Welcome to Exploring Computing.
Today's video is Computer Networks Part One, Hardware.
So we've been looking at individual computers
and how individual computers work.
But one of the things that makes computers extremely useful,
particularly given our current situation,
is the fact that they're all working together
on the network.
And so, we're gonna start by looking at computer network
hardware, and then we're going to take
a look at how to identify computers on the network.
And then we're going to move up to the different rules that
regulate how networks work together,
and then finally, we're going to end
by talking about how the world wide web works.
And so, this is going to take several different lectures.
But for starts, let's take a look at the actual physical way
that computers are hooked together to form a network.
So what I'm going to form a network of computers,
I need to worry about two different things--
I need to worry about what the network topology
is, which is to say what is the geometric configuration
of computers, and then I need to worry about what medium
I'm using to connect the computer together.
So that's talking about are the computers being hooked together
using electromagnetic waves.
Am I using optical cables.
Am I using copper fiber.
What specific medium am I using to connect these computers
together.
Here are three different classic topologies,
and there are many, many more.
These are just three classic examples.
So I've got a star network, a ring network, and a bus
network.
You can see that the star network has
a single note in the middle and then a bunch of nodes
in the outside.
And actually, you are all quite familiar with this network
because it turns out that this particular network is the type
of network that Wi-Fi uses.
So if you think about it, you've got
a bunch of people in your house, or you've
got a couple of people in the classroom,
and they're all on the internet.
And somewhere in that room or in a nearby room
is the Wi-Fi router.
And so, all the computers talk to
that individual Wi-Fi router.
And if the individual Wi-Fi router goes down,
nobody can talk to each other.
So that's a star network.
A ring network, as you could see,
here we have a bunch of computers,
and they're all hooked with one following the next following
the next following the next.
And so, depending upon how many nodes you have,
this actually takes less connection medium
than our star network does.
Ring networks are commonly used, for example,
with optical cables.
One of the problems with the ring network, at least,
is the way they typically work, is if one of the nodes
goes down, this message is stop traveling around the ring.
You can certainly imagine having a ring where
if one of the nodes went down, the messages started going
in the opposite direction.
But that's not typically how they work.
And then finally, our last example here
is the bus network.
And at a bus network, all the computer
connected to a common cable.
This network used to be quite common, connecting computers
together, say, in a classroom.
Bus networks also have their own set of problems.
And in fact, each of these networks
is going to have to have different roles for how
they govern and how they work.
And so one of the problems with the bus network
is if multiple computers on the network
try to send out a signal at the same time,
you have a problem where there's contention on the line.
And the network needs to have rules
for what happens when two computers are simultaneously
trying to use that bus.
So one question we can ask ourselves is what kind of--
so we've seen three different types of networks here.
What kind of network is the internet?
And that actually turns out to be a trick question
because the internet is not a single network.
It is actually a network composed of different networks,
so this is what we refer to as an internetwork.
And so here I've got a drawing of network of networks.
This is the thing that might connect different people
on campus, so we might have set of optical fiber
cables connecting the different buildings in the university.
And so we've got a ring connecting each of those,
and then we might go from there to a bus network
connecting different floors on a particular building.
And then those individual floors might have their own bus
networks, so we have a bus network
connected to a bus network.
And ultimately, for convenience, somewhere on that bus network,
we need to have our Wi-Fi router,
and then we have different people
connected to the Wi-Fi router.
So this is just a small example of an internetwork.
But in fact, the internet itself is
composed like this with different networks connected
to different networks connected to different networks.
Now there's different types of hardware
you may run into when we're working with our internetworks.
And so one of the devices is a bridge,
and a bridge is used to connect to different networks that
are of the same type.
So you can imagine this is simpler
than connecting different networks of different types.
And so, say we have one bus network that's
running across all of the dorms in a particular complex.
So say we have a bus connecting all the dorms in Stern
or all the dorms in FloMo, and then
each of the individual houses might have their own network.
And so, we would need a bridge to connect
one of those networks to the other network.
If we have two different types of networks connected together,
we need a fancier piece of equipment.
And so, a router connects to different types of networks.
This is probably most famous for wireless router,
so you hear people talk about wireless router.
That is a device that connects the wireless network,
the Wi-Fi network, to some other type of network.
You may also hear the term gateway or gateway computer.
A gateway computer is a computer that
basically acts similar to a router or a bridge.
It connects different networks together.
The difference being that the router or the bridge
is a very special purpose box that that's
its only purpose in life is handling these messages.
Whereas a gateway computer is a general-purpose computer
that, among other things, handles network traffic.
Now, we're not going to talk too much
about the different mediums that are available to connect
our computers together.
There's a wide range ranging from plain old telephone
service or POTS, which is the traditional copper
cable is used for telephone services ranging up
to cable networks to fiber optic cables to Wi-Fi to satellite.
These all are different mediums, and they're are different ways
we can connect the computers in our network.
One of the issues that comes up when
we're talking about different mediums is the bandwidth.
And so what the bandwidth is-- what the bandwidth originally
referred to--
I suppose technically still does,
although computer scientists tend
not to talk about it this way--
is the band of frequency that set aside
for that particular communication to take place.
So the wider the frequency band we have available,
the more messages we can send through.
Computer scientists and generally,
colloquially, most people refer to bandwidth is just
the speed of the network.
So if I have a lot of bandwidth, that
means I can spend a lot of information quickly.
And if I don't have a lot of bandwidth,
then I can't send messages very quickly.
You may also hear the terms broadband and narrow band.
Narrowband basically is plain old telephone line,
and plain old telephone service are POT as it's technically
referred to.
I actually think it's a cute name, POT.
That has very low bandwidth, and we can't send much information
through it.
And in fact, as we saw in one of our previous videos,
if we want to send a five-minute CD song over POT,
it's going to take about five hours.
So that's pretty bad.
The term that you'll hear more frequently is broadband,
and this just refers to something
that's not narrowband.
Generally, these terms aren't in widespread use
anymore except for there's a fair number of companies that
have a name broadband in it because it
used to be really cool to be broadband instead
of narrowband.
But now, everybody's broadband, so you will still
hear the term broadband banding around
mostly in the names of various companies that are out there.
But that's what they're referring to.
Here's some more terms you might run into.
LAN stands for Local area network.
That's a small network.
So if we have a classroom, we have
a bunch of computers hooked together in a classroom,
we might have a LAN.
If you play computer games or you
have friends that play computer games,
you may hear them refer to a LAN party.
This is a party where people bring their computers
and get them networked together to play computer games.
Well, we used to do these back in the day
when Stanford changed its drinking policy at one point.
We were trying to provide students with things
to do other than drink.
And so one of the activities the RCC
started having LAN parties for campus.
That was kind of cool.
You may also hear about a wide area networks.
These are networks that connect cities across continents
or even across continents.
You may also hear the term MAN, metropolitan area network.
This is a network within a city.
So if you have a friend that's saying
I need to find a good man, they might
be referring to wanting to find a good guy,
or they might be thinking about which city
they want to move to after graduation
and making sure that that city has a good metropolitan area
network.
I have to admit.
I've actually never seen man in use in this usage other
than in-network textbooks.
But I'm fond with this joke, so you guys
will have to put up with it.
Another term you might run into is hops.
As we talked about earlier, the internet
is not a single network.
It's actually a network of networks.
And so hops is referring to how many networks my message needs
to go through in order to get to its destination.
So if I'm trying to communicate with a friend, say at Harvard,
my message is going to have to go through many hops
as it goes from the Stanford networks
out to Stanford's connection with the wider internet.
It goes through a bunch of internet backbone providers.
It goes back at the other side to get
into the Harvard network.
It probably has to hop through a couple of different networks
at Harvard.
So these are all different things
that the message has to do in order
to get to its destination.
And what's going to happen here is
there's a number of different things
that's going to determine how fast a message or data travels
through the internet.
One is going to be the connection medium.
So if I've got fiber optic cable,
that is going to be much faster than, say, plain old telephone
service POT.
But the other issue that's going to occur
is every time message traffic goes from one network
to the next network, there's also
going to be delay because whatever we're
using to connect those two networks together
needs to spend some time looking at that message,
figure out is this a message that's supposed to be
going to someone on my network.
Is this getting passed to another network.
How does this need to get from this network
to the other network.
And so there's a delay because this extra processing
has to take place.
So the more hops we go through, the longer the message
is going to take to get there.
On a related note, you may hear people
referring to lag and latency.
Generally speaking, lag refers to the perception
that the message is getting passed very slowly.
For example, one of the things that I
used to do as a resident fellow was when we were interviewing
for new arrays, we would sometimes
interview people at various Stanford
overseas being overseas programs.
And you would notice sometimes there
would be a long lag between when you--
someone spoke and when you actually heard their words.
And so this is lag.
This is the appearance that the network is not doing well.
Another time you will hear the term lag
is you have a friend who plays first-person shooters,
and they're like, that wasn't my fault. It was the lag.
Actually, it was their fault. They sucked.
But the idea here is they tried to move their character
in a particular direction and because of the delay in network
traffic, they ended up getting shot.
It wasn't because they didn't move quickly enough.
It was because the message had to travel through.
Too many slow connections or too many
hops, or there's just too much-- we'll
talk about this in lecture.
There was too much--
too many other packets traveling to the same network.
Anyway, so the end result was the information
that they did not get to the server in time.
The term latency generally is slightly more technical term
referring to the amount of delay involved in the transmission,
but these two are often used more or less synonymously.
I should mention that lag and latency are not necessarily
directly related to the speed of the connection.
And so, the idea here is you can have a lot of bandwidth,
and you could still have lag and latency.
So here's an example here.
Suppose we are sending a message from Earth
to a spaceship traveling around Mars.
We've got several different things
that we need to think about here.
How much bandwidth do we have.
Let's say I'm sending a new copy of the latest movie out.
I don't know.
I guess there aren't any movies out right now,
but let's pretend Mulan is out.
And I've got an astronaut out there,
and she really loves Mulan.
She loves the cartoon version of Mulan,
and the new live-action version of Mulan is out,
and we want to send her a copy, and she's out of Mars.
So if we have slow bandwidth, it is
going to take a long time for that entire movie
to reach this destination regardless
of how far away that person is.
So let's say two of us are on the same network.
There's no hops between us, but we're connected
by a plain old telephone line.
It's going to take a long time for that movie to transmit
over that telephone line.
On the other hand, if we have a really fast connection
because this person is really close to us,
it's going to transfer very quickly.
Now, let's look at the situation.
We're sending the data to Mars.
Regardless of how much bandwidth we have,
it is still going to take a very long time for those first bits
to reach Mars.
And we have high bandwidth, once those first bits
reach Mars, the rest of the bits will fall very quickly.
If we have low bandwidth, we still
have the same delay because it just
takes a certain amount of time for light to travel to Mars.
But if we've got low bandwidth, once those bits start
reaching Mars, they're going to have
to wait a long time between when the first bit hits there
and when the last bit hits there.
So the point of this is there's different issues involved here.
It's not just a matter of, hey, I have a really high bandwidth
connection, or I have a really low bandwidth connection.
Yes, those are important, but there's also a question
of how far away is this person.
How many hops does this have to go through and things
like that.
And that's actually going to be an issue when
you start worrying about things like web servers.
We typically don't get into this, but since I've
got a captive audience here.
One of the issues that comes up is if I've got a web server
and that web server is, say, in California
and my customer is in Australia, there's
a pretty long delay where the message needs
to get transferred from my server
here in California through a whole bunch of hops
to get to Australia.
And so, a better way to handle this is to use something
called a content delivery network or CDN.
And what a CDN does is it takes the files that would normally
be in my server here in California
and distribute this to a whole bunch of different servers
around the world.
So that would be a server in Australia or Oceania.
That would be a server in Asia that would server in Europe,
and there'll be a server in the United States.
There'll be a server in Africa.
And wherever the person is trying
to access our information from, they
will get it from the nearest server.
And so, there will be fewer hops to get to that server.
There will be less lag.
There will be less latency, and the information
will get to them much faster.
So if you're going to come up with a server
and you think that you're going to have a global audience,
contracting out to content delivery network
might be a great idea.
All right.
So that's it for now.
In the next picture, we're going to talk
about how to identify individual computers on the network.
And the next Friday, we're going to take a look
at different protocols that are used governing how information
travels through the internet.
I'll see you again soon.