Category : Tutorials + Patches
Archive   : CLASS.ZIP
Filename : CHAPTER.07

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CHAPTER VII. The Switched Communication Network

Simple 2 wire Local Transmission Line Echo Path

³ ³ \ \ ³ ³
³ ³  -2-  ³ ³
³ ³ ÄÄ / ÄÄ / ³ ³

-1- represents the transmit carrier from one modem.
-2- represents the echoing back of the transmit signal
to the other wire. (Talkers echo)
-3- the echo on the 2nd wire cause another echo on the
other direction (Listeners echo)

For short distance communication, your two wire line goes in and
out of Telco Offices with any major transformations.

Among the first things we encounter is the ECHO. Irregularities
inherent in transmission cause a portion of the signal energy
to be reflected back toward the originating end. This is referred
to as Talkers Echo. When using a Phone, you can here your own
Voice on the receiver but at a much lower Level.

NOW get this !!! "If the Talkers Echo encounters Irregularities
(by the way I am quoting a Bell System Manual here) on its way
back to the originating end, still another echo is produced which
will propagate in the same direction as the desired signal" but
on the other line.

Think about it for a minute. You MODEMS receiver might see its own
TRANSMIT Carrrier !! AND !! Meanwhile the 2nd echo is aimed at
the other modems TRANSMITTER.

500mv ³
³ xxxxxxxxxxxxxxxxxx Normal Modem Receive Level
400mv ³
300mv ³
200mv ³ xxxxxxxxxxxxxxxxxx Nominal Echo Receive Level
100mv ³ xxxxxxxxxxxxxxxxxx Background Noise Receive Levels

Normally, the echo's are not a problem. Their strength is usually
so low that they can not be picked up by the receiver.

However should these Phone Line Irregularities become so
prominent that they generate loud echo's, Data WILL BE interfered
with. Loud echo are obvious. Connecting a handset to a line
and checking for echo's is easy.

Remember This !!!!!

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 . . .

1170 Hz 2125HZ

This kind of effect is know as "Distortion"

Distortion occurs when the loss or delay of a transmission line
varies as a function of frequency. In the above example,
we see that we can pass certain frequencies but can not pass
other frequencies.

If we were to transmit a test signal at one end and monitor the
receive level at the other end, we would see a sharp loss when
we attempt to transmit through the BAD area.

As an example, if at 1170Hz we were to receive the test signal at
400 mv (milli volts) or (400 x 10 -3) or (.4 volts)
then at the 2125Hz in the Bad area, the receive level might
be 100 to 200 mv.

The most common used term in the communications industry is the
DB (not to be confused with PCB's).

The Decibel (DB) is a unit for measuring the relative loudness
of sound OR unit for expressing the ratio of two amounts of
electric or acoustic signal power.

Since we are after all dealing with Sound transmissions, the
DB is apropriately used.

Dont let it confuse you. Decibels may be a distinct unit of measure
however it can be converted to Voltage potential (Volts and milli

You should think of it as an alternative language.

It is important you learn DB's for in the world of Data Comm it
separates the people who know a little from the people who know
even less.

We transmit at a certain DB level,we receive at a certain DB level,
Interferring noise is measured in DB's and Frequency response is
also measured in DB's. For this reason, next week I will included
a chart showing DB to Voltage conversion with respect to Power &
impedance of the voice grade line. With this chart, you will be able
to measure the actual voltage level of received or transmitted
signals and by referring to the chart, know what DB level you are
dealing with.

The opposite will also be true. If someone tells you to look for
a specific DB level, you can look at the chart and know what
voltage level you are looking for. When you get this chart;
Keep it. Copy it. Laminate it. Dont ever lose it.

DECIBELS were designed to help determine the rate of change of
sound. In the Data Comm world, that use is not consistent. We use
the DB in the comm world as a reference point. A voltage level
actually. However since the DB is the language of the Phone
company, we are forced to accomodate them.

In the DB world a 2volt peak to peak signal (2.2 actually) is the
ZERO reference point for our application. It represents the maximum
permissible signal level that can be or should be put on a Telco
line. From there, all voltage levels get smaller. However since
ZERO is as HIGH as we can go, we represent these smaller voltages
in NEGATIVE DB numbers or MINUS DB.

The following table will not be found in a library or bookstore.
I tried. I got it from General Data Comm Tech support. These levels
represent DB's of a Balanced Telephone Line Load.

Voltage Level Voltage Level Voltage Level

2.2 0 120 mv -17 16 mv -34

2.0 -1 100 mv -18 14 mv -35

1.8 -2 90 mv -19 13 mv -36

1.6 -3 80 mv -20 11 mv -37

1.4 -4 70 mv -21 10 mv -38

1.2 -5 60 mv -22 9 mv -39

1.1 -6 55 mv -23 8 mv -40

1.00 -7 50 mv -24 4.5 mv -45

900 mv -8 45 mv -25 2.5 mv -50

800 mv -9 40 mv -26

700 mv -10 35 mv -27

640 mv -11 30 mv -28

570 mv -12 28 mv -29

500 mv -13 25 mv -30

450 mv -14 22 mv -31

400 mv -15 20 mv -32

350 mv -16 18 mv -33

If you were to measure with an oscilloscope, the p-p voltage
at the Telco D mark, and see a signal that is 1 volt from peak
to peak, you would say your signal strength is Minus 7 DB.

On the other hand, if you were to use a VF MOnitor that registers
DB's and it registered -7DB you would know that you have present
a 1 Volt Peak to Peak signal. (Peak to Peak is refering to the
wave form I displayed as carrier. From the most positive voltage
to the most negative voltage.


The Telephone Company provides Voice Grade lines for
telecommunication use. We use these lines for our Personal
Computer Modems.

You can also get condition Voice Grade Lines for Data use
commonly referred to as Switched Network Lines. These lines
operate the same way as the Voice Grade lines, but they are
conditioned for lower noise levels and have a Fixed Loss Loop.

Line Loss can be said to include the impedance of your "In
House wiring" added to the distance of the telephone pole
outside your house to the nearest Phone Company Office.

Fixed Loss Loop (FLL) is a preset factor which the Telephone Company
creates by taking the already exisiting Line Loss and adding or
subtracting impedance to insure that your modems transmit carrier
arrives at the Phone Company office at a Level they prefer to see.

Each Phone Company office may vary as to what it wants to see.
However by using a predetermined Modem Transmit level, any phone
company can adjust the FLL to suit its own need.

Normally the FLL is set such that if your Modem were to transmit at
-11 db it would reach the Phone Company office at the correct Level.
If your modem were to transmit at a higher level (-9 db -8 db etc)
you begin to risk jamming the phone company with a signal that is
to strong.

If you transmit at a lower level (-13db -15 db etc.) you risk
your signal being to weak to be received.

Of course there is a tolerance factor. Most Modems in the PC
community are designed to transmit at -9 db. There are few problems
with that level.

The alternative to the FLL is the Programable Jack. The Phone Company
creates sort of a bridge effect. It puts into your Phone Jack a
resistor that allows your MOdem to determine a balance of impedance
with the phone line. The Modem can thus adjust its transmit level
as the Phone line impedance changes insuring a more accurate
transmit voltage.

Electrical properties are Non-stable. The impedance of Lines can
vary by their length (distance), by their gauge (width of wire),
by the temperature and other outside influences.

If we expect to Transmit in the -9 DB range, then what do we
expect to receive our incoming signal at ??

The phone company is required by Tariff to insure no greater
that a 16 db loss from tranmitter to receiver. If we transmit
at -9 db that we expect the other modem to receive the signal
at -25 db.

Modems are capable of receiving downward to -40 db, However since
Line noise does exist, any receive level below -30DB can be
suspect as a problem child.

These levels are true for Dial Up Modems over switched
Networks ONLY.
In the world of Industry, Modem transmit levels must be set by
the customer or installer. The installer must know if the
Jack being connected too is a FLL Jack or programmable.

Some modems have an option for Programmable & Permissive to allow
the modem to make the automatic adjustment.

Others assume Permissive and give you NO option.

Home PC modems do not have these options YET but they will have as
they attempt higher and higher speeds.

When our switched carrier phone line connects us long distance,
we no longer operate over just a switched network. Our call is
placed on a trunk or multiplexer for lengthy travel. Since noise
and deterioration would occurr normally, our signals are
separated and individually handled allowing for better
amplification or regeneration without loss.

2 Wire to 4 wire Transmission Line

³ Modem ³
³ ³ Local Phone Company
³ ³ Wires
³ ³
³ ³ ³ ³
³ ÚÄÄÄÁÄÄÄÄ¿ ÚÄÄÄÁÄÄÄÄ¿ ³ Phone Company Converts 2 Wire to
³ ³ Hybrid ³ ³ Hybrid ³ ³ 4 wire. A separate 2 wire Line
³ ³ Trans ³ ³ Trans ³ ³ for the transmitter and a
³ ÀÄÄÄÒÄÄÄÄÙ ÀÄÄÄÒÄÄÄÄÙ ³ separate 2 wire line for the
³ º º ³ Receiver
³ º º ³

Four Wire Trunk Line Or Multiplexed
On a T1 Facility

³ º º ³
³ º º ³
³ º º ³
³ ³ Hybrid³ ³ Hybrid³ ³
³ ³ Trans ³ ³ Trans ³ ³
³ ³
³ ³ Telephone Line
³ ³
³ Modems ³

Most of our work has been involving dial up Modems over switched

Dedicated Lines are also used, mostly by Industry. These lines
run from point A to point B. They are not accessible by outsiders.
No dialing is required. The line is always connected to the modems
and the modems are always connected to each other.

These dedciated 4 wire lines are Two pairs of phone lines. One
used exclusively for the Transmitter and the other used
exclusively for the Receiver.

Each tranmitter now having a full line, a full band spread, of
its own not needing to share it with anyone.

³ O ³ ³ O ³
³ E ³ ³ E ³
³ M ³ ³ M ³

Since these lines are DEDICATED, they can be shared with other
specific locations HARD WIRED into the Line.

Thus we get our two definitions of DEDICATED LINES.

1. POINT to POINT - A four wire line running from point
A to point B

2. MultiDrop - A 4 wire line connected from a Master
Modem to 2 or MORE remote modems (drops)

ÉÍÍÍ͵Rec M ³
º ³ O ³
º ³ D ³
º É͵Tx E ³
º º ³ M ³
º º ³ ³
A ³ O ³ º º Multidrop Line
S ³ D ³ º º
T ³ E RecÆÍÍÍ» º º
E ³ M ³ º º º ÚÄÄÄÄÄÄÄÄ¿
R ³ ³ º ÈÍÍÍ͵Rec M ³
º º ³ D ³
³ M ³
³ ³

Wait a second. Is This possible ?? Three Modems on a Line ?????
Wont the transmitters interfer with each other.

This and other exciting information in Chapter ??, The Wonders
of the Multidrop Line.
When you utilize a Dedicated Line, you are always using the same
circuitry. If the line goes bad, it must be repaired.

On a dial up line, if it goes bad, you hang up and call again.

When you call locally, you carrier goes through the local phone
comapny switch to its destination.

When you dial non local but no too far, you call goes via microwave
or trunk to the proper Phone company office onto its destination.

When you call very long distance, or coast to coast, there is now
a good possibility that your call will be via satellite.

Satellites make for poor Data Comm facilities. There are massive
delays in transmission, horrible echos, and something called the
DOPPLER effect.

The DOPPLER effect is simply a reference to the fact that the
satellite is in motion and that during prolonged transmissions,
this motion can cause the carrier to become distorted.

Bear with me on this one.

You have a dedicated circuit (this problem occurs on dial up also
but since you can dial a new line, you will never know it) that
wont let you pass data. The Phone company has swept the line
and found no problem.

Your Data Comm engineer has such a sincere face, you know he is
right. What might the problem be ??


In a switched network, the amplifiers on the phone circuit, sees
the carrier turned on and then they turn on to allow the signal
through. HOWEVER it is possible that some circuit, switch, amplifier
or whatever "turns on toooo slow".

You Transmit a Data Block. An amplifier turns on toooooo slow and
the beginning of you carrier gets clobbered turning your Data
Block to mince meat.

The phone company can not detect this problem because they use a
Continuous Tone to sweep the line. Only an engineer or a good
guesser can determine that problem.

The biggest threat to Data Circuits is NOISE. There are
a number of different types of Noise.

IMPULSE NOISE is sporadic, Low Frequency Voltage spikes caused
by older type Phone Company Equipment. There are still many
many parts of the country with Non-state of the art Phone
Equipment. A VF line monitor or telephone handset can pick up
impulse noise. It sounds like a pop, or a crackle. (sorry
Kellogg, no Snap)

BACKGROUND NOISE is present in every circuit. Usually it is
filtered to such a degree that it is rendered harmless. It
becomes harmful when its signal strength increases to a
point where it can compete with your carrier. It usually
has the same sound you here when you have your radio or TV
tuned to NO station. When you amplify your Data Signal you
amplify the background noise with it. If the Data Signal is weak
and Telco tries to compensate by increasing the circuit Gain,
they will increase the noise level as well.

INTERFERENCE - Outright interference is a real problem. It comes
from many sources sneaking its way in at the Phone Company Office,
along the telephone line route or from the wiring in your house.

I have heard Military CW, Radio Stations, Foreign Exchange lines
and engine noise interfere with Data Communications. Idealy your
Computer and Modem should be on its own circuit breaker (Mine is).
An Air Conditioner turning on and off can do incredible things
to your data.

Your computer and Modem should share a Common Ground. DO NOT USE
a Ground Eliminator Adapter. This can be hazardous to the health
of your Hardware.

Interference must be tracked down and eliminated. If the source
is the Phone company, it is their problem to correct. Otherwise
it is your problem.

SHIELDED TWISTED PAIRS are the best Data Comm cable routing.

Another interesting device is called an echo suppressor.
When making a long distance call, the phone company will
sometimes insert an echo suppressor designed to control
the amount of echo on the line. This has the effect of
shutting down one side of the line. This of course is only
used in the 4 wire state after your call has been transformed
from a 2 wire state.

The echo suppressor, depending on the type, can have a 100
millisecond delay in getting out of the circuit. Carrier
generated during that time may be destroyed.

Some commercial modems have what is called a "Dither Tone".
The purpose of the dither tone is to deactivate the echo
suppressor by keeping both side of the line in constant use
with a low frequency continuous wave outside the carrier
band width.


Every Modem or similar device that is designed to connect to
a Phone line must conform to FCC standards. One of those standards
is the amount of load placed on the line by the device.

Most Modems and related devices use a Transformer to not only
provide for a balanced load but also to act as an isolator to
separate potentially damaging voltages from passing back and

These transformers are designed to provide a 600 ohm Balanced
load at the line. A 600 ohm load will

1. Help minimize echos

2. Provide for proper Gain of the received signal

3. Insure a proper Transmit Level to the line.

Improper impedance WILL adversely effect Data Communications.

Impedance is not controllable by the customer (You) in most cases.
More advanced Modems and Telephone equipment do provide such


Please remember that Commercial Modems are far superior that Modems
designed for the Home PC and thus are capable of doing far greater

One of those things is called the "Clear To Send Delay"

This is how it works.

Your computer raises RTS (Request to Send).

In preparation for sending Data, the Modem responds by turning
carrier on, HOWEVER the Modem does not provide the Computer
with Clear To Send. You have carrier being transmitted without
any Data being sent.

If there are any TURN ON problems, echo suppressors or whatever,
they will be clobbering carrier without Data.

After a desired delay, CTS is then turned on and your Computer
begins sending Data, ONTO a clean, cleared line.

The Clear To Send Delay can be optioned from a fast 5 to 10
milliseconds to cope with minor problems or expanded to
500 milliseconds (1/2 a second) for sever problems such as those
created by satellite transmissions.

While we are on the subject of telephone lines......

It is ILLEGAL to connect and Non-Registered Device to a
Phone Line. The connecting of "home made" filters, equalizers
or amplifiers is subject to severe Penalties.

Should you desire to connect an Unregistered device to a Phone
Line, you must request the Phone Company to install DAA
Data Access Arrangement or equivalent. This device protects
the Phone Line should your equipment not perform as you expect
it to.

  3 Responses to “Category : Tutorials + Patches
Archive   : CLASS.ZIP
Filename : CHAPTER.07

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