-We will now talk about protocols. You may have wondered where the digitized information that Nathalie presented during week 3 came from, that is to say the series of 0s and 1s. Data no longer comes directly from the application, such as a video or an image. This is not what is being sent directly. Why not? Imagine that you arrive in China and that you tell the first person you meet "restaurant" in Swedish. I doubt that they would show you the closest restaurant where you can have a decent meal. With communication, you cannot do anything you want. We must set certain rules in order to communicate. Some of these rules are pretty basic. First of all, you must be able to speak the same language. It seems simple, but it is not always obvious, especially with information technology. This means switching the information you want to send to a common form. You can mention what type of information you are sending, what kind of protocol you are using, so the other can speak the same language and understand what they receive. This was the first thing. Secondly, everyone cannot be speaking at the same time. If everybody was talking on a single medium, the conversation would be inaudible and you would not hear anything. In networks and telecommunications, this is called media access, the access method, access control, etc. Another thing to mention is the need to address the right interlocutor. If I teach this class while looking at the pillar next to me, you will have trouble relating to what I am telling you and if I call you "Robert" instead of "George", it will not do either. All these rules also apply to networks. We must be able to name the person or the entity we want to communicate with. You can use a name, such as what is called a URL, www.fun.fr. We can use an address, an IP address, a MAC address, and so on. We can also use phone numbers, circuit IDs, a whole lot of different numbering systems. Lastly, we must be able to speak loudly enough to cover the noise, but not too loudly so as not to be deafening to our interlocutors. There are a few rules to follow some of which are a little more complex and subtle. For example, we must not speak too fast. If I was speaking six or ten times faster or giving too much information at one time, you would be saturated. It is the same with networks and telecommunications. If the transmitter is sending data much faster than the receiver is capable of processing them, it will be saturated. This is what is called flow control. There is also the issue of detecting whether the other has received what you were telling them. For me, it will be difficult to do, which is why at the end of the week, you will receive a feedback form. In everyday life, visual contact and this sort of thing allow you to know if you have been understood, but this does not exist in networks. In order to remedy this issue, the easiest solution is to ask, "Did you receive what I sent you?" To do so, when the other receives the information, they will tell you so by acknowledging receipt, as it is called, "I acknowledge receipt of the information you sent me," as with registered mail. In networks, we call it an ACK. Problems arise when the information is not received. No ACK will be sent since nothing has been received. In this case, the solution is for the transmitter to set up a timer. After a set amount of time, the data will be sent again because I believe that my interlocutor has not received it. This is not necessary for every media. If your medium is reliable, you may not have to set up this sort of rule. There are many other issues. But with all these rules, all the details I have given you, there may be something to do. A protocol is comprised of a set of rules. So a protocol is all the rules which allow two entities to communicate with each other, one entity, as we can talk to ourselves, or several entities in the case of broadcasting, for example. As far as the Internet is concerned, the protocol has a very simple name, "Internet Protocol", which is usually called "IP" for short. I refer you to the "Going Further" episode following this one if you want to learn more about IP and its model. But you should know that if a satellite system wants to be interconnected with other networks and if these networks mostly use IP, the easiest solution is to use IP. In other words, for a satellite system to interconnect with the Internet network, it must know how to speak IP.
