Now we're at a point where we can begin to understand one of the most prevalent and predominant wireless network that you see all around us today, the proverbial mobile network or the cellular network. Now, if you compare a modern cellular network to a legacy wireless network such as FM radio, we'll notice that FM radio and a cellular network over the CP in entirely different manner. An FM radio usually broadcasts its signal from just one common tower, let's say towards the center of a town and the whole town gets the exact same FM signal because they are trying to listen to the same song or the same broadcast at the same time. However, the mobile network covers the same city differently. A mobile network's coverage area is divided into what are called cells, the small areas which are called cells. That is where the name cellular communication or cellular network derives from. Because the area is covered into many different cells of small size and each of those individual cells is served by what is known as a base station. Something that we have learned about already. In proverbial terms, it is also known as the cell tower or simply the tower. Those are the same cell towers that you see all around us. Sometimes they are mounted a top a tall structure or sometimes they are just on the facades of residential or commercial buildings. But nonetheless, no matter what shape or form they take, in a cellular network, one tower or one base station is responsible for providing coverage to a limited geographical area that is called the cell. Different cells are designed in such a way that they have perfectly oscillating coverage. Oscillation is a concept from advanced geometry. It essentially means that you are trying to cover a certain area by using smaller pieces of different shapes so that those shapes nonetheless fit hand and glove perfectly with each other, and even though those shapes may have a different area or a different size and shape than the underlying surface that you are trying to cover, because those pieces fit together hand-in-glove, you'll be able to cover the entire surface using those tiny pieces like that of a jigsaw puzzle. Cellular networks are designed in an analogous manner to provide oscillating coverage and in most cases, if not all, no matter where you are, you are provided a cellular service from the nearest base station. For example, if you phone happens to be somewhere in this area, you will be served by this base station. However, if you happen to be somewhere in this geographical area, you will be served by this base station, which sounds good as long as you are sitting in one place. But what if you start moving from one point to another, especially if you are going from one point to another which are served by two different cells? Well, you as the end-user don't have to worry about it a bit because the cellular network and your phone take care of it seamlessly in a manner that is completely transparent to you, the end-user. Let's look at a little example over here. Let's say you are sitting in a car that was parked here, so it was covered by this base station. But then your car started moving and it ultimately ended up in this location, which was closer to this base station. While your car is moving from this point to another, your phone will communicate with this cell tower as well as the cell tower in order to coordinate so that the service gets seamlessly transferred from this tower to this tower without you, the end-user, having to be aware of it. In technical terms, this procedure is called a handoff or a handover, and it is an integral part of modern cellular networks that feature mobility on different counts and different levels. Now, that said, there are a couple of points I want to emphasize however. First of all, we have shown a neatly hexagonal oscillating pattern as the shape of a cell. But in reality, that is not how cells look like. In reality, hardly any cell will ever have such a neat hexagonal pattern. Cells have somewhat different radiation and coverage pattern. However, we stick to this hexagonal pattern as the first step because it serves as a very powerful illustrative model of the concept of oscillation and the concept of cellularization. Once we have this basic concept in mind at a theoretical level, with the help of these hexagons, we can translate that understanding into real networks wherein cells tend to have slightly irregular shapes. But even though they have irregular shapes, network operators make sure that the entire geographical area is covered by at least one cell if not more than that. That said, in reality it is possible for certain small areas to lack cell coverage, which are proverbially also called coverage rules. For example, there is a base station over here. But if there is a building in the middle that is taller than the base station itself, then stands to reason that the signal from this base station may have a little difficulty reaching this point beyond a tall building. Or if there's a mountain or a hill, you can have a similar effect. In case of such obstructions, it is possible for certain points in a cellular network to be devoid of a cellular or wireless coverage and it is possible that if your phone goes into such an area that lacks sufficient wireless coverage, your phone can temporarily lose wireless service. So if you're in the middle of a voice call, your voice call will drop. If you are streaming a video, that video might stop streaming and would need to buffer to play further. This example is not exactly out of the left feet in that if you might remember, in old days, if you walked into a basement or deep inside a shopping mall or inside an elevator in an office building, chances are you will lose cellular coverage and your ongoing voice call would drop. The fundamental reason behind that is that in those situations, you would have walked into the proverbial coverage for a small area where wireless coverage is either very weak or non-existent. That is where the fundamental tenet of modern cellular network design comes into picture. The basic principle of modern cellular network design involves minimizing the number of dropped calls, while at the same time, maximizing the data rate that you get on your phone and the coverage area, while also limiting the number of towers that you have to build. I think we are all in agreement about these two points. That is, we have to minimize the number of dropped calls and maximize data rate and coverage area. But the question might arise in your mind that is, if there is a proverbial coverage wall here, why not just build a new base station over here? Well, the reality is that base stations are not expensive, neither is realistic on top of which those base stations are built. If we had to build a dedicated base station for every coverage wall, that deployment might get prohibitively expensive and that is the reason why operators have to minimize the number of base stations that a particular geographical area has, and that is where the fundamental trade-off of cellular network design comes into picture. You have to maximize the data rate and coverage area while keeping the number of base stations at a minimum.