-You may not have realized

that I left out a major challenge of satellite communication systems,

the issue of the return channel,

which Laurent mentioned in a "Common Thread" video.

What is a return channel?

It is the communication channel between a satellite terminal

and the transmitter station or another station.

It refers to being able to transmit from one user terminal,

unlike the traditional forward channel,

which connects a gateway to terminals.

What is its use?

For television, it is not very useful except for interactive television.

However, currently,

so called interactivity on satellite or terrestrial television

is not actually true interactivity.

There is no interactivity with the television's transmitter,

only with your decoder.

The decoder preloads information and communicates with you.

It is very different with voice

since voice is not just a monologue put out there for an interlocutor.

We must be able to answer.

In this case, there is a symmetrical communication

between two people talking to each other.

With the Internet, things may be less symmetrical.

It depends on the application used.

Voice applications are symmetrical.

When surfing the Web, a user will send requests,

Even if they are small pieces of data,

we must be able to send it to request what we want

since it would be impossible to preload everything in advance.

Finally, if you are uploading things or using peer-to-peer,

it is also more symmetrical.

This is why we need a return channel to have bidirectional communications.

What are the challenges?

The first one, which you already saw, is the transmitting device.

You cannot afford to have the same transmitting device

as the one from the Toulouse Airbus Defence & Space teleport,

which you can see on the left.

You need devices which can be used by anyone, by the general public,

and are thus relatively smaller.

Here is an example of a 1.2-meter antenna.

These devices can be rather sizeable

but must cost less than large gateways, use less power, and so on.

This is the first challenge, if I may call it that.

These are very-small-aperture terminals.

The second challenge is the communication method.

It is very different to transmit

from the gateway to the terminals.

You only have one source on a given frequency range or carrier,

the gateway which will transmit toward all terminals at the same time,

which is called broadcasting.

With a return channel,

you have several terminals which want to transmit toward the gateway

to reach deeper into Internet, since we are talking about this,

for example to contact their Internet access point, or POP.

It is a multisource and point-to-point channel.

We do not want to broadcast the information to everyone.

This is already an issue to consider.

Several users transmitting at the same time means

sharing the resource between all the users.

This is a major issue.

To prevent one user from constantly using the resource

and depriving the others of it, we must divide it up.

It can be divided based on time. This is time division.

Each user, here represented by different colors,

sends their messages at specific moments which were assigned to them.

We can also divide it based on frequency.

Each frequency range will be used by one given user

who can then transmit information on this frequency.

Even better and more commonly used with satellite return channels,

is a division based on both time and frequency.

The available space will be divided based on time and frequency

into slots or bursts which will be assigned to each user.

Each user will use them differently.

The major issue for the devices

is that they have to be perfectly synchronized with regard to frequency,

but especially to time.

If one is slightly out of sync with another,

and if they have neighboring slots or bursts,

they may overwrite the neighboring slot when transmitting.

Their transmissions will collide since the neighboring slot,

which is supposed to be in its place, will be out of sync

and the information will be overwritten.

They must be precisely synchronized,

which explains why transmitters can be more expensive.

Sharing the resource means that the resource must be distributed.

Distribution is not easy. There are two main types of solutions.

You can either decide that once the user has logged in,

they can use such and such slot.

It is static, the user is logged in and has something fixed.

The issue with fixed solutions

is that they cannot be adapted or adjusted.

You cannot adapt to the user's needs. When a user is surfing the Web,

they can send out small requests or nothing.

If the user is uploading a video, they will need a higher throughput

but will be restricted by the throughput they were assigned.

With 5 kbps, it will take a long time to upload a video.

In order to deal with this issue, there are dynamic methods,

which are more complicated to implement.

There are various methods which fall into two main categories.

Some will use signaling to solve the issue

and will distribute the resource through a control center.

One center will distribute it to the users.

The users request resource and it is assigned to them.

The drawback is that latency is introduced

by the demand between the client and the control center

for resource and capacity.

Users have to wait for the right satellite to obtain the resource.

Then they can send their data, and the answer will return.

This explains why, especially at the start,

satellite communications can be slow.

When you click on a Web page, with the time to ask for resource,

to send out the request, then for its answer to return, and so on,

1 second or 1.5 second may have passed.

So signaling can be annoying for the user.

In addition, signaling leads to higher throughput consumption.

How is signaling sent out? Which resource is used?

This is not useful data.

Another solution, which is not really better, is contention.

Instead of signaling, you try to make do with what you have.

An old protocol called Aloha was used for this purpose.

It was used to communicate via satellite with Hawaii, I assume.

With this, everybody was trying to communicate at the same time,

so it did not work, but with luck

your message would reach its destination, and

you would receive an ACK in response, as we saw in the first sequence.

If you received an ACK, it meant that you did not need to send it again.

Otherwise, you tried until it worked.

Since there were a lot of users, it was not very performant.

This was just to give you an idea of how it worked.

To finish on the subject, I will present three technologies

which integrate return channels.

We already talked about the GMR, originally created for voice.

Voice is bidirectional

so it incorporates the capacity to communicate both ways.

As I explained, the GMR is similar

to our current mobile telephone networks.

You also have two return channels for television,

including DVB-Return Channel for Satellite Terminal, DVB-RCS.

It was originally made by ASTRA,

and then standardized and somewhat improved.

It is a cutting-edge and quite expensive product,

and a second version came out recently.

Opposite this performant but expensive product,

you have a lower-end product

which is about 100 times less expensive, DOCSIS.

It provides a satellite return channel.

It was inspired by a type of cable in the United States,

which is also called DOCSIS.

Need for a return link

Introduction to Satellite Communications

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