Category : Tutorials + Patches
Archive   : CLASS.ZIP
Filename : CHAPTER.06

 
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VI. Intro to Telephone LInes




Before we can discuss more about the modem, we need to learn
a little about telephone lines.

Telephone lines are wires.
All wires have resistance.
Wires are used to allow electrons to flow from one point to another.

BUT the resistance in these wires helps to impede our electron
flow. Impedance can be caused by the physical properties of the
wire itself, the width of the wire in respect to how much current
is flowing, and the length of the wire.

At the end of our wires, at the phone company office, are
electrical circuits and switches all designed to permit
transmission over these phone lines but all with limitations
of their capabilities.

Just as your computer has its limits, so doesn't the phone
company's Lines.

The lines are designed to allow transmission of voice grade
signals back and forth with minimal and predetermined loss.

What this means is that the phone company has designed its
circuits to allow passage of certain frequencies associated
with voice transmissions. Frequencies outside this range
are not included or desired.



"Under IDEAL circumstances" this chart illustrates how
the frequency response of the phone line allows passage
of frequencies between 300 through 3000 HZ but PREVENTS
passage of frequencies below 300HZ and above 3000HZ


³
Amplitu³de
³
Gain ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ³ ³
Volume ³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
ÃÍÍÍÍÍÍÍÍÏÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÏÍÍÍÍÍÍÍÍÍ
³0 300 3000
³ Frequency in HZ (cycles per second)


How can this be accomplished. VERY simple. We design our
amplifiers to amplify any signals between 300 and 3000HZ
and we do not amplify anything outside this range. Signals
placed on the line outside our 300 to 3000 limits will die
off as they travel over the phone line.

Of course we live in an imperfect world. Full of tolerance.
Tolerance means that when we set out to do something, we
do it right, ALMOST.

Resistors have tolerance, Capacitors have tolerance,
Amplifiers have tolerance. About the only thing I can think
of with no tolerance is the Ayatollah Khomenini (sp).

Not to mention tolerances change with temperature and humidity.
Two identical pieces of wire will have different resistance
levels if we were to measure resistance down to micro ohms.

As a result, our perfect Band Pass Filter (and amplifier designed
to amplify only desired frequencies) has flaws. Instead of
perfection we find that some ampliifcation takes place outside
our 300 to 3000 Hz dome and that inside our specs our desired
frequencies are not amplified all equally.

³
³
³ . . . . . . . . . .
³ . .
³ . | .
³ . | .
³ . | | .
³ . | | .
³ . | | | .
³ . | | | | .
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³...... | | | | .
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
0 300 800 2000 3000


What this chart shows us is that at 2000 HZ we get maximum output
or maximum gain while at 800 HZ our gain is less and at 300 HZ even
less. Obviously it would do us better to operate at 2000 HZ.
But that would leave us with too small a range within which to
operate.


I can best illustrate the results of this by explaining to
you one of the more common gain tests used to determine
the usability of a line.

If you were to connect an audio oscillator to one end of
a line and transmitted an exact audio level

While at the other end of the line, you connect up a Monitor
that will let you see the level of that signal as it arrives,
you would see the signal that was transmitted MINUS any loss
of strength from traveling through wires and circuits.

Going back to our diagram of Frequency Response, we would expect
to see a smaller signal arrive at 800HZ than at 2000Hz.


Suppose we transmitted our signal at 1.0 volts.
At 2000Hz we would expect to see arrive a strong signal near
the 1.0 volts we transmitted.

At 800Hz however, we know that there was less gain OR more loss
so the signal we would expect to see would be below 1 volt.

At 300 Hz, well below 1 volt.

Example:


Transmit Receive
Level Level


2000HZ 1.0 v .99v

800HZ 1.0 v .85v

300HZ 1.0 v .50v



One side note hear. The frequency of 2600HZ has been set aside
as the disconnect frequency. If you were on the phone talking to
someone and had an audio oscillator set to 2600HZ, your call
would be disconnected if the tone we to be picked up be the
telephone.

OK, so where were we.

Oh Yeah. Ever wonder how the cable company can send you so many
different channels over 1 cable. Simple, they use a different
frequency for each channel.

FDM or Frequency Division Multiplexing. We use that big range
of frequencies from 300 to 3000 to send multiple tones or multiple
CARRIERS at different, non interfering frequencies.



Transmit ³ ³ ³ ³ Receive
Frequency ----- ³ ³ ³ ³ ----Frequency
Range ³ ³ ³ ³ Range
. . . . . . . . . . . . . . . . .
. . . .
. . . . . .
. . . . . .
. . . . . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
... . . ³ .. ³ ..... . . .
³ ³
³ ³

1170 Hz 2125HZ


Gimme a break. ASCII wasnt meant for superior graphics.

In the diagram, I have as the outer shell, the Phone company
Frequency Response graph and inside I show the two frequency
bands we use, one for transmit and one for receive. They
pass each other in opposite directions without interference.
Thus allowing us to transmit and receive simultaneously over
one phone line.


The reason why a Voice Grade line has a frequency range of 300
to 3000 HZ is because those are the nominal frequencies of our
hearing capabilities (but not the full range).

It should seem obvious that a Voice grade line would do just what
it implies, operate at voice frequencies.

Thus the phone company designed and built phone lines that would
amplify and carry Voice frequencies while ignoring or eliminating
outside frequencies.

The term "Frequency Response" refers to how well an amplifier
amplifies at a specific frequency. If it amplifies all that we want
it to, it has a good frequency response. If it amplifies poorly
it has a bad frequnecy response.

Idealy we would like out telephone line to have a consistent
Frequency response over the entire 300 to 3000Hz range and look
like this..........

³
Amplitu³de
³
Gain ³ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ³ ³
Volume ³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
³ ³ ³
ÃÍÍÍÍÍÍÍÍÏÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÏÍÍÍÍÍÍÍÍÍ
³0 300 3000
³ Frequency in HZ (cycles per second)


But we know, that because of tolerances our frequency response
looks like this............






³
³ . . . . . . . . . .
³ . .
³ . | .
³ . | .
³ . | | .
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³ . | | | .
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ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
0 300 800 2000 3000



When you compare the top figure (ideal band pass filter) and the
actual filtering, it should be obvious, that if we could have our
ideal band pass filter, we would have alot fewer line problems
to worry about. Nice clean lines with all frequencies being
received at the same signal strength (signal level).


As we earlier stated. The problem with the real world filter
is that the signal level RECEIVED at 400 HZ and 800 HZ and 2000 HZ
will all be DIFFERENT ;

BECAUSE the frequency response at 400, 800 and 2000 Hz
is different.


Thus if we TRANSMIT a 400 HZ frequency at 1 volt and a 800 Hz
freqency at 1 volt and a 2000 hz frequency at 1 volt, we would
end up with varying results at the RECEIVE end

(example levels only)

400Hz receive .6 volts (loss of .4 volts)

800Hz receive .8 volts (loss of .2 volts)

2000Hz receive 1.0 volts (loss of 0 volts)

If we cant improve the quality of the telephone line. How about if
we increase our transmit level at those frequencies we are losing
voltage.


(examples only)



Transmit Transmit Receive Loss
Frequency Voltage Voltage

400 1.4 1.0 .4

800 1.2 1.0 .2

2000 1.0 1.0 0


In other words to receive a CONSTANT receive level all we have
to do is vary our TRANSMIT voltage adding more voltage where there
is loss.

This function is called EQUALIZATION. OR EQUALIZING the line.

Equalization can be accomplished by varying the level of the transmit
voltage at the transmitter OR it can be accomplshed via a PRE-
AMPLIFIER at the RECEIVER (altering the levels before it gets to
the receiver).

How modems equalize is different depending on manufacturer and
modulation type. However, one interesting way it is accomplished is
via a training sequence. Prior to transmitting carrier, a modem
transmits a test pattern which is something like a sweep of the
frequency range. The receiving station has stored in ROM exactly
what the test pattern looked like when it was tranmitted by the
originating modem. It then compares what it receives against the
template of the test pattern it has stored in memory.

It now computes what the telephone line characteristics realy are,
How much loss and at what frequency. It then adjusts its transmitter
to vary the voltage of its transmission at all the different
frequencies.

Below (sample levels only) we show the response of the phone
line and below below, how we alter our transmit levels to
Equalize the line.






. ..............
. .
. . Telephone
. .
. . Line
. .
. . Frequency Response
. .
. .
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
300 3000


--------------------------------------------------------------------




2 volt . . Transmitter
. .
.
.
. . Voltage
1 volt . .. ............
Levels

0 voltsÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ





If we equalized properly, if we add Graphs 1 & 2 above, we should
idealy get a FLAT frequency response as we saw in the earlier message.

Transmit ³ ³ ³ ³ Receive
Frequency ----- ³ ³ ³ ³ ----Frequency
Band ³ ³ ³ ³ Band Width
Width . . . . . . . . . . . . . . . . .
. . . .
. . . . . .
. . . . . .
. . . . . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
. . ³ . . ³ . .
... . . ³ .. ³ ..... . . .
³ ³
³ ³

1170 Hz 2125HZ
Center Center
Frequency Frequency


Above we see how we make maximum use of the Band Pass. We are
transmitting out of modem # 1 at 1170Hz while we are simultaneously
transmitting out of modem # 2 at 2125Hz. The Band Width of each
transmitter is small enough to allow BOTH carriers to co-exist on
the same circuit without interferring with each other.

When we use a different scheme, Transmitting in only ONE direction
at a time, we can utilize more of the available Band Width.




Transmit ³ ³ ³
Frequency ----- ³ ³ ³
Range ³ ³ ³
. . . . . . . . . . . . . . . . .
. .
. x x x x x x x x x x xx .
. x x .
. x ³ x .
. x ³ x .
. x ³ x .
. x ³ ³ ³ x .
. x ³ ³ ³ x .
. x ³ ³ ³ x .
. x ³ ³ ³ x .
... x ³ ³ ³ x . . .
³ ³ ³ x
³ ³ ³

Center
Frequency


It is this Difference that allows us to break the barrier of 600
Baud. With two Modems transmitting simultaneously, each Modem
takes up 1/2 the Band Pass and of course a GAP exists between the
two. Add the two and the gap together and you get a much bigger
BANDwidth.



"Well, Howcome I can tranmit to them but they can not transmit
to me"

It is possible that a telephone company equipment problem
can effect certain frequencies without effecting others.
Below you see an example of the Telephone line's frequency
response WITH A PROBLEM. We have effectively lost half of
our line. However we still have half of the line remaining.


Transmit ³ ³ ³ ³ Receive
Frequency ----- ³ ³ ³ ³ ----Frequency
Range ³ ³ ³ ³ Range
. . . . . . . . . .
. x . . .
. x x . . .
. x x . . .
. x x . . .
. x ³ x . . .
. x ³ x . . . .
. x ³ x .
. x ³ x .
. x ³ x .
. x ³ x .
... x x ³ x . . .
³
³

1170 Hz 2125HZ


If we transmit a carrier at 2125HZ thru the degraded part of our
line, the carrier may not get through and if it does, it is be
such a weak signal that the modem will not be able to see it.

I dont know how many times I have gone onto a customer site "after"
the phone company tested the line and found a line problem. Trying
to convince a customer that the phone company missed something is
not always easy. BUT it used to be that the only TEST (if you want
to call it that) the Phone company would perform is to send a
1000 Hz tone down the line and loop it back.

Well thats great for the 1000 Hz range but what about the 300's
500's 1500's 2000's etc etc.

Unless the Phone Company has done a FULL SWEEP of the entire
frequency spread of the BAND, they have DONE NOTHING.


  3 Responses to “Category : Tutorials + Patches
Archive   : CLASS.ZIP
Filename : CHAPTER.06

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