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TUCoPS :: Phreaking General Information :: mlt.txt

MLT Test Sequence

                             MLT Test Sequence:
                       Interpretation of MLT Test Results
                               by Don McCarty
             (This column first appeared in Outside Plant Magazine May 1998)

We receive many questions on the operation of the MLT machine when the field
technician is staring at his access unit reading the words "Still Processing."
We hope a review of the test sequence will help. Remember that the MLT takes a
snapshot in time and gives the results. Also, MLT can be fooled. "Test OK" means
that of the tests the MLT makes, those tests passed. It does not mean that there
is no problem with the circuit.
For you good folks who do not have computerized testing, there are some fine
units out there. The ability of the field technician to do single ended testing
justifies the cost of the equipment. Technicians can access the system to send
tone, short the pair, open the pair, and so on. This eliminates the need for a
second technician in the central office.
MLT cannot test loop current, loss, circuit noise, power influence, slope,
station ground, or circuit design. If there is a series resistance fault close
to the customer, MLT will test OK even though the customer hears static.
For those of you who do not use MLT, bear with me. We'll cover other CO test
units at a later date.
Following is the 19-step test sequence that takes place when someone requests a
"Full Test" from the MLT system. I'll go over individual sequences in future
MLT Test Procedures
    MLT makes decisions by comparing the present values on a circuit to ones
    that it already knows about.
    MLT is programmed with a list of DC and AC values that correspond to certain
    line conditions. For example, the list specifies what a short looks like in
    terms of DC resistance and what a POTS telephone set looks like in terms of
    AC Resistance.
    MLT "communicates" with the LMOSdatabase to get customer line records.
    The user types the telephone number on the "Customer Request" portion of the
    mask and "sends" that information.
    MLT goes to the LMOS database to retrieve the customer line records. The
    information MLT retrieves includes type of service, type of equipment and
    central office switch.
Once the line record information has been retrieved, MLT tests the line,
interprets the test results and presents the interpretation via a VER Code.
When anyone requests a full series of tests on a line, MLT begins testing in a
particular sequence. MLT makes decisions about what to do next by comparing the
results it gets with what it expects to see. For example, MLT expects a standard
POTS line to have a certain AC resistance. If the line has the correct values,
it decides there are no AC problems and moves onto the next test in sequence.
However, if the values do not match, MLT determines there is a problem and moves
to a different sequence of tests.
This will be easier to understand as we look at the sequence of tests that MLT
performs on a circuit.

For you good folks who do not have computerized testing, there are some fine
units out there. The ability of the field technician to do single-ended testing
justifies the cost of the equipment.

Test Sequence
To begin, MLT connects a test trunk to a line and begins the following series of
1. Access Test:Checks for high voltage on the line.
If: Hazardous voltage is present,
Decision: MLT drops access and stops testing.
If: OK, goes on to next test.
There are three basic causes of hazardous potential on a circuit:
    Someone may be calling the customer at the time of the test. MLT does not
    differentiate between the 105 volts of AC ringing current from the central
    office and high voltage. VER 13, Hazardous Potential will be displayed. When
    re-testing, if a good test is displayed, this was probably the cause of the
    initial VER 13.
    The customer may be fed by "C" rural wire, an unshielded wire used in rural
    areas. This wire, because of its close proximity to power lines, may carry a
    high AC potential.
    There could be hazardous voltage entering the circuit from the customer's
After re-test, if VER 13 continues to be displayed, a call to the Maintenance
Center may be appropriate to advise a supervisor before you visit the customer's
network interface. That way, if you are gone for too long, at least we know
where to find the body.
Remember:Use all safety equipment (voltage tester, rubber gloves, etc.) until
you are satisfied that there is no hazardous voltage present at the network

2. Busy Line:Checks to see if the line is busy.
If: Line is busy.
Decision: MLT drops access.
If: OK, not busy, MLT goes on to the next test.
3. Foreign Electromotive Force Test (FEMF): Checks for high voltage (not
If: Voltage exceeds certain level.
Decision: MLT stops testing but stays connected to the line.
If: OK, MLT goes on to the next test.
MLT stops testing if voltage is over a certain level because high voltage will
adversely affect the results of let er tests.
4. Line in Use:Checks for off-hook signature (each switch has a different
off-hook signature).
If: Off hook, MLT checks for conversations. If there is conversation,
Decision: MLT drops access.
If: OK (no conversation), MLT goes on to next test.
5. ROH: The ROH test distinguishes between a hard short and an off hook
condition. MLT puts current on the line and waits for a signal that the current
has passed through the telephone or other CPE. If there is a short, there is no
signal. If the signal comes, MLT determines the receiver is off hook.
If: Off-hook.
Decision: MLT stops testing.
If: OK (on-hook), MLT goes on to next text.
6. Intercept Test Intercept lines, lines that are purposely put out of service,
have a distinctive signature.
If: On intercept.
Decision: MLT stops testing.
If: OK (not on intercept), MLT goes on to next test.
7. DC Test MLT removes the line circuit from the line to measure DC resistance,
short, and ring to tip ground.
If: The fault is very low resistance, 3 K Ohms DC resistance on any one of the
three sides, T-R T-G, R-G,
Decision: MLT stops testing.
If: OK or fault identified but not "very low resistance," MLT goes on to next
8. Voltage:DC voltage values are used to identify and cross to a work ing pair.
There should be no voltage on a good line.
If: Hard battery, 48 volts or more,
Decision: MLT stops testing.
If: OK or fault identified but not "very hard," MLT goes on to next test.
9. PBX Identification:DC resistance tests are also used to validate non POTS
telephone signatures. MLT compares DC values it measures to those it expects to
see via the LMOS line record. A PBX, for example, will have a value similar to
the following:
            K Ohms   Volts
              100             T-R
              12              T-G
              400             R-G

When MLT measures these values and the line records indicate the presence of a
PBX, MLT test results will show a valid PBX signature.
If: Valid PBX signature,
Decision: MLT stop testing. MLT does not do AC, Longitudinal Balance,
Thermistor, or Open tests since the presence of a PBX on the line leads to
inaccurate results from these tests.
If: No PBX signature, MLT goes on to next test.
10. AC Signature:AC resistance is used to identify POTS and other terminations.
The T-R value for a standard ringer is between 8 and 18 K Ohms. The T-G and R-G
values are greater than 25 Ohms and are about equal.
If: High AC resistance value (does not see a valid signature),
Decision: MLT does an open test.
If: Valid AC signature, MLT goes on to next test.
11. Longitudinal Balance Test:Measures how likely it is that the line is noisy.
This test is performed only if a termination is seen and the line is unbalanced.
The results are expressed in decibels (dB). If the balance is less than 40 dB,
the line is probably very noisy. If the balance is above 60 dB, there should be
no noise problem.
Decision: MLT gives test results and goes on to next test.
12. Thermistor Test:Checks for the presence of a thermistor on the line. A
thermistor is a resistor that is part of some PBX and key systems telephones. A
thermistor test is done if the line records indicate that there should be a
thermistor on the line.
Decision: MLT gives test results and goes on to next test.
13. Open:AC capacitance is used to measure and analyze the location and type of
open on a line. When MLT determines a line is open, based on results from the DC
tests, AC signature test or thermistor test, it does an open test to determine
if the open is in or out of the central of fice If the length of wire (tip or
ring or both) is ''short" (less than 350 feet), the open may be in the central
office. If the length of the wire is "longer," the open is out of the central of
fice In the case of an open out, MLT also gives the distance in feet from the
central office to the open.
Decision: MLT gives test results and goes on the next test.
14. Capacitive Balance Measurement:MLT uses AC capacitance to compare the
capacitance (length) of the tip wire to the capacitance (length) of the ring
wire. For a good pair, the length of the wire is equal and the balance
percentage is 100%. In this case, the MLT test result will be:
Balance: Cap 100%
This means that both sides of the loop are open at the same place. However, when
the lengths are not equal (one wire is shorter than the other), more than likely
the shorter wire is open. MLT determines which side is shorter and reports
either "open tip" or "open ring."
Decision: MLT gives test results and goes on to next test.
15. Line Circuit Test:This test checks for the proper arrangement of the battery
and ground in the central office line circuit. The line circuit is the equipment
    provides battery to the loop
    detects the phone is taken off-hook
    provides dial tone
    connects the loop to the switching equipment
    accepts dialed digits and switches the call to its destination.
All of the tests described so far are conducted without the line circuit
(battery and ground) present. Remember that MLT removes the line circuit at the
start of the DC tests so that all the tests are made without the line circuit.
At this point in the test sequence, MLT reconnects the line circuit to the loop.
MLT expects to see 51 volts of battery on the ring side and central office
ground on the tip side. If this is the case, MLT test results show the
Central Office: LINE CKT OK
If battery and ground are not present, MLT test results show "open in."
Central Office: OPEN IN
16. Draw and Break Dial Tone:MLT simulates a telephone going off-hook and checks
for the presence or absence of dial tone. Then it removes the simulated off-hook
condition to ensure the dial tone breaks or stops.
17. Soak Test: The soak test measures DC resistance over time to determine if a
fault is swinging. The term "swinging" comes from testing a circuit with a
meter. When either the tip, ring, or both tip and ring wires have DC faults due
to moisture, the meter needle jitters or moves slightly as the moisture affects
the intensity of ground on the wire. Thus, when a ground is swinging, it means
that moisture is present and is affecting the fault.
18. Ringer Test:This test determines the number of ringers on a line and whether
the results are consistent with the line records.
19. Length of Loop Measurement:This test is the last performed in the series. It
uses AC capacitance to measure the length of a good pair. It functions similar
to the open test and reports the distance from the central office to the
telephone. This test is run only on single-party POTS lines that have already
been determined to be "test OK."
All of the above tests, 1 through 19, are part of the full series MLT tests.
Most MLT test requests run through all or some of these tests.
Additional MLT Tests
There are other MLT tests that are more specialized, requiring interaction with
the customer. Let's take a look at these tests.
Touch-Tone Test:This test checks the condition of the customer's touch-tone pad
by analyzing the tones that are produced when the customer presses the buttons
on the pad. MLT checks the tones and returns a signal to indicate whether the
pad is good or not. This test is run whenever you suspect a problem with the
touch-tone pad. For example, when the customer complains of getting wrong
Coin Tests:There are two coin tests that check two primary mechanisms in the
coin set: the totalizer and the coin relay. The totalizercounts the coins a
customer puts in. The totalizer must be in a certain starting position when the
coins are dropped into the phone. When the totalizer is in the correct position,
it is said to be "homed." Each coin deposited causes the totalizer to send tones
to the central office When the central office hears enough of the required
tones, the customer is allowed to make a call.
When a coin test is run, MLT looks for a short. If there is a short, MLT
suspects the totalizer is not "homed" and attempts to "home" it. If MLT does not
find a short, it checks the coin relay.
The coin relay is the mechanism that returns or collects the coins deposited by
the customer. The relay sends the coins to either the coin box or the return
slot. If MLT sees a T-G fault, it suspects a problem with the relay. MLT
attempts to operate the relay, measures the relay's timing and measures how much
current was needed to operate the relay.
Signing Off
Most field technicians do not use MLT to its fullest capabilities. Learn the MLT
test sequence, learn how to interpret the tests, and use the VER codes that
accompany the tests. Your job will be easier in the field.

Let's Talk
We have received a tremendous response from our readers about our free monthly
newsletter.. We are I diligently working on it and the first) issue will be out
soon. If you are I interested in the newsletter, call or e-mail us.
I appreciate hearing from you with questions or comments on any topic that you
think is of general interest to outside plant technicians and mangers. And, as
always, I look forward to talking with experts in the field. That expert may be
one damn good field technician whom you respect. So please call (408) 341-8990,
e-mail, or send a fax (408) 341-8970 to me. (Our web
site is in development and should be completed soon, so keep watch at:

Donald E. McCarty is president of McCarty & Associates, Inc., an international
consulting firm offering classroom, on-site and field training courses in
outside plant cable and fault locating. For more information on McCarty &
Associates training seminars, please call (408) 341-8990.

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