The stations a letter passes through

It can't go in one jump

My computer and my friend's computer
each have a unique address.
That number address we saw before.
But the two are not joined
by one direct wire.
We cannot tie every computer
in the world together with cables.
So the data cannot go straight
and needs places to pass through on the way.

Tap between the start and the end. It looked like a straight line, but stations were hiding in between.

The path that looked like a straight line
actually had several stations.
Like a letter going through post offices,
data passes through stations on the way.
This kind of in-between station
is called a router.
So what do these stations
do with what they receive?
Let's look inside one station next.

A station hands it over

What one station does is simple.
It takes the data that came from before
and hands it to the next station.
That is all.
Like a clerk at a post office
sending it on to the next post office.
The station does not open the letter.
It just receives, and passes it on.
This simple action is the heart of it.

Tap the middle station. It takes the packet from the left and hands it to the next station on the right. (station = router)

When you tapped the station,
the packet moved to the next one.
Receive, then hand over.
Those are the only two things a station does.
But with just one station
it still cannot go far.
The friend is not one station away
but many stations away.
So we need several stations.

It passes through many stations

When one station hands it off,
the next station hands it off too.
That station hands it to the next as well.
Repeat this
and the packet moves from station to station.
One step at a time, closer to the destination.
Like crossing on stepping stones.
One stone is short,
but step on many and you cross the river.

Tap send. The packet moves from station to station, one step each, until it reaches the end.

The packet stepped on each station in turn
and at last reached the end.
Even if there are many stations,
there is nothing to worry about.
Each one only has to hand it on.
That simple job, chained together,
spans a long distance.
But let's look closer at what happens
the moment a station hands the packet on.

Receive, then send again

A station does not bounce the packet straight off.
First it briefly receives the packet,
holds it in hand a moment,
then hands it on.
Receiving and sending
are two separate actions.
You must receive before you can pass it on.
Like holding a letter in hand
before you can drop it in the next mailbox.

Tap receive and the station briefly holds the packet. Then tap send and it goes to the next station.

Receive, then send.
The two actions made a pair.
Repeat this at every station
and the packet is carried all the way.
But while the packet rests at a station,
the station must decide one more thing.
Which station to hand it to next?
Choosing the road when there are many forks,
that we will meet in the next lesson.

Let's wrap up

Gathered on one line, it's this.
Data cannot reach its destination in one jump.
It passes through several in-between stations, the routers.
Each station receives the packet and hands it on.
Receive and send, repeated at every station.
Getting one step closer that way,
it reaches even the farthest place.
So even from afar, it arrives step by step.

Tap the key points in order to review. (can't go in one jump → a station hands it over → through many stations → receive then send again)

Now you can see the path
data takes from station to station
to reach a far place.
One question remains.
At a fork, which next station
does a station choose?
That road-choosing is called routing.
In the next lesson, let's follow together
how a station decides the way.