One of the things that few people appreciate is that the Internet was able to expand so quickly because
well we already had a network ready to go.
And that was our long distance telephone system.
Now to pass the Network+ you really need to understand how the telephone system works.
It's actually kind of cool so let's take a moment.
We're going to go way back in time.
Ever watch any of these old movies where they have the phone on the wall telephones made out of wood.
They're picking it up and they turn a crank and they're like: hello shady.
Yeah.
Give me Don over at the butcher shop and all that type of stuff.
Well that is actually how phone systems originated over 100 years ago.
So do appreciate that.
Let's take a look at our blocks here.
What would happen in the old days is that you would have telephones installed into individual locations
and then a physical line from each one of those telephones would be brought in to a central office.
These central offices were usually no more than three miles away from any particular phone.
The reason for that was that analog signals had a bit of a problem going more than about three miles
at the voltage they used back back over 100 years ago. In the central office was this usually a lady
that's where Sadie was and she would hear you as you crank.
You were actually bringing voltage onto the line she would see a light come on or a buzzer come on.
She would connect to you and then make a physical connection between you and whoever you were trying
to call.
Now this system worked great.
And after a while by World War II Saidie had disappeared and we had replaced her with electronic systems.
But it was still an analog system and it worked well.
The challenge we ran into is when you wanted to talk to somebody further away.
So if we look at a situation like this here we have some people connected to one central office and
other people connected to another central office.
Now if I'm going to go make a call from here to here we could do that but we would use something called
trunk lines.
Now trunk lines worked in a very interesting way.
What they would do is they would use something called frequency division multiplexing to allow you to
handle more than one call.
So if you've got lots and lots of people calling between these two central offices what the phone company
would do is they would take your phone call and then they would shift the frequency.
So all of a sudden your phone calls like this and then somebody else is I can't even get as high as
it would be.
But that multiplexer would split through frequency division multiplexing all of the different calls
into these different frequencies it would come over the trunk line over to the other end which would
bring it back down and then it would be handled that way.
This worked really really nicely except for one problem and that is analog by definition really has
a problem with long distance.
And I'll bet you've even seen in some of those old movies where the guys trying to call from New York
to let his mom know he's going to be in Wichita in a week or something so get ready and be like they
couldn't hear each other and it was difficult.
So what happened is that starting in the 1950s and pretty much completed by the 1970s in the United
States we switched from an analog to digital system.
Now with a digital system you're going to see a couple of things change. With the digital system the
conversation between your location and the central office was still analog.
But once it got into the central office it would be broken up into individual digital samples that would
be moving along and building up and making the conversation.
Once it gets to the other central office that was sent back to analog.
So you would have these little digital pieces that were being sent across. All of these little pieces
are being sampled with 8 bit samples eight thousand times per second.
So each little piece of this conversation is 64 K now you would have lots and lots of these going over
so it's not just one you just keep going and going until the conversation is done.
Now the tricky part to this is that everybody hears the word 64 K in the telephone world and if you haven't
you're going to be hearing it a lot shortly.
So the reason 64 K came along was simply because the telephone company thought that that was the proper
sampling rate to be able to hear a good phone conversation.
And if any of you ever use a landline any more you can appreciate that compared to most cell conversations.
Landline sound pretty good.
Anyway if we come back down here and take a look, the trunk line is going to change dramatically.
Instead of being a frequency division multiplexer it's going to go into time division.
So the conversations are being broken up into the 64 K chunks but that one big line is carrying a lot
of different conversations.
So we had to take these individual chunks which are known as a DS0 signal and we're going to combine
them into a great big chunk which is called a DS1. A DS1 signal is 24 DS0 signals all
going down the same piece of wire.
Keep in mind that we're not doing frequency division multiplexing anymore.
We're doing time division multiplexing and anybody who's been working in networking particularly Ethernet
understands that we don't have frequency issues.
We just have everything in little blocks. So the whole idea of frames and all that was happening back
in the 1950s.
OK.
So we've got a DS1. Now DS1 is just a signal type and it runs on a very specific type of cabling
system called T1.
So if you've got 24 DS0's on a DS1, 24 times 64 K comes up to about 1.5 megabits per second.
So in the networking world we say we have ethernet frames and ethernet cable. In the telephone world
we use the term DS1 frames but it's on a T1 system.
Now that's great but you can even go faster than that.
The other type.
And these are all copper carriers.
By the way the other type that you can use is called a DS3. A DS3 signal runs 28
DS1 simultaneously so it does a lot of signaling going on.
Now in Europe we have E1 and E3 which are very very similar to T1 and T3 but not exactly the same.
Make sure you memorize all of this for the Network+
Now the thing you need to appreciate about all of these T carriers is that they're point to point connections.
There's no telephone number associated with them or anything like that.
They were originally designed just to interconnect central offices.
However over time we've discovered that these are pretty good ways to haul data as well.
Now if you want to get say 1.5 megabit per second throughput you can order yourself a T1 line.
And what they'll do is they will literally run a line from their central office to your location.
In most cases there's so many lines already installed they just pick one of a zillion that are already
there.
So they run this line and on each end of that T1 line is what we call a CSU/DSU.
By the way a T1 line the inter connections look pretty much like just shielded twisted pair.
So looks pretty much like Ethernet.
So these ends are plugged in and you have a CSU DSU.
Now the CSU DSU acts as the endpoint and it can be you can do a lot of stuff with CSU/DSU. For example
you can plug your CSU/DSU directly into a router.
Any good Cisco router knows how to speak T1 no problem.
Those are plug in and it just runs forever so once it's turned on it is a permanent connection it's
always running.
Now there's one other little quickie term I want to throw in and that is something called a BERT test.
Whenever you have a problem with a T1 line you have a BERT test. It usually manifests itself as
a button on the CSU DSU and it's a bit error rate test and it's just a quick and dirty way if you think
you've got a problem with the T1 you do a BERT test on it to make sure that that inter connection is
good.
Now also on the Network+ is the term T1 crossover. A T1 crossover is a pretty weird little thing
and you'll see it a lot of times where people are experimenting with routers.
You can take two routers that have a built in CSU DSU and you could literally take one little piece
of T1 crossover plug it into each one of the CSU/DSU's and it's a fun quick way like if people are
learning Cisco certifications and such they can use this as a way to emulate a full blown T1 connection.
So that is on the test so make sure you're aware of that.
So that's the basis of telephone communication and how it affects ourselves in terms of the Internet.
The big thing I want you to take away from this more than anything else is an appreciation of the difference
between frequency division multiplexing versus time division multiplexing.
And make sure you know your T connectors. It's on the exam.