-We have seen what interconnection and interoperability are by using IP in the first instance. Then I talked about a key service for satellite, television. I will now deal with another key service, telephony. I split this sequence in three main parts. The first part will talk about independent systems, that is to say systems designed for telephony. We will then see how we can interconnect telephone networks with other telephone or terrestrial networks. Eventually, I will tackle voice over IP, Internet Protocol. Voice is the other name given to telephony in networks. This is the evolution I have already mentionned in sequences 1 and 2. Let us start with independent systems: the ones aiming at providing voice services. We have to analyze the needs for voice services. I mentioned these needs at the beginning of this lesson: calling from anywhere to anywhere. The satellite coverage is important since we can imagine a system covering the whole Earth allowing us to call from anywhere to anywhere. It can be used for niche applications that I already explained. For example a mother working on an oil rig who wants to call her family in the evening can use satellites. But also general-public applications. In this case it is slightly more difficult. We will see that independent systems will first want to offer to all users the ability to give calls. In the middle and at the end of the '90s, there is a kind of big boom in the terrestrial networks of mobile telecommunications for private users. At the beginning of the '90s, it only existed analog systems and mobile telephony was dedicated to a professional use. All at once, with 2G, that is to say GSM, systems opened themselves to the general public, mainly in cities where there was a large public. In France, it was first France Télécom with Itineris. Then came SFR, Bouygues. Per-minute billing has been replaced by progressive flat rates with 2 hours of telecommunications for example. There was a huge boom at the end of the '90s, in 1999 for example, with the "Millénium" contract which is a good example of the change in contracts. More and more users from the general public use mobile telephony services, mainly in covered zones such as cities. At the same time, at the beginning ofÏthe '90s, satellites want to offer this telephony where terrestrial systems have not really advanced yet since there are large uncovered zones called "white zones". They want to compete with terrestrial infrastructures quite fast at the beginning of the '90s. Projects are launched to try and offer telephony to the general public via satellites. First, which satellite system must we choose? To choose a satellite system, we must first look on what communication quality depends. This communication quality is called "Quality of Service", QoS. For telephony, we must first know how to measure it. It is rather complicated. One solution is to use MOS. MOS means Mean Opinion Score. It is a subjective value. We ask users to grade the quality of their call. They use their phone, listen to a conversation, maybe talk, and grade it from 1, the worst, to 5, the best. This is quite expensive because measures need to be made, it is quite subjective, it takes time and money. So today we tend not to use MOS to assess communications but automatic algorithms that work instead. Regarding satellites, the propagation delay has an impact on user interactivity, as the jitter also may. I will explain jitter in the "going further" episode about QoS at the end of this part. You are welcome to watch it if you wish to learn more about quality of service. In fact, to solve these issues, to reduce the delay for example, we will rather use LEOs. The big project of these times will be to use LEOs in order to provide telephony. They have a small coverage compared to geostationary satellites so you waste less when sending from one person to another, which is the standard usage of telephony. But constellations will have to be designed to cover the whole world.
