Average Users Monthly Vol 1 Issue 1 - FILE 2
[Disclaimer: As much as I'd like to take credit for this informative piece of work on phone lines and frequency, I found this text file on the Internet, and cannot take credit for it. Would the original author please step forward?]
MEMO TO MARTY HEETDERKS & MIKE TUCKER.....MOFFET INMATES!
INTRODUCTION TO PHONE LINES
Before we can discuss 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 300 HZ and above 3000 HZ
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Amplitu|de
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Gain | ⁄----------------------ø
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| Frequency in HZ (cycles per second) How can this be accomplished. VERY simple. We design our amplifiers to amplify any signals between 300 and 3000 HZ 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 amplification takes place outside our 300 to 3000 Hz dome and that inside our specs our desired frequencies are not amplified all equally.
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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 travelling through wires and circuits.
Going back to our diagram of Frequency Response, we would expect to see a smaller signal arrive at 800 HZ than at 2000 Hz.
Suppose we transmitted our signal at 1.0 volts. At 2000 Hz we would expect to see arrive a strong signal near the 1.0 volts we transmitted.
At 800 Hz 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
2000 HZ 1.0 v .99v
800 HZ 1.0 v .85v
300 HZ 1.0 v .50v One side note hear. The frequency of 2600 HZ has been set aside as the disconnect frequency. If you were on the phone talking to someone and had an audio oscillator set to 2600 HZ, 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 | | | | Receive
Frequency ----- | | | | ----Frequency
Range | | | | Range
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1170 Hz 2125HZ
Gimme a break. ASCII wasn't 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 frequency response.
Ideally we would like out telephone line to have a consistent Frequency response over the entire 300 to 3000 Hz range and look like this ..........
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Gain | ⁄----------------------ø
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|0 300 3000
| Frequency in HZ (cycles per second) But we know, that because of tolerances our frequency response looks like this .......
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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 a lot 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 frequency 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)
400 Hz receive .6 volts (loss of .4 volts)
800 Hz receive .8 volts (loss of .2 volts)
2000 Hz 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 accomplished 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 transmitted 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 really 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.
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300 3000
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2 volt . . Transmitter
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1 volt . .. ............
Levels
0 volts--------------------------------------------------------- If we equalized properly, if we add Graphs 1 & 2 above, we should ideally get a FLAT frequency response as we saw in the earlier message.
Transmit | | | | Receive
Frequency ----- | | | | ----Frequency
Band | | | | Band Width
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1170 Hz 2125HZ
Centre Centre
Frequency Frequency Above we see how we make maximum use of the Band Pass. We are transmitting out of modem # 1 at 1170 Hz 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 | | |
Frequency ----- | | |
Range | | |
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Centre
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, How come I can transmit 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 | | | | Receive
Frequency ----- | | | | ----Frequency
Range | | | | Range
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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 don't 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 that's 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.
