Instructor: In this lesson, we're going to talk
about three different types of technology:
dial-up connections, ISDN connections, and DSL connections.
The first one we need to talk about is what's known
as the plain old telephone service, or POTS,
which runs as a dial-up connection.
These type of connections are going to be used
on the public switched telephone network, or PSTN.
This consists of all the telephone carriers
from around the globe.
I can make a phone call
from my house to my neighbor's house,
or from my house to the other side of the world,
and it's still running on the PSTN using POTS service.
This is what we call analog phone service,
and all based on beeps and bops.
Now these analog connections can be voiced when I'm talking,
or it can be data that's being converted
from ones and zeros into a modulation
of that sound using those beeps and bops.
This is how PSTN works.
Now, these are often called dial-up connections
or POTS connections.
Now, if you're an older person like me,
you might have used a dial-up modem back in the day.
You might remember that dial-up modems have
a maximum bandwidth of 53.3 kilobits per second,
which means they're really slow.
The reason for this is the phone system
worked on these T1 channels,
and each one is only 64 kilobits per second.
And so essentially, POTS ran over the same technology
as a T1 connection or one-24th of a T1 connection.
So when you made a phone call, you only had access to 64K,
and part of that was being eaten up by overhead,
so you got a maximum of 53.3 kilobits per second.
Now I just give you a lot of information about dial-up,
and you really don't need to memorize all the specifics.
Because honestly, most people don't use dial-up anymore.
In fact, if you're still running dial-up,
you're not watching this video
because it's just not fast enough, right?
It's too darn slow for anything you really want to do
on the real internet nowadays.
But you may find it
in some very specialized, unique situations,
or what we call legacy systems in your network.
Now legacy system means it's an old system,
but we still need it for some critical function,
and it just can't work on the internet itself,
so it still relies on something like a dial-up modem.
Now, for the home and office user,
you're just not going to find these dial-up modems anymore
because most people just don't use them.
You may find something known as ISDN though,
which is the Integrated Service Digital Network.
It supports taking multiple 64k bearer channels
and putting them together.
So it works a lot like dial-up,
but you can bind those channels together
and get speeds up to a T1 connection.
Now, again, this is an older technology designed
to carry voice, video, or data over these channels.
Now we're not going to go too in depth into ISDN here,
because honestly you're probably never going to run
across one in the real world here in 2020,
because most people have already moved
to cable or fiber or even a T1 connection.
But if you work for some organizations,
they may have some legacy thing out there
that still has an ISDN.
I actually ran into one about six months ago
where I was doing some contracting.
Now these, again, were popular back in the late nineties.
So it is something you may come across, but not likely.
Next we have DSL, which is a Digital Subscriber Line.
Now, when I first started building networks
back in the late 1990s and early 2000s,
DSL was all the rage.
It was everywhere because it was a really inexpensive way
to get high speed data to our end users
and small offices and home offices.
Now, instead of having to pay for a T1 connection,
which could cost you 1000 or $2,000 a month
you could actually go ahead and get a DSL connection
for 50 or $100 a month.
Now there was three different types of DSL.
You have ADSL, SDSL, and VDSL.
ADSL is asymmetric DSL.
That means there's a different speed
on the download versus the upload.
The maximum download speed was about 8 megabits per second
according to the current textbooks,
although there were some
that were getting a little bit faster than that.
The upload speed was equivalent to a T1
at about 1.544 megabits per second.
So I have 8 megs down 1.5 up.
Now, why would it be okay
to have different uploads and download speeds?
Well, this went back to the 80/20 rule.
When you're online, do you upload more or download more?
If you're like most people,
you download a lot more than you upload.
For example, when you're watching this video,
you selected the video with your mouse
and you told the server what you wanted to watch.
That may have been just a couple of kilobits in size.
Then the video sent you back
100 or 200 or 300 megabytes in size,
and it's a really large file,
and you're downloading all of that.
So your upload was very, very small,
but your download was a lot.
And for most users, they upload very little,
but they download a lot of information.
And so ADSL maximizes the download
and minimizes your uploads.
Now SDSL, or symmetric DSL, works just like a T1.
You'd have an equal upload and download speed.
The speeds are much slower here for symmetric,
but they were dedicated access,
so you're pretty much guaranteed
to get that speed all of the time.
With ADSL,
if there was a lot of people online at the time,
it could slow down your speeds.
Now, ADSL was very popular with home users
and that started getting a lot more funding,
and because of that, ADSL speeds increased quickly
and got into those 1.5 megabits per second
and then 4 megabits per second
then 8 megabits per second.
And they kept going up from there.
When they first came out, all of this was very slow.
We were talking about DSL speeds
of 256 kilobits per second or half a mega per second.
As advancements in technology went though,
they kept moving more and more towards ADSL
and most people abandoned the symmetric DSL.
Now the last type of DSL we have
is known as the very high bit-rate DSL.
This is where you could get very, very high speeds.
In fact, you could have downloads
as high as 50 megabits per second or more,
and uploads over 10 megabits per second or more.
The big limitation here though
was your distance from what's known as the DSLAM.
Now, what is a DSLAM?
Well, this is the point of presence that's owned
by the telephone company.
With variable DSL,
you had to be within 4,000 feet from that DSLAM,
which is less than a mile.
Now, not all home users or offices could get VDSL,
because most of them them were within 18,000 feet.
And if you're within 18,000 feet, you could get ADSL
but if you were lucky and you were within 4,000 feet,
you could then use VDSL and get those higher speeds.
Again, because DSL started declining
in popularity in recent years
as more people moved towards cable and fiber,
that's what ended up happening.
And so DSL didn't get a lot more DSLAMs put in
and it ended up fading away
as we started moving towards cable and fiber,
which we're going to talk about in the next lesson.