So, why doesn't your cellular phone call that you're on doesn't get dropped, or at least, most of the time it doesn't get dropped when you are traveling in a car? That's because the mobile operator is switching you from one base station to another as you are traveling in your car. We are going to see how this whole process works, how do cellular networks actually work. So, in the past, for the television broadcasting networks, they would have these large antennas, and they still continue to do, which would have very powerful transmitters that would provide coverage over a really wide area. Your television set and the antennas would pick up the signals from the television broadcasters. In the context of cellular networks, instead of having this one large and powerful antenna providing coverage over a large area, the idea was to create many small cells, also are many small regions that this area can be segmented into, and have smaller antennas providing coverage over those cells. So, in the context of cellular, instead of having a very large antenna providing coverage over a large area, the area was broken up into smaller cells. Each of these are called cells, and hence the term cellular network and each of the cell will have smaller antennas that would provide coverage in that area. It's called cell because as you can see conceptually these have a honeycomb pattern that is used by cellular network providers because it provides them a way to have three antennas in three directions providing them with 120 degrees of coverage like this. So, each side is 120 degrees covered by one antenna, so this side would have another 120 degree another antenna, and the same for the other side. So, that's why conceptually these are seen as hexagonal cells, in a cellular network. So, each cell will have a cell tower providing coverage over that particular cell. The nice thing about having these cells is that previously the single antenna was using all the frequencies to transmit. Instead, over here, since we have smaller antennas covering smaller areas, you can reuse the frequencies in the different cells in such a way that the frequencies that are used in this particular cell are different from the frequencies that you're using in this adjacent cell. So, cellular networks you would have cell towers, these cell towers are actually connected with a wired network. So, although when we talk about wireless network we think of it as being wireless, the part that is wireless is really the part between your phone and the cell tower. So, only this part is wireless. From that point on once the data has been transmitted to the cell tower, then the data goes over the wired network. So, now let's say somebody is trying to place a call to you from their landline phone. So, how does that work? So how does AT&T or Verizon manage to route telephone calls to you? So, if somebody is placing a phone call to you, so here you are with your mobile phone, if somebody is trying to call you from a landline phone, that call with first go through the central office and will be routed through the Public Switched Telephone Network, also known as the PSTN. So, this is the traditional telephone network. So, the call will be routed through it and since it's a mobile number it will be sent to a mobile telephone switching office. This mobile telephone switching office is connected to all the base stations with very high-speed links. These are wired links, physical links through which the call can be routed to you. So, the mobile telephone switching office will query the base stations and it will find that your phone is in the zone of this particular cell tower. So let's say, you have moved here and this is the particular cell tower that you're talking to at this time or which is in contact with you. So, the mobile telephone switching office is going to route that call, that incoming call to you, to the cell tower that you are connected to or nearest to you and this will then place the call to your phone. So, this is how the telephone calls go from the landline phone over to the public switched telephone network, to the mobile telephone switching office, then it's going to create at the base stations find where you are where the base station has the strongest signal from your phone, and it will route it to that particular base station. So, most of the path is actually over the wired network. It's only in the edge of the last mile does it become truly wireless between the base station of the cell tower and your device, your mobile device. So now, let's say, you have been moving, you are in a car and your car is traveling across these different cells, so you are going from here to this particular cell. So, how does the switching happen? So, each of these cell towers will be in contact with your mobile phone, those cell towers that are nearby will be able to get the signals from your mobile phone, the cell tower that detects the strongest signal is the cell tower to which your mobile phone will be attached for the time being. So, previously, you are attached to this cell tower because you're closest to it, but as you move across these different cells, the signal strength from your phone as detected by this cell tower is going to decrease as you move away from it, and it's going to increase as you move closer to the other cell tower. So, once the signal strength becomes higher for this other cell tower, that is when you have moved into that cell towers coverage area, the network is going to switch you from this particular cell tower that you had been previously associated with to the new cell tower that you are currently attached to without dropping your call. This process is called handoffs in cellular networks. Sometimes the calls may be dropped because of changing the cell towers but the technology's setup in a way that it can handle these transitions between different cells in a seamless manner and this process is called hand-offs. The handoff is typically successful and that's why you're calls shouldn't get dropped when you are traveling in a car. Now, one thing to note here is as I mentioned before, the last mile that is the edge is only the cellular part, the rest of the network, the path that the data takes is typically on the wired links. Now, cellular networks provide a much a larger area of coverage than Wi-Fi network. Wi-Fi only provides coverage of about 10 to 100 meters so it's much smaller in coverage area than compared to cellular networks which can provide you outdoor coverage. One of the key difference between cellular networks and Wi-Fi is that cellular networks operate on licensed spectrum, so the cellular network operators need to have the license to use the spectrum that they are using to transmit the data. But in the case of WiFi, this is a reserved spectrum called the unlicensed spectrum. This spectrum is open to anybody so you don't need to have a license to use the spectrum, and it's about 2.4 Gigahertz bandwidth. This part of the spectrum has been allocated as unlicensed spectrum so that we can have innovations in the mobile technology, in wireless technology, and it's open to everyone to use. So, this is how data actually gets transmitted over cellular networks and between telephone networks and cellular networks. This is the infrastructure for the companies like AT&T, Verizon, T-Mobile, and others. So, now that we have looked at how voice gets converted into data packets and how data packets are sent over a cellular network, we are going to look at some applications of different types of mobile technologies; that is technologies that are not just about cellular networks but also RFID, Bluetooth and other technologies and how these are applied in different industries.
