what is a T1 Line?
In principle, a T1 Line is just a direct connection between two locations.
As simple as it sounds,
there's a lot of science behind that sentence,
which I shall try to explain in this tutorial.
The T1 is based on the old Telephony technology
which was developed in 1960s.
Its original intention was to carry
24 Pulse Code Modulated Speech Signal streams.
Each at 64 Kbit/s.
Right now, I know exactly what you're thinking!!
I thought the same.
Guys, bare with me because as the video goes on,
this will make a lot of sense,
and it's the foundation of digital communications.
And by the end of this video,
you will understand exactly what all this means.
A T1 is a very reliable connection
with a maximum signal speed of 1.544 Mbit/s.
A T1 is also known as a DS1.
It can carry both voice and data.
A good example of its usage is:
many mobile carriers commonly use T1 lines
to connect base stations to transmit data
back to the Mobile Switching Center or MSC,
which is also known as a Telephone Exchange.
Pulse Code Modulation
or PCM
is used to digitise a voice conversation
and to insert it on a T1 line.
The PCM process take samples from the analog voice signal
which in this diagram is the Red sine wave.
A Sample is defined as a measurement
of a level of the analog signal,
and the samples here are
represented by the Yellow vertical lines.
With this example
we can verify that the quantised values
are the sampling moments,
which are indicated with the blue dots.
Let's just write up binary table.
So the first sample is 8,
which has the binary value of 1000.
The next sample is 9,
with a binary value of 1001.
And the next sample is 11,
which is a binary value of 10 11, and so on.
I'm not going to do them all as I'm sure you get the idea.
These 8-bit binary values provide 256
different levels of the analog signal.
Remember this is done 8000 times per second.
The analog spectrum is mapped
using the G.711 MU-Law algorithm.
Once digitised, these 8-bit binary
are then transmitted at a rate of 64 Kbits/s,
which is called a DS0,
which means Digital Signal 0.
If we multiplex 24 DS0's
it becomes a DS1,
which is referred to as a T1 line.
The binary data is then transmitted
using Time Division Multiplexing (or TDM)
into a single stream.
This is also known as a Time Slot or Time Slice.
So that the receiving end knows the beginning of each stream,
a framing bit is added to the sequence.
We now have a complete frame.
And if we do the math,
we have 8 Bit,
multiplied by the 24 channels,
plus the framing bit,
equals 193 bit;
multiplied by 8000,
(which is the amount of samples taken each second),
give us a total of 1,544,000
which, in technical terms, is written as 1.544 Mbit/s.
The receiver must be able to detect the framing bit
in order to synchronise.
The bit framing pattern sequence is
calculated to have a low probability of reoccurring
and therefore a low probability of being mistaken in a data payload.
I hope this video has been informative for you and thanks for watching.