-To take the subject one step further, let us look at quality of service, specifically the quality of service of a telephone communication via a satellite system, and let us see what can impact this communication. Let us start with delay. I have already seen many questions about delay on the forum, so let us start by defining what the delay of a communication is. This delay is called end-to-end delay, and it is the time between the transmission of a message and its reception. It originates from three things, the propagation of the signal, the throughput of the medium used, and the load of the network through which the signal travels. As a simple analogy to help you understand, take for instance a journey, you take to go on vacation or to work. The distance that you cover takes more or less time depending on the vehicle that you use, particularly on its speed. This is propagation delay, but you know that other delays must also be taken into account, as unexpected events can occur, which account for the load of the network you are going through, the urban network, the cars, the traffic jams, these sorts of things. This is what I called network congestion, network overload can cause a more or less long delay. There is also transmission delay, which is the time you will take to load your car. If you are going to work, you can simply hop on a moped, grab your briefcase, and that is it. However, if you are going on vacation, with a big car and two young children, playing Tetris for hours with all their equipment in order to load the car as best as possible takes much longer, and it is the same with networks. This is the transmission delay, which is the time required to transmit the message, which depends on the medium throughput, 512-kbps ADSL is not the same as 300-mbps fiber optic, and on the size of the message you are sending, a 1500-byte message takes longer to send than a 50-byte message. I will come back to propagation delay. In traditional communication systems, the maximum speed of a signal is necessarily the speed of light. An electromagnetic wave propagates in a vacuum at the speed of light, which is 300 000 kmps. This means that there is a limited and incompressible time, and communications take at least this amount of time to travel from a point A to a point B. Therefore, in the case of a satellite, its altitude determines a fixed amount of time required to go to the satellite and back, do a satellite hop, a round trip between the Earth and the satellite. This picture from Wikipedia is a reminder of the various satellite altitudes, and as you can see, some satellites are very close to Earth, about 500 to 700 km from Earth, while others are much further away, like geostationary satellites, which are 36 000 km away from Earth. This means that the propagation delay is linked to the altitude of the satellite you are considering. In the case of Globalstar, which is about 1 400 km above ground, the round-trip time is 10 ms, and in the case of Iridium, which is only half as far, it takes only half the time to reach it. However, this doesn't take into account the hops between satellites or going through the various satellites, or the system load, only the propagation time. So if you add all this you end up with far more than 5 ms. And for a geostationary satellite, about 36 000km above ground, it takes approximately 124-125 ms to reach the satellite at light speed, then also 124-124 ms to come back, so the satellite hop takes approximately 250 ms. The impact of delay is clear, it is not poor audio quality, signal loss, or things of that kind. Of course attenuation can occur due to the distance, but it is not caused by delay itself, the impact of delay is an impact on user experience, and particularly on interactivity. The longer the delay, the poorer the user interactivity. Hailing you from 5 meters away is not quite the same as calling you via a geostationary satellite, with the 250 to 500 milliseconds it takes to reach you, then with the same time for the information to come back. Receiving the answer to your question takes up to 1.5 second, which might disturb and annoy you, and lead you to use other communication systems, if any are available. How can the delay be altered? As I explained, regarding the propagation time, the satellite altitude must be changed, since the speed of light is incompressible. This is why LEOs were used for voice at first, as I explained earlier in this sequence. Regarding the throughput, the system capacity simply needs to be increased, but it is not always easy, and can be expensive. Finally, regarding congestion, one solution is to control the access to the system, to control the resources of the system so that it is not overloaded. Applying this system to highways, which happens sometimes, closing the access roads prevents people from taking the highway when it is overloaded, for example. Something even more important, that has much more impact, not in terms of interactivity but in terms of the quality of the communication itself, is what I called jitter, which is the variation in end-to-end delay. I showed it on this picture, so that you can clearly see that when the delay does not vary, all messages take as much time and arrive at regular intervals, but the propagation delay can vary, causing some sort of accordion effect. The transmission delay can also vary, with message transmission taking more or less time, also causing an accordion effect, which distorts the audio signal that you receive, turning it into something quite inaudible, as if I were speaking twice as fast, three times slower, and so on, as if I were compressing my words, but since they are actually sounds, it get complicated as they have different frequencies if it happens directly in analog form. So, jitter distorts things, and it usually comes from network congestion or overload, but also from going through a different number of Iridium satellites, from the mobility of the users, who move, and therefore influence the delay, from the access time to the medium, the time it takes to request the resources and be able to transmit the information, which may vary, meaning that the information takes more or less time to arrive, which is unpleasant. Other factors include for example the quality of the medium. So what is the impact of medium quality? If the medium is significantly prone to loss, to errors and to noise, you risk losing messages, and having gaps or missing parts, or even not being able to receive the information. Another factor is network quality, if the network is significantly overloaded, or if throughput fluctuation occurs, you will have the accordion effect that I explained to you. The transmitter's equipment can be of poor quality, or the receiver's, its display, watching this video on a smartphone is not the same as watching it on a 50 or 51-inch HD television set. To conclude this section on service quality, here is an illustration, devised by students from Télécom Bretagne, during a project, the references are given in the pictures. We send an audio signal via a satellite, and monitor what happens, so let us start by listening to this track. -"You will have to be very quiet." -We will now send it via an emulated satellite, first without implementing any protection mechanism. -"You...have...be very...uiet." -We cannot hear very well, and we can tell from its spectrum that the signal is somewhat garbled, that there are gaps, and that things must be missing compared to the following one. So we will implement some protection, service quality mechanisms, to protect the signal in the network, and you can hear the result. -"You will have to be very quiet." -It is a bit better this time, it is much clearer, we can understand. I will now leave you to think about the importance of service quality in networks, and I shall see you again for the last session of the last week, following this episode.
