-We saw how telephony has evolved in satellite systems, how these systems interconnect with the rest of the world by using terrestrial mobile phone technologies. Let us now see how voice itself has evolved, how systems have evolved toward IP. This is what we will see in the last part of this sequence with voice over IP also called VoIP. First, what is voice? At the beginning, it was circuits. In fact, it is a cable from one person to another. I added a picture showing how circuit switching was done by hand by cabling one part to another. To fully understand this circuit notion, I drew a small illustration. Imagine person A who wants to make a call somewhere in the world. If there is only a single other person in the world, B, for them to call each other, you just need to install a cable between A and B. "Just"... Well, it depends on the distance separating them. If you add another person in your network, you go from 1 cable to 3. If you add yet another, you get to 7 cables. Yet another, you reach 11 cables. So you keep on adding endlessly. So if there are many people, it is not possible to install cables everywhere if you want everyone to communicate with each other. One of the solutions is to implement circuits with switching at the heart of the network. Now imagine that you set one cable for each user going to one common switch. If A wants to communicate with D, you just need to create a link, by hand like they did at the time, between A and D. It is the same if C communicates with B. You will have a link between C and B. But then, you cannot have E communicate with A, B, C or D because communication is already busy between these persons. This is the circuit notion. In the beginning, this switch was done by hand. As soon as 1888, automatic switching also began. But for a very long time a part of switching remains manual. For example, in France in the mid '50s there was still manual switching, in particular in hotels, etc. This switching is the circuit mode which is very far from the IP mode. The Internet Protocol does not work in the same way at all. It is not a switched mode with voice travelling along a pipe. Internet Protocol uses the packet mode that I will explain in the last sequence of this lecture. What are the benefits of circuits? Resources are monopolized for the called and calling users. These resources are thus dedicated to this communication, so the quality of service is linked to the cable you created with a good throughput and fixed delay. Nothing changes. It is also hierarchical. Each user can be numbered according to your switches, so you can dial phone numbers as before with a hierarchy according to your region, county, city. But there are major flaws. First, when you are communicating with someone, you cannot communicate in another way. There is no multiplexing or resource sharing between several communications. Once you are calling someone, your cable is dedicated to it. This is the first issue. Second issue, it is not flexible at all. You cannot easily add any information other than voice in this system. So it will be difficult to change things. This is why we try to get into the IP world. Instead of creating a system fully dedicated to telephony, we create a system which works with data, among which there will be IP. But there are needs for this because telephony does not tolerate a variation in the arrival of two packets. It is very unpleasant for the user when there is such a variation as it will be explained in the "Going Further" part. So we need to implement quality of service in networks in order to make sure that voice goes through correctly and that the user is satisfied of his service. The problem is that you will use more resources compared to a dedicated cable. But on the other hand, you do not physically need a dedicated medium. This is one benefit. The major benefit is that it becomes interoperable and can interconnect with all the other networks. This allows you today to use Skype, to have voice tools in games, etc. This was not possible before. Here is a small slide summing up the whole sequence. We saw that telephone systems underwent a kind of transition. They went from being satellite systems dedicated to telephone communications to systems that interconnected with the current telephony. It can be in the wired or mobile world, or even systems that will become full IP. As I told you, eventually we will migrate these different standards toward standards that rely on IP instead of specific technologies. Circuits have come to an end. Voice becomes data like any other, except it requires quality of service. I invite you to watch the "Going Further" episode right after this one.
