[MUSIC]. Good day viewers, in this segment we'll
take about different kinds of media, wires and so forth.
This is material at the bottom of the physical layer.
Okay, so there are three kinds of media which are very common that we'll talk
about. Wires, fiber optic cables and also
wireless as a kind of media for propagating signals.
The issue with all of these kinds of media is that they propagate signals.
These signals carry bits of information. In this segment, we're just going to talk
about these basic types of media and get a little bit of a feel for them.
Then we'll move on to understand how signals that which carry the information
are propagated across these media. So, the first kind of media we have is
just wires, twisted pair wire. That's your garden variety wire.
It's quite common. this is the kind of wire which runs to
your house for your telephone, assuming you still have a wired telephone.
you will also probably seen it in enterprises.
This cable showing here, the category 5 UTP stands for unshielded twisted pair.
that is a, a common Ethernet cable. You can see in here that there are four
pairs of twisted pairs. The twisted pairs, each one is literally
two wires twisted around. The signal is carried as a voltage
differential across those wires. And they're twisted just to help reduce
the interfering RF signal that's radiated.
At least they reduce the radiation of that signal.
And by reducing that, we reduce the interference between adjacent pairs of
wire. Another kind of wire here is shown in the
picture in the middle. It's a coaxial cable.
It's also fairly common. It used to be used to carry video
signals. Still is for cable to your home.
This is a these kind of wires generally have better performance than twisted
pairs. Meaning that they're able to carry a lot
faster amounts of data, higher data rates, longer distances because of their
physical properties. You can see here it's built instead of
just twisting a couple of pairs of wire it has a metal core in the middle.
And that you can see here. And also an outer conductor round here,
around the edge, instead of the two wires.
And it's put together in a nice way. both the twisted pair and the coaxial
cable you'll see that around they're the most common kinds of wire, and they're
designed for communications. There are many other kinds of wire we
might use too. One interesting kind you could just have
a look at is household electrical power lines.
These are starting to be used for instance to carry information around your
home. Just for reusing existing infrastructure.
We can do that nowadays. You can read a little about it in the
text if you'd like. A different kind of media is fiber or
fiber optic cables. A picture shown here just to, the
operation of fiber. Fiber is here in the pink.
Now a fiber is really just a very long, thin, pure strand of glass.
The way it operates is when you si-, shine light in from a light source, that
could just be a laser or an LED, a light operating diode.
The fiber is so thin that the light bounces around, it doesn't come out of
the fiber. It bounces around until it gets to the
other end. And then it goes out, then you can detect
it with some kind of instrument, a photo detector.
Fiber compared to wire is able to transfer enormous amounts of bandwidth,
very high data rates over very long distances.
This is because it is it, well fiber because of its physical properties allows
very wide ranges of frequencies through. And it also attenuates signals very
little because very pure light just tends to zap right through it.
So signals can go for a long way before they're attenuated.
There are just like different kinds of wire, there are different kinds of fiber.
The two most common varieties are what is called multi-mode and single-mode.
Multi-mode is the cheaper version, so it's good for shorter links, maybe links
that are less fast. Single-mode is the more pure, higher
quality version. In a, a single-mode fiber, light can only
go straight down the middle because it's so thin.
And single-mode fibers can be used to carry signals up to 100 kilometers or so.
Just like the wires, there's a lot of construction here.
This portion in the middle, this is actually the fiber.
The rest is just all the padding and packaging that goes with it, the cladding
and the jacket and then over here there is a bundle with three different fibers
in that, in that one cable. And the third kind of media that we'll
look at is wireless. Wireless is fundamentally different than
wires and fiber optic in one important respect.
And that's because, and that is, that the sender is radiating a signal in many
directions, it's radiating across a region.
So the signal, rather than being confined to a wire or to a fiber optic cable,
radiates in many different directions. This has an advantage that can reach
potentially many different receivers at the same time, everyone in the vicinity.
But it also causes a sizable complication for using wireless systems.
That nearby systems, near, sorry, nearby signals which are being transmitted on
the same frequency can interfere with one another at the receiver.
This picture shows the setup. Here we have a laptop in the middle.
So this is the receiver. And you can see that there are two
transmitters. I've shown one as a little closer so
there's a fairly strong signal that arrives at this laptop.
The other one is further away, but it's signal still propagates and reaches the
laptop. So this laptop receiver will receive two
signals at the same time. Two signals superimposed on one another.
So it will be somewhat jumbled. Now because these signals interfere.
We need to be careful and coordinate the way the wireless spectrum is used.
Guess how we do that? Well, here's one answer.
Look at this chart, this is quite amazing.
This is the United States frequency allocation chart.
I don't expect you to be able to read all of that.
the, the key issue here is that this chart shows how regulation is used to
manage the different parts of the spectrum.
Different frequencies are used for TV stations, radio stations, police
communications, aircraft as well as computer networking.
So the government regulates who's allowed to use what frequency.
You can't just use any frequency you like.
In fact, just these small circled regions are the ones which you use for Wi-Fi
computer networking. There also some other 3G frequencies but,
but only very small portions of the spectrum are actually used for for data
communications, even within these circles it's just a small portion.
The portions of the spectrum that are used for Wi-Fi are actually fairly
interesting in terms of a story. they are, the frequencies that we like
for data communications tend to be in the microwave band, from hundreds of
megahertz up to several gigahertz. Now that's where you'll find Wi-Fi as
well as 3G cellular network communications.
Wi-Fi is actually transmitted in the what's called the ISM band.
you can see here this figure just shows different frequencies, and frequency goes
up to the right, so this is frequency. And some portions of the frequency were
reserved as part of these ISM bands, they're shown here.
Wi-Fi actually uses portions of these bands.
In I think 1985, the FCC decided to allow anyone to use these bands without a
license. so you don't have to buy an expensive
license, as TV stations and, and mobile network providers, providers, cellular
network providers do. That's why these these bands became
unlicensed. These were actually the the junk bands,
garbage bands just left over with a lot of interference and so forth.
They were considered undesirable. But in these circles, people began to use
them for computer networking. That was part of the intention.
Now this means that you know, within this area many different computers may be
using the Wi-Fi. So they will have to coordinate
themselves, since the government's not doing it for us by exclusively allocating
us a frequency. But these bands have been a major
success, as you know by the, the prevalence the Wi-Fi today.
There's been a huge amount of innovation in different kinds of computer networking
technologies using just this tiny niche of the spectrum.
We'll learn more in the next lecture about how signals propagate through
different kinds of media.