- [Instructor] In this video, we want to talk
about a few different types of digital circuits,
and based on the CLCOR exam blueprint,
we need to know about the configuration
of things like a T1 circuit or an E1 circuit.
And when we talk about T1 and E1 circuits,
these are digital.
In other words, they are clocked circuits.
We might see a T3 circuit or a T1 circuit
in North America or Japan.
E1 and E3 circuits, they're most popular outside
of North America and Japan.
And we could use these for voice, video, data.
If we're using them for data, they're going to terminate
in a synchronous modem, if you will,
that we call a CSU/DSU,
a Channel Service Unit/Data Service Unit.
If we're running data over these different digital circuits,
we might be using a Layer 2 protocol
such as the point-to-point protocol.
However, that's not our focus in this course.
We're focused on using these digital circuits
not to carry data, but to carry voice.
And you see on screen as a reference
the bandwidth events available
for those different circuit types,
and we're going to get into a very extensive configuration demo
in our next video where we will configure a T1 circuit.
The total throughput of a T1 circuit is
1.544 megabits per second,
and if we're sending different voice calls
over that T1 circuit, 1.544 megabits per second is
way more bandwidth than we would need
for just a single voice call.
So how does that work?
How do we send multiple voice calls over the same circuit?
We're going to be using something called TDM,
Time-Division Multiplexing.
To paraphrase Whitney Houston,
each channel or each voice call gets one moment in time,
and we basically take turns.
Channel 1 can send a sample,
Channel 2 can send a sample, Channel 3, and so on,
all the way through Channel 24.
And by sending individual samples from 24 voice calls
in succession like that, we can simultaneously be sending
24 calls over a single circuit.
And there are some parameters for these digital circuits
that we might want to configure.
In our next video, we'll actually see the configuration
of these parameters for a T1 circuit,
but let's think about how framing is going to work
on a T1 circuit.
We said it's going to take up 1.544 megabits
per second of bandwidth.
Let's break down the math and see how that works.
We said we had 24 different channels,
and as we discussed very early on in the course,
when we take a voice sample,
we often do that using eight bits.
So we're using eight bits per channel,
a single voice sample being taken
from 24 different channels.
Here's what a lot of people miss when it comes to the math.
This frame, which contains eight bits per channel
for 24 channels, it's got a framing bit at the end
to say when one frame stops and another frame starts,
so we need to include that framing bit.
And how many of these samples are we going to be sending
per second from each channel?
Well, that goes back to the Nyquist theorem
that we discussed early on,
which said we should take double the highest frequency
that we're trying to send,
and in a regular PSTN environment,
we're not trying to send more than 4,000 hertz
as our highest frequency.
And Mr. Nyquist said, "Let's double 4,000,"
and that gives us 8,000.
We should be taking 8,000 samples per second.
How big is a frame?
And we're going to be sending 8,000 of those per second.
A frame, as we discussed a moment ago,
is going to be 24 channels.
We're sending eight bits for each channel,
and we're adding a framing bit.
That means one frame contains 193 bits,
and Mr. Nyquist said we should send 8,000
of those 193 bit frames every second,
and if we do that math, that's where we get it.
That's where the number comes from
of 1.544 megabits per second.
But when we're doing framing on let's say a T1 or an E1,
we don't send just one frame by itself.
We group them together.
It's almost like we're gluing some
of these frames end to end.
And in the T1 world, we could have either a Super Frame
or an Extended Super Frame.
A Super Frame is where we've taken 12
of those 193 bit frames,
and we've stuck those together end to end,
and we send them all together.
With an Extended Super Frame,
which is what I've seen most in the real world
in North America, an Extended Super Frame is
where we're gluing together 24
of those 193 bit frames end to end.
And if we want to use all 24 channels,
we're doing something called Channel Associated Signaling,
or C-A-S or CAS.
Channel Associated Signaling allows us to have some bits
to indicate things like an on-hook condition
or an off-hook condition.
Specifically what we might have in something
like a PBX environment is a channel bank,
and a channel bank is going to take in
a T1 circuit on one port, and it's going to break it out
into its 24 individual circuits.
We could send one circuit to a phone,
we could send another circuit back
to the central office, and so on.
And when that phone goes off hook,
we talked about earlier the concept of loop current.
We said loop current would start flowing over that circuit.
How do we represent that off-hook condition
on the T1 circuit?
It's digital.
Well, we can do that using these signaling bits.
The way Channel Associated Signaling handles
these signaling bits is that it will take a bit
from every sixth frame.
So if we have Super Frame framing,
we're going to go into frame number six and frame number 12.
And from each one of those 24 channels,
we're going to take the low order bit, meaning for that sample,
we're only sampling using seven bits, not eight bits.
Does that hurt voice quality?
Technically, yes, I probably would not be able
to tell the difference, but technically, yes, it does.
And two bits for signaling, that's fine
if we just have a phone going on-hook or off-hook.
But if we have something a bit more complex,
we're connecting into a PBX,
we might need extra bits for signaling it,
and that's what I typically see.
And that's with an Extended Super Frame.
We're still going into every sixth frame,
and we're using the low order bit as a sampling bit.
All 24 channels in that sixth frame,
they're giving up a bit to represent a signaling bit.
And if we have these four bits in Channels 6, 12, 18, 24,
we call those signaling bits the A, B, C, and D bits.
However, with only four bits at our disposal,
there's not a lot of information we can send
with just four bits.
We could send things like on-hook or off-hook information,
but we're not able to send something
like caller ID information, where information indicating
if this call was an international call
or a long distance call or a local call.
We simply don't have enough bits
to store that much information.
That's where Common Channel Signaling, or CCS, comes in.
With Common Channel Signaling, we can say, "You know what?
Why don't we just take one of those 24 channels in a T1
and just dedicate it to carrying nothing but signaling."
And typically we say, "All right, channel 24,"
the very last channel in a T1 circuit,
"if we're using Common Channel Signaling,
it's going to be dedicated
to only carrying signaling information,
and it's going to carry more than just four bits per channel.
It's going to run its own signaling protocol."
And the CLCOR exam wants you to know about ISDN,
Integrated Services Digital Network,
and we'll dig a bit deeper in that in just a moment.
But with ISDN, we use signaling protocols
that are running on the 24th channel,
leaving the other 23 channels
at full capacity to send voice.
Well, we're sending a lot more information in Channel 24
because we have more bandwidth available,
and this is the way it works on a T1 circuit,
but what about an E1 circuit?
On an E1 circuit, we don't have the concept
of a Super Frame or an Extended Super Frame.
Instead we have an E1 Multiframe.
That's where we have 16 E1 frames glued end to end.
And while a T1 has 24 channels,
an E1 has 32 channels,
but the first of those 32 channels is always used
for framing and synchronization.
So it's not going to be carrying voice.
It's not going to be carrying signaling.
Channel 17, it is dedicated
to carrying nothing but signaling.
If you think about it, we've got 32 channels,
but two of them are dedicated.
That leaves us 30 channels we can use for voice,
and we want to send signaling information
for those 32 channels.
And yes, we could use channel 17
to carry a signaling protocol,
like we did with our T1 circuit,
but how do we use those A, B, C, and D bits?
Well, there would be a series of bits
in that very first frame in the multiframe to declare,
dum, dum, dum, this is an E1 multiframe.
And then in Channel 17, in the next frame,
frame number two of that 16-frame multiframe,
there's going to be eight bits in that channel.
We would use four of those bits
as the A, B, C, and D bits for Channel 2.
We would use the other four bits
as the A, B, C, and D bits for Channel 18.
And the next frame in the multiframe,
we would use four bits for Channel 3,
and four bits for Channel 19,
and on and on it goes like that.
That allows us to send four bits of signaling information
for 30 different channels inside
of this 16-frame multiframe,
and that's the way that framing works on T1s and E1s.
Something else we might need to configure is line coding.
How do we represent with voltage
a binary 1 or a binary 0?
On a T1, one option is
to use Alternate Mark Inversion, or AMI.
With AMI, the goal is to have an average of zero volts
on the wire that has a nice electrical characteristic.
So let's say we're coming into the beginning of this frame.
We're about to start eight bits.
And at the start of this byte, we want to send a binary 1.
Well, we might do that with a positive voltage.
That would represent a binary 1.
Now we want to send a binary 0.
We could do that by sending no voltage.
Now we want to send another binary 1.
Well, with Alternate Mark Conversion,
we're alternating the polarity of the voltage.
Now when we want to represent another binary 1,
it's going to be a negative voltage.
You see, on average, we have an average
of zero volts on the wire, and let's say we want
to send binary 0s for the rest of this byte.
Well, it works fine
by just alternating the polarity like that.
We have an average of zero volts on the wire.
We're representing binary 1s with the presence of voltage,
binary 0s with the absence of voltage.
However, AMI could run into an issue.
If we have a byte containing all zeros with AMI,
it's kind of hard to tell sometimes where one bit starts
and another bit stops because it's just silence.
We're not hearing any voltage at all on the wire.
And even though it should work most of the time,
and vendors have come up with some vendor proprietary ways
of handling an all-zero byte,
there's another way of doing line coding
that represents a byte containing all zeros
in a different way, and that line coding is called B8ZS
for Bipolar Eight-Zero Substitution.
In other words, we're substituting eight zeros
with a couple of what are called bipolar violations.
Remember, we want to have an average
of zero volts on the wire,
and we do that by alternating the polarity for binary 1s?
However, if we have a bipolar violation
where we send two consecutive positive voltages
or two consecutive negative voltages, that's a violation
because we're supposed to be alternating.
However, with B8ZS, we intentionally create a couple
of bipolar violations.
Let's say before this byte started,
the previous binary 1 was a positive voltage.
Well, at a very specific bit position with B8ZS,
we send another positive voltage followed
by a negative voltage, then no voltage,
then another negative voltage.
That's another bipolar violation.
And then we even things out with another positive voltage.
We're not representing ones with those voltages.
We're trying to make sure that this byte is interpreted
as a byte containing all zeros,
and we do that by creating bipolar violations
at these very specific bit positions.
I guess you could say with B8ZS,
two wrongs really do make a right.
Now that's on a T1 circuit.
Now on a E1 circuit,
there is a similar approach to line coding called
High Density Bipolar of Order 3, or HBD3.
They have slightly different bipolar violations,
but it's the same basic concept.
Now we're going to take in our next video
one of these T1 circuits,
and based on the CLCOR blueprint,
we're going to configure that T1 circuit
to act as an ISDN circuit.
Let's talk a bit about ISDN
before we go into the configuration in our next video.
ISDN, Integrated Services Digital Network is
what that stands for.
And there are two basic types of ISDN circuits.
We have BRI, or basic rate interface,
and PRI, primary rate interface.
And with BRI, we have three channels.
There's a D channel that carries the signaling information.
It only has 16k of bandwidth
because it's only carrying signaling for two channels.
Then we have two B channels, or two bearer channels,
that carry the voice or the data or the video.
They're 64k each.
They could each carry, for example, a single voice call,
and the signaling for those voice calls would be handled
by that 16k D channel.
And you don't see these BRI
or basic rate interface circuits as much as you used to
because higher bandwidth amounts have become
more widely available.
For example, I have gigabit ethernet in my home,
when years ago I might have been happy
to have a BRI connection giving me 128K
of combined bandwidth.
So we don't see BRI as much as we used to, but for the exam,
I do want you to know about the configuration of a PRI,
a primary rate interface.
This is where we take one of those T1 or E1 digital circuits
and we configure it to act
as a primary rate interface ISDN circuit.
And we said that a T1 had 24 channels.
Well, off those 24 channels,
we're going to dedicate one channel, the 24th channel,
to carrying nothing but signaling,
and that's going to be our D channel.
And because it is carrying signaling for 23 other channels,
it needs more bandwidth than the D channel
for the BRI circuit, which was only 16k.
Here the D channel is 64k,
and we've got 23 bearer channels, 64k each.
Or we could use an E1-PRI.
This is where we still have a D channel. It's 64k.
And remember, this is going to be in the 17th channel
on an E1 that's dedicated to signaling,
and the first channel is dedicated
to framing and synchronization,
leaving us 30 channels that can carry voice.
So we've got 30 bearer channels, 64k each.
Now let's identify specifically
what signaling protocols are happening in that D channel.
We've got a couple of main signaling protocols with ISDN.
There's a Layer 2 signaling protocol called Q.921,
and there's one at Layer 3 called Q.931.
And we're actually going to be doing a debug
of Q.931 in our next video, and we can see a lot
of information about a call being set up and torn down.
And that's a look at digital circuits.
Now join me in our next video as we take a Cisco router
and convert it into an H.323 gateway known
to our communication manager cluster,
and we're going to configure on that gateway
an ISDN PRI circuit based on T1.
I'll see you back for that video next.