Voice over IP

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Из курса от партнера Institut Mines-Télécom

Introduction to Satellite Communications

214 оценки

Institut Mines-Télécom

Introduction to Satellite Communications

214 оценки

How is a satellite built? How do they fly? How do they communicate and how does the network operate? You will get all the answers in this course from teachers and researchers from three schools associated with Institut Mines-Télécom. The course is made of : teaching videos, equipment demonstrations and simulation programs. They will guide you through the discovery of satellite communications. Professionals in the space field will share there vocation for this scientific and technical sector. Have you ever wanted to know more about transponders, the geostationary orbit, QPSK modulation, channel coding, link budget, TCP over large bandwdith x delay product links ? This course is for you! This course is available in English: French-speaking lecturers with English subtitles and fully translated contents (slides, practices). This MOOC is supported by the Patrick and Lina Drahi Foundation.

Из урока

Networking and services

This module explains how services such as voice, television and Internet access are provided by means of communications satellites.

Independent systems5:10

Globalstar vs Iridium7:12

Interconnection with mobile telephony networks5:51

Voice over IP6:13

Going further: Impact of a satellite link on QoS10:19

Познакомьтесь с преподавателями

Laurent Franck
Professor
IMT Atlantique, on leave to Airbus Defence & Space as of September 1st, 2017
Tarik Benaddi
Associate Professor
IMT Atlantique
Julien Fasson
Associate Professor
INPT/ENSEEIHT
Nathalie Thomas
Associate professor
INPT/ENSEEIHT
Marie-Laure Boucheret
Professor
INPT/ENSEEIHT

-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.

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