Hello, this is Song Kuk Kim. Today, you're going to study already about the LTE services. I know that you have studied a lot about the communication theories. How we can encode all the signals into the waves? And was how we can deliver this data between cellular phone and the base station. I think you may wonder how the signals and data are delivered beyond the base stations. And also, how we get all the datas from the public Internet to your cellular phone? Today, you are going to study or read about this mechanism. Also we will study about the packet switching and mobility support, which is the one core concern with LTE data services. Telephone has developed the best on the circuit switching. Yes, you may have made this can payphone when you were in kindergarten. This is basically, a demented type of a telephone. One can you speak in to capture the sound and convert it into wave signals. The signal travels through the string. And then, finally, the other side, the other signal, the waveform in the string will be converted into sound that's the mechanism of the telephone, basically. And you can talk to another person with one telephone. But the problem is, if you want to talk many other friends, then you need to have many of these cans between all your friends. So, it's almost impossible to have one dedicated phone per each your friend. So, people developed the switching mechanism. You can see the person on the right side, who is the telephone operator, who switched each circuit. Seen here, if the person on the blue telephone want to talk to the friend on the red telephone. Then first, another signal goes into the telephone operator, long time ago. Then the operator manually switches the switching gear to connect from the blue to the red. Then this is the socket is switched, basically this is a test from blue side to the red side. Then you can talk to these other side without any problem. Then, another person on the green telephone may want to talk to the yellow side. Then you can switch one more line to connect them. Then you can think, okay, if we have this telephone link service in one town, you may talk to all your friends in your own town. Then, another problem is how can you cover all the country with only one telephone exchange service? It's almost impossible. So we need to expand this skin to cover all the area. So here we can see the many distributed telephone switches. Switch A is connected to Cellular user A, and then the Telephone Switch B is connected to Cellular user B. Then, if you want to talk to from Cellular phone A to Cellular phone B, then first you are calling the Telephone Switch A. Then Switch A switch one line between Switch A and Switch D, yeah, that's the red line. Then Switch D will connect to the cellular user B. This is how you are connected from your phone to another side of the area to your friend. Then, each, the telephone switch need to have lots of lines between another switch's. Then, you need to search all the lines. This is basically the circuit switching. because really switch each socket for each cause. The advantage over this skin for the circuit switching is guaranteed bandwidth, because we dedicate one line per each cord, the bandwidth. You already studied bandwidth, the concept of bandwidth is in the previous lectures. There is a guarantee between two users. And it's actually simple, you can guess how it works with a simple diagram. Also, based on the telephone numbering system, it's easy to switch the area code that means where this code should go. So, [INAUDIBLE] simple. Also, the forwarding mechanism is very simple, as you can see here, at the beginning of the call, you need to switch the line. But, after the switching is done. You don't need to do any further work much. Basically, the socket is dedicated, so all the signal will travel through the all your dedicated lines without any further operation. So, the socket switching is easy to implement and also very efficient to guarantee bandwidth. But the problem is this mechanism, this model works with telephone ware however it's not very well suited for the computers. The reason is a video form, typically you talk to only one person at a time. Whether you do your smart phone, or you do your computer, you may want to communicate with the multiple concurrent peers at the same time. You want to send some data through some messaging mechanism also you also want to download at the same time from other server. So, in that case to circuit switching it is not enough to solve all of the data traffics. So people developed another mechanism. We will study this packet switching mechanism in the next session.
