to electrical signals that are vibrating over time. This is an analog signal. Analog signals are continuous signals that are varying continuously over time just like, as we're speaking, it's a analog signal, and those are being converted into electrical signals, which are also analog signals. Now, we need to convert this into the digital form that is in the binary format. So, how do we convert them into the digital format? Now, let's suppose that we have only two bits that we can use in order to represent the signal. So, if you have two bits or two binary digits, they could be 00, 01, 10, and 11. So, those will be the four different values or levels for this signal. So, if we have two binary digits, then we can create four levels: 00, 01, 10, 11. So, these are four binary values that we want to use in order to represent this signal. So, how do we do that? So, initially, the signal is staying flat, and then it jumps up to the highest level. So, let's say the highest level here corresponds to 11. Then it falls down, and so we're going to represent this level by 10. Then it further comes down to 01 and goes back up to 10. So, this is how the signal is varying over time when this analog signal is transferred into equivalent digital signal. So, this process of taking an analog signal and discretizing, so that is taking these values and plotting them all at discreet intervals, given the number of binary digits we have to represent the data is called discretization. So, here we have taken this analog signal, and we have discretized it into four levels, and this is the resulting digital signal. So, now for this digital signal, when we need to transmit the data, if we break this time axis into slots, then the first unit of time will transmit this level, which is 00. So, those are the first bits that I want to transmit. Then we need to transmit 11 for the next slot. After that, in the next slot is going to be 10. After that, it will be, again, 01. Then, finally, in the final time slot, we're transmitting 10. So, this is going to be my bitstream that I want to transmit corresponding to this signal. So, how do we do that? So, let's look at that now. So, we converted the voice signal that we had, which was varying over time depending on the fluctuations of what we are saying, we converted that into a bitstream that is our stream of zeros and ones by digitizing and discretizing that signal. So, this was the bitstream that we obtained, and we need to now transmit this bitstream from the sender to the receiver. So, now let's see how that happens in the context of a wired network, such as a fiber optic network. So, in a fiber optic network, we have LED that is a light emitting diode and that serves as the transmitter of the information, and it's going to transmit this bit sequence through this fiber, this is a fiber optic cable, onto the other end, which is a receiver, which has a detector to detect the light signals. So, we're going to convert these electrical pulses, the zeros and ones that were created from discretizing that signal, into light signals. So, to do that, when it is zero, this LED is not going to glow. So, the detector will not detect any light. When it's one, this LED is going to come on and this light will be transmitted through the fiber based on the principle of total internal reflection, and it will be picked up by the detector, and the detector will detect one. So, in this case, if we want to transmit this bit sequence, the LED is going to be off for the first two time slots, then it will come on and remain on, and then it will go off for two time slots, again on for two time slots and finally, it will be off. So, the detector will be able to pick up these patterns of light being on and off and at the other end, it will be able to reconstruct this bitstream. So, that's how data is transferred in a wired network, in a fiber optic network. Similarly, for a copper network, instead of light, it is an electrical signal that is sent whenever there's, one, we send an electrical signal that will be picked up by the detector. If it is zero, there is no electrical signal that is sent. So, that's how the detector would pick up, whether it's zero or one, at different time slots, and that's how the bitstream will be transmitted from the sender to the receiver. So, how do we do this in the context of a wireless network? So, in wireless network, again, this is the bitstream that we want to send from the sender to the receiver, and we have some electromagnetic wave on which we want to transmit this data. The process of putting this data onto a wave is called modulation. So, there are different ways of modulating our wireless electromagnetic wave. One way to do it is amplitude modulation. What that means is that the sender is going to send. So, this is the sender transmitter and this is the receiver of the detector. So, if it's sending zero, then at that time slot is not going to send out any signal. If it is trying to send out the binary digit one, it's going to send out a signal with a certain amplitude. Again, the next slot, it's again one. So, it's going to continue sending the signal with that amplitude and then when it hits zero, it again stop sending anything. Again at one, it will start sending a signal and not anything at zero. So, this pattern is transmitted from the sender to the receiver. So, the receiver will be able to detect these changing patterns of zeros and ones. So, when there is some amplitude present, that is when there is a signal with a positive amplitude coming in to the receiver, then they will pick up one. Otherwise, it's zero. So, this is a simple way to do transmission of binary data over a electromagnetic wave, and this is called amplitude modulation. There is also another way to send data from the sender to the receiver, this bitstream from the sender to the receiver, using frequency modulation. This is a term that you may have heard. It's called FM, for frequency modulation. So, in frequency modulation, what happens is that the sender will be sending out a signal of a different frequency when it's sending out zero and it will send out another signal of a different frequency when it's sending out one. So, when it's trying to send out zero, maybe it will use a signal with a larger frequency and then when it is trying to send out one, it is going to send a signal with a higher frequency. So, as you can see here, depending on zeros or ones, the frequency of the signal that the sender is sending out is varying over time. So, the detector will be picking up these signals and it will look at the frequency of the signal. So, if the frequency is small, then it will know that it is zero that is being sent. If the frequency is high, then the detector will know that it corresponds to one. So, by varying the frequency of the electromagnetic wave that is used for the transmission of this bitstream, the sender could send information to the receiver, and this is called frequency modulation. So, this is how data actually gets transmitted from a sender to a receiver. The voice of our vocal cords would be first converted into sound waves. Those sound waves will be picked up by the sensors of the electrical circuitry in our phones and other devices that we use for communication. Those devices will convert the vibrations into electrical signals. Those electrical signals will be discretized to create bitstreams depending on how many bits you have available. You can create different levels, and you would reconstruct the analog signal into a digital format and get a bitstream. Then to transmit that bitstream, for a wired network, you can use copper or fiber and vary the presence or absence of a signal, whether it's light or whether it's electricity corresponding to zero and one that you are trying to send. In a wireless network, you would send this bitstream over electromagnetic wave by changing either the amplitude of the wave corresponding to differences between zero and one or the frequency of wave to indicate differences between zeros and ones. So, this is how data actually gets transmitted over a physical medium. Our voice data will be converted into bitstreams and then transmitted, with the wired or wireless mediums. So, now that we have seen how voice gets converted into data packets, we are going to look at how these data packets get routed from you to the receiver over a cellular network. That is how cellular networks work.
