U.S. patent application number 09/285954 was filed with the patent office on 2001-08-09 for method and apparatus for qualifying loops for data services.
Invention is credited to FAULKNER, ROGER, SCHMIDT, KURT E., ZHANG, YUN.
Application Number | 20010012333 09/285954 |
Document ID | / |
Family ID | 26804088 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012333 |
Kind Code |
A1 |
FAULKNER, ROGER ; et
al. |
August 9, 2001 |
METHOD AND APPARATUS FOR QUALIFYING LOOPS FOR DATA SERVICES
Abstract
A method and automatic test system for determining qualification
of a twisted pair transmission line to propagate data signals. The
method includes measuring phase imbalance in the twisted pair
transmission line. The phase imbalance is determined by resistance
imbalance in the twisted pair transmission line. The resistance
imbalance is determined by applying a common mode voltage to the
twisted pair transmission line; and, determining phase imbalance if
the twisted pair in response to the applied common mode voltage.
The method includes applying a common mode voltage to the twisted
pair transmission line; determining phase imbalance if the twisted
pair in response to the applied common mode voltage; detecting a
peak in the determined phase imbalance; determining a frequency of
the detected peak; determining line qualification in accordance
with the determined frequency. Methods are provided using series
resistive imbalance and phase measurements to discover the type of
imbalance existing on a twisted pair transmission line which is
unable to support data transmissions. Methods are provided using
series resistive imbalance and phase measurements to determine
where an imbalance occurs as well as the magnitude of the
imbalance.
Inventors: |
FAULKNER, ROGER; (SWINDON,
GB) ; SCHMIDT, KURT E.; (BURLINGTON, WI) ;
ZHANG, YUN; (WHEELING, IL) |
Correspondence
Address: |
Legal Department
Teradyne, Inc.
321 Harrison Avenue
Boston
MA
02118
US
|
Family ID: |
26804088 |
Appl. No.: |
09/285954 |
Filed: |
April 2, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60106845 |
Nov 3, 1998 |
|
|
|
Current U.S.
Class: |
379/27.01 ;
379/27.03 |
Current CPC
Class: |
H04M 3/306 20130101;
H04B 3/46 20130101 |
Class at
Publication: |
379/27.01 ;
379/27.03 |
International
Class: |
H04M 001/24; H04M
003/08 |
Claims
What is claimed is:
1. A method for qualifying a transmission line to propagate data
signals comprising: measuring phase imbalance in the transmission
line at a terminating end of the transmission line.
2. A method for qualifying a transmission line to propagate data
signals, comprising: measuring resistance imbalance in the
transmission line from a terminating end of the transmission
line.
3. A method for qualifying a transmission line to propagate data
signals, comprising: applying a common mode voltage to a
terminating end of the transmission line; and, determining phase
imbalance in the line in response to the applied common mode
voltage.
4. A method for analyzing a transmission line comprising: applying
a common mode voltage having a frequency changing over a range of
frequencies into the transmission line; determining phase of
signals produced on each wire of the transmission line, in response
to the applied voltage, relative to the applied voltage; detecting
a peak in the determined phase; determining a frequency of the
detected peak.
5. A method for qualifying a transmission line to propagate data
signals, comprising: applying a common mode voltage having a
frequency changing over a range of frequencies into the
transmission line; determining phase imbalance in response to the
applied common mode voltage; detecting a peak in the determined
phase imbalance; determining a frequency of the detected peak.
6. A method for automatically qualifying a plurality of
transmission lines, comprising: feeding signals from a controller
to a switch connected to termination ends of the transmission
lines; coupling test signals to the transmission lines through the
switch selectively in accordance with control signal fed to the
switch; detecting resistance imbalance between each of the legs in
a selected one of the transmission lines.
7. A system for automatically qualifying a plurality of
transmission lines, such system comprising: a switch coupled to
terminating ends of the plurality of a transmission lines; a
controller for feeding signals to the switch; a measurement unit
coupled to the switch and the controller, such measurement unit
being adapted to feed test signals from the measurement unit to a
selected one of the transmission lines through the switch, such one
of the transmission lines being selected in accordance with a
control signal fed to the switch by the controller, such
measurement unit isolating resistance imbalance between each of a
pair of lines in the selected one of the transmission lines in
response to the feed test signals fed to such selected one of the
transmission lines; and wherein the controller, in response to the
isolated resistance imbalance, is adapted to determine
qualification of the selected one of the transmission lines for
data signals.
8. A method for analyzing a transmission line recited in claim 4
including: comparing the frequency of the detected peak in the
determined phase with one or more reference frequencies.
9. A method for analyzing a transmission line recited in claim 4
including: determining, in response to the frequency comparison,
whether a resistive imbalance is present on the transmission
line.
10. A method for analyzing a transmission line recited in claim 4
including: determining, in response to the frequency comparison, a
type of resistive imbalance present on the transmission line.
11. A method for analyzing a transmission line recited in claim 4
further including: determining, in response to the frequency
comparison, presence of a capacitance imbalance on the transmission
line.
12. A method for analyzing a transmission line recited in claim 4
further including: determining, in response to the frequency
comparison, presence of an inductive imbalance on the transmission
line.
13. A method for analyzing a transmission line recited in claim 10
further including: determining whether the resistive imbalance is
series a series resistive imbalance.
14. A method of analyzing a transmission line recited in claim 13
further including: determining whether the resistive imbalance
varies with time.
15. A method for analyzing a transmission line recited in claim 9
further including: computing admittance of the transmission line at
varying frequencies; deriving capacitance of the transmission line
from the computed admittance; dividing the derived capacitance of
the transmission line by per-unit length capacitance for type of
transmission line under test to produce a quotient; location the
position of the imbalance on the transmission line from the
produced quotient.
16. A method for analyzing a transmission line recited in claim 9
further including: measuring the magnitude of the voltage on each
one of the legs of the transmission line in response to the applied
voltage; comparing the absolute value of the measured magnitude
voltage and a detected frequency peak with a list of reference
data; determining the location of the imbalance based on the
comparison.
17. A method for analyzing a transmission line recited in claim 9
further including: comparing the frequency of a detected peak to a
list of reference data; determining the magnitude of an imbalance
on the line based on the comparison.
18. A method of analyzing a telephone line having at least a first
leg and a second leg, the method comprising the steps of: a)
applying test signals to the first leg and the second leg to
determine, at a first frequency and a second frequency: i) the
capacitance between the first leg and ground; and ii) the
capacitance between the second leg and ground; b) determining that
no resistive imbalance exists when: i) the difference between the
capacitance between the first leg and ground measured at the first
frequency and the second frequency is below a threshold; and ii)
the difference between the capacitance between the second leg and
ground measured at the first frequency and the second frequency is
below a threshold.
19. The method of claim 18 additionally comprising the step of
identifying an imbalance when the difference between the
capacitance between one of the legs and ground measured at the
first frequency and the second frequency exceeds a threshold.
20. The method of claim 18 additionally comprising the step of
identifying the leg containing the imbalance as being the leg in
which the difference between the capacitance between that leg and
ground measured at the first frequency and the second frequency
exceeds the threshold.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] Under 35 USC .sctn.119(e)(1), this application claims the
benefit of prior U.S. Provisional Application 60/106,845, filed
Nov. 3, 1998.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to communication networks
and more particularly to systems for qualifying telephone lines for
data transmission.
[0003] As is known in the art, public switch telephone networks,
i.e., so-called plain old telephone service (POTS) lines, were
originally designed for voice communications which cover a limited
frequency bandwidth (i.e., about 4 KHz). Today, it is desired to
use the same POTS lines for data transmission. Data signals,
however, generally have different frequency characteristics than
voice signals. As a result, a POTS line that works well
transmitting voice signals might not work well, or at all, for data
signals. Telephone companies need to know which lines are suitable,
i.e., qualify, and which lines are not suitable for data
transmission. Telephone companies also need to know why particular
lines are unable to support data transmissions and where such
faults occur so they can determine whether the transmission line
can be corrected.
[0004] The telephone network was originally designed for voice
communication. Voice communication covers a limited frequency
bandwidth. In some cases, telephone lines were optimized for
signals in this frequency range. Even where the lines were not
optimized for voice signals, there was no incentive to make the
lines operate at other frequencies and often they did not.
[0005] Now, it is desired to use those same lines to carry data
signals. The data signals generally have different frequency
characteristics than the voice signals. As a result, a line that
works very well transmitting voice signals might not work well or
at all for data signals. Phone companies need to know which lines
will work for data signals and use those lines for data.
[0006] Line Qualification is the overall ability to make statements
about the quality of a subscriber loop as it relates to its ability
to deliver voice communications (i.e. POTS), or data services.
Disqualification is the ability to make a statement with a high
degree of confidence that a subscriber loop will not support a data
service without remedial actions. Pre-qualification is the ability
to make a statement with a high degree of confidence that a
subscriber loop will support a data service without remedial
actions.
[0007] Telephone operating companies (TELCO's) have two problems to
solve in qualifying subscriber loops for delivery of data. The
first problem is strategic. Telco's are reluctant to deploy
emerging technologies for the delivery of data (e.g., ISDN or ADSL)
because there is uncertainty in their knowledge that sufficient of
the subscriber loops are of high enough quality to make deployment
economically successful. This discourages early adopters because
there is significant risk in being first to deliver a technology
that may not work in their access network. If Telco's could be
given a technology to take much of this risk out of initial
deployment, they can secure market share and lead in the face of
competition
[0008] The second problem is tactical and comes after a Telco has
made a decision to deploy a particular technology. There is a need
to qualify, either pro-actively or reactively, specific lines for
service as that service is requested by subscribers or targeted by
the Telco for delivery. For example, if a Telco is to market and
deliver the new service, they would like to target those subscriber
loops most likely to support the service out of the box and/or with
a minimum of work. As another example, a Telco receiving a new
service request from a subscriber desires information to either
accept or reject that request for new service based on the
condition of their line.
[0009] 4TEL, a product sold by Teradyne, Inc., of Deerfield, Ill.,
USA, has been used in the past in support of line qualification for
delivery of POTS. Techniques in 4TEL lend themselves to the
accurate detection and location of conditions which impair both
voice and FSK modems. Modern data transmission techniques (such as
those used in V.34, V.90, ISDN, and ADSL) encode data in part by
shifting the phase of the carrier frequency(s). As such, these
technologies rely upon there being fixed end-to-end and
differential transmission characteristics (e.g., phase and
echo).
[0010] A telephone line is made up of a two wire pair, called Tip
and Ring. Ordinarily, the Tip and Ring wires should have the same
electrical properties. It is desirable for the lines to be
balanced. In a balanced line, the resistance, capacitance and
inductance of each wire are equal. Imbalances exist if capacitance,
inductance, or resistance of one of the wires differ from the
other.
[0011] A particularly difficult type of condition to identify on a
telephone line using single point measurements is called a series
resistive imbalance. A series resistive imbalance introduces a
differential phase shift between the two wires of the loop. The
cause of series resistance is likely due to non-cold welded wire
wraps, IDC, or dry solder joints. The oxidation created at the
junction of the failing connection causes the series resistance to
be unstable, thus modifying the phase shift with time due to
changes in current flowing through the junction, further oxidation
of the junction, mechanical movement of the junction, and the like.
Higher speed modems encode many bits into phase shifts on these
carrier frequencies. Thus even minor instabilities of the series
resistance cause reduced data throughput, errors, and retraining.
With ISDN, the shifts in phase cause energy from one pulse to
overlap into the synchronization signal or into the time occupied
by another pulse, thus causing inter symbol distortion and/or loss
of synchronization. As can be seen, there is quite general
degradation of both analog and digital transmission methods, both
being susceptible to minor instabilities in series resistance.
Stable series resistance, even when values get very high can often
be successfully compensated for by internal circuitry in analogue
modems or at the U interface for ISDN.
[0012] It is important to detect series resistive imbalance because
large imbalance values affect POTS by reducing loop current levels.
It is possible that the imbalance might be so large, (2 kilo-ohms
or more) that seizing a dial tone may not be possible, or the
ringing current might not be sufficient to activate the bell
circuitry in the telephone or modem. It is equally important to
detect imbalance at values below 2 kilo-ohm when data transmission
is concerned. Any series resistance and the noise that it causes in
terms of phase shift have a detrimental effect on the data
throughput that may be achieved on that subscriber loop.
[0013] A telephone company would like to pre-qualify a line for
high data rate operation, such as ISDN and ADSL. Lines that have
been pre-qualified can be leased at a higher price. Lines with
imbalances would not be made available for these high data rate
services.
SUMMARY OF THE INVENTION
[0014] In accordance with the present invention, a method is
provided for qualifying a transmission line to propagate data
signals. The method includes measuring phase imbalance in the
transmission line from a terminating end of the line.
[0015] When the wires get out of balance, a human user of the
telephone line might notice a degradation in performance in the
form of audible noise or reduced voice quality. When the line is
used for data transmission, imbalance can limit the data throughput
at which the line can operate. However, we have recognized that it
is the change of imbalance that has most significant effect on data
transmission.
[0016] In accordance with another feature of the invention, a
method is provided for qualifying a transmission line to propagate
data signals. The method includes measuring imbalance in the
transmission line from a terminating end of the line.
[0017] In accordance with another feature of the invention, a
method is provided for qualifying a transmission line to propagate
data signals. The method includes applying a voltage in common
(i.e., a common mode voltage) to the transmission line; and,
determining phase imbalance in the line in response to the applied
common mode voltage. The phase imbalance being representative of
the difference in phase between the phase of a signal produced in
one of the legs in the transmission line and the applied voltage;
and, the phase of a signal produced in the other one of the legs in
the transmission line and the applied voltage.
[0018] In accordance with another feature of the invention, a
method for analyzing a transmission line wherein a common mode
voltage having a frequency changing over a range of frequencies is
applied to a pair of wires of in a transmission line; measuring the
phase or magnitude of the signals in each wire of the transmission
line relative to the applied common mode voltage in response to the
applied common mode voltage over the range of frequencies;
determining phase imbalance in the pair of wires in response to the
applied common mode voltage over the range of frequencies;
detecting a peak in the determined phase imbalance over the range
of frequencies; determining a frequency of the detected peak.
[0019] In accordance with another feature of the invention, a
method is provided for qualifying a transmission line to propagate
data signals. The method includes applying a common mode voltage
having a frequency changing over a range of frequencies into the
transmission line; determining phase imbalance in the transmission
line in response to the applied common mode voltage over the range
of frequencies; detecting a peak in the determined phase imbalance
over the range of frequencies; determining a frequency of the
detected peak; determining line qualification in accordance with
the determined frequency.
[0020] In accordance with still another feature of the invention, a
method is provided for automatically qualifying a plurality of
twisted pair transmission lines. The method includes feeding
signals from a controller to a switch connected to termination ends
of the transmission lines, such switch being coupled to a
measurement unit. Test signals from the measurement unit are
coupled to the transmission lines through the switch selectively in
accordance with control signals fed to the switch by the
controller. In response to the test signals, the measurement unit
isolates resistance imbalance between each of the wires in the
selected transmission line. The controller, in response to the
isolated resistance imbalance, determines the qualification of the
selected transmission line for data signals.
[0021] In accordance with still another feature of the invention, a
system is provided for automatically qualifying a plurality of
transmission lines. The system includes a switch coupled to
terminating ends of the plurality of transmission lines. A
controller is provided for feeding signals to the switch. A
measurement unit is coupled to the switch and the controller. The
measurement unit is adapted to feed test signals from the
measurement unit to a selected one of the transmission lines
through the switch. One of the transmission lines is selected in
accordance with a control signal fed to the switch by the
controller. The measurement unit isolates resistance imbalance
between each pair of wires in the selected transmission line in
response to the test signals fed to such selected transmission
line. The controller, in response to the isolated resistance
imbalance, is adapted to determine the qualification of the
selected one of the transmission lines for data signals.
[0022] In accordance with another feature of the invention, a
method is provided for determining the type of imbalance on a
transmission line having a pair of wires. The method includes:
feeding a frequency varying signal to the pair of wires;
determining the phase imbalance in the pair of wires in response to
the applied common mode voltage over the range of frequencies;
measuring a frequency at a peak in the determined phase imbalance
for a selected paired transmission line; and comparing the
determined frequency to a pair of reference frequencies expected
with a phase balanced pair of wires to determine the type of
imbalance between the wires.
[0023] In accordance with yet another feature of the invention, a
method is provided for locating the position of an imbalance on a
selected test line. The method includes: applying a common mode,
frequency varying voltage to twisted pair transmission line;
measuring the phase of the voltages on each wire of the twisted
pair transmission line relative to the applied voltage; computing
the admittance of the twisted pair at the varying frequencies;
deriving the capacitance over a selected transmission line from its
measured admittance at the varying frequencies; dividing the
derived capacitance by the per-unit length capacitance to ground
for the transmission line under test to produce a quotient;
computing the distance of the imbalance from the produced
quotient.
[0024] In accordance with still another feature of the invention, a
method is provided for locating the magnitude and position of an
imbalance on a selected test line. The method includes: determining
the presence of a series resistive imbalance; and if present,
establishing the location and/or magnitude of the imbalance. The
position of the imbalance is located by: applying a frequency
varying, common mode voltage to the transmission line; measuring
the magnitude and phase of the voltages on each wire of the
transmission line; determining phase imbalance in the twisted pair
in response to the applied common mode voltage; detecting a peak in
the determined phase imbalance; determining a frequency of the
detected peak; comparing the absolute value of the magnitude of the
measured voltages and the detected peaks to a list of reference
data for a transmission line of the type under test; determining
the location of the imbalance based on this comparison. The
magnitude of the imbalance is determined by: applying a common mode
voltage to the twisted pair transmission line; measuring the
magnitude and phase of the voltages on each wire of the twisted
pair transmission line; determining phase imbalance in the twisted
pair in response to the applied common mode voltage; detecting a
peak in the determined phase imbalance; determining a frequency of
the detected peak; comparing the frequency of the detected peaks to
a list of reference data for a transmission line of the type under
test; and, estimating the magnitude of the imbalance based on this
comparison.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] These and other features of the invention will become more
readily apparent from the following detailed description when taken
together with the accompanying drawings, in which:
[0026] FIG. 1 is a diagram of a POTS system having a twisted pair
transmission line data signal qualification testing system
according to the invention;
[0027] FIG. 2 is a simplified diagrammatical representation of a
measurement unit of the test system of FIG. 1 according to the
invention, such unit being coupled to a selected one of a plurality
of twisted pair transmission lines of the POTS system of FIG. 1,
such transmission line being shown by the equivalent circuit
thereof;
[0028] FIG. 3 is a block diagram showing a preferred embodiment of
the measurement unit of FIG. 2;
[0029] FIG. 4 is a flow chart showing the steps taken to disqualify
a transmission line for data service according to the
invention;
[0030] FIG. 5 is a flow chart showing the steps taken to
pre-qualify transmission line for data service according to the
invention;
[0031] FIG. 6 is a flow chart indicating the steps taken to
disqualify a transmission line for V.90 modem service according to
the invention;
[0032] FIG. 7 is a flow chart showing the steps taken to identify
the type of imbalance present on a transmission line according to
the invention;
[0033] FIG. 8 is a graph showing the relationship between phase and
frequency for either of the two wires in a balanced 6 Kilo-foot, 24
gauge, twisted pair transmission line;
[0034] FIG. 9 is a graph showing the phase difference over
frequency and the relationship between F2 (balanced) and Fpk for a
capacitive imbalance at 3 Kilo-foot on a 6 Kilo-foot 24 gauge,
twisted pair transmission line;
[0035] FIG. 10 is a graph showing the phase difference over
frequency and the relationship between F2 (balanced) and Fpk for an
inductive imbalance at 3 Kilo-foot on a 6 Kilo-foot, 24 gauge,
twisted pair transmission line;
[0036] FIG. 11 is a graph showing the phase difference over
frequency and relationship between F1, F2 (balanced) and Fpk for a
500 ohm series resistive imbalance at 3 kilo-foot on a 6 kilo-foot,
24 gauge, twisted pair transmission line;
[0037] FIG. 12 is a graph showing the phase differences over
frequency for resistive, inductive and capacitive imbalances, and a
balanced phase on a twisted pair transmission line;
[0038] FIG. 13 is a flow diagram indicating the steps needed to
locate the position of an imbalance on a transmission line using a
measurement of capacitance to ground method according to the
invention;
[0039] FIG. 14 is a flow diagram indicating the steps needed to
locate the position and estimate the magnitude of an imbalance on a
transmission line using a phase difference peak method according to
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Referring now to FIG. 1, POTS telephone network 10 is shown.
The network 10 includes a plurality of subscribers 11 connected to
a switch 12 (which is, or is connected to, the central office)
through transmission lines 14, which in many instances will be a
twisted pair. A centralized test system controller (TSC) 16 is
connected to one, or more, measurement units 18, and is adapted to
determined whether the twisted pair transmission lines 14 are
qualified for data signal transmission. The measurement units 18
are connected to the switch 12, as shown. The test control system
controller 16, measurement unit 18, and switch 12 are
interconnected as described in U.S. Pat. No. 5,699,402 assigned to
the same assignee as the present invention, the entire subject
matter thereof being incorporated herein. The measurement unit 18
will be described in detail hereinafter. Suffice it to say here
that the unit 18 is adapted to test the twisted pair either on
demand, or automatically, from a preprogrammed list of lines. It is
noted that a subscriber's transmission loop can be tested from the
central office because each measurement unit 18 has access to every
subscriber through the switch 12 and the techniques employed herein
use test signals that pass through switch 12 without undue
distortion. The unit 18 gains access to test a subscribers loop
through a switched test bus located in the switching element 12.
The switched test bus disconnects the line to be tested from the
switch 12, and connects it to the measurement unit 18.
[0041] More particularly, a system 13 is provided for automatically
determining qualification of the plurality of twisted pair
transmission lines 11. The system 13 includes the controller 16 and
the measurement unit 18 which are coupled to the switch 12. The
switch 12 is also coupled to the terminating ends of the plurality
of twisted pair transmission lines 14. The controller 16, here a
computerized work station, such as is commercially available from
SUN Computers, Inc., is provided for feeding signals to the switch
12 and to the measurement unit 18. The measurement unit 18 will be
described in detail in connection with FIG. 2. Suffice it to say
here that the measurement unit 18 is coupled to the switch 12 and
the controller 16 and that such unit 12 is adapted to feed test
signals from the measurement unit 12 to a selected one of the
twisted pair transmission lines 14 through the switch 12. The one
of the twisted pair transmission lines 14 selected is in accordance
with a control signal fed to the switch 12 by the controller 16.
Further, measurement unit 18 isolates resistance imbalance between
the pair of wires, T and R, in the selected one of the twisted pair
transmission lines 14 in response to the test signals fed to such
selected one of the twisted pair transmission lines 14, in a manner
to be described. The controller 16, in response to the isolated
resistance imbalance, is adapted to determine the qualification of
the selected one of the twisted pair transmission lines 14 for data
signal transmission.
[0042] To qualify a subscriber loop for data transmission, the
centralized test system controller 16 gathers information from many
sources, one of which is the measurement unit 18. The test system
controller 16 applies this information using the appropriate
hardware and software to a set of rules described below which
determine whether a tested line is capable of carrying data
transmission signals (i.e, the line is qualified). The following
steps are directed by software programmed in controller 16 using
known programming techniques.
[0043] One method for making a determination about the suitability
of a subscriber loop for data transmission, particularly either
ISDN or ADSL type data transmission, is the Disqualification
method. The Disqualification method allows a telephone company to
test its transmission lines to determine which lines may support
data transmission, and to disqualify those lines which do not.
Under the Disqualification method, the test system controller 16
gathers several factors about the test line including: (1) using
any known technique to determine the length of the line; (2) using
any known technique to determine the magnitude of any DC metallic
faults present on the line; (3) using any known technique to
determine the capacitive balance of the line; (4) using any known
technique to detect the presence of load coils on the line, such as
the one described in U.S. patent application Ser. No. 08/929,842 by
Yun Zhang entitled "Fast and Noise-Insensitive Load Status
Detection" which is hereby incorporated by reference; (5) using any
known technique to determine the composite noise on the line; and
(6) using the technique described below to determine the resistive
balance on the line. However, it will be appreciated that a line
might be disqualified by using less than all of these techniques or
by using additional checks.
[0044] Referring now to FIG. 4, the test system controller 16 then
executes the following rules, using the appropriate hardware and
software, to determine whether a line should be disqualified for
data transmissions. A line is disqualified if the test system
controller determines:
[0045] That the line length is greater than some threshold,
preferably in the range of 4 to 6 kilometers, and more preferably
5.5 kilometers (step 401); or
[0046] That metallic faults are less than some threshold,
preferably in the range of 80 to 200 kilo-ohms, and more preferably
100 kilo-ohms (step 402); or That capacitive imbalance is greater
than some threshold, preferably in the range of 0 to 5% and more
preferably greater than 0% (step 403); or
[0047] That load coils are detected (step 404); or
[0048] That noise is greater than some threshold, which is
preferably empirically determined (step 406); or
[0049] That resistive imbalance is greater than some threshold,
preferably in the range of 0 to 50% or that the series resistive
imbalance is unstable, meaning that the measured series resistance
imbalance changes more than some threshold since a reference
measurement was made.
[0050] It will be appreciated that not all of these measurements
might need to be made to disqualify a line. Further, it should be
appreciated that the thresholds used for each test might be
different, depending on the type of data service. For example, ISDN
data service can operate at a lower error rate than V.90 at a given
level of instability in the series resistive imbalance. It is
contemplated that the thresholds will be empirically determined,
taking into account such factors as actual experience and the
acceptable bit error rate specified by the user or other
factors.
[0051] Another method for qualifying a subscriber loop for data
transmission is the Pre-Qualification method. The Pre-Qualification
method allows a telephone company to test its subscriber loops to
determine which ones are capable of supporting ISDN and ADSL type
data services. Under the Pre-Qualification method, the test system
controller 16 makes the same measurements as described above for
the Disqualification method.
[0052] FIG. 5 illustrates the method by which a subscriber line can
be pre-qualified for data services. Note that the system of FIG. 1
can, by appropriate programming and commands input into controller
16 test all or some subset of the lines attached to switch 12. Very
simply, if a line is not disqualified using the tests described
above in conjunction with FIG. 4, it can be concluded with a high
degree of confidence that the line is qualified for data services.
Notably, all of the measurements needed to qualify the line can be
made from one end of the line and can also be made through a
switch.
[0053] A third method for qualifying a line for data transmission
is the V.90 Disqualification method. The V.90 Disqualification
approach enables a central test system controller 16 to test a
transmission line to determine whether it may handle a V.90 analog
modem.
[0054] As shown in FIG. 6, a line will be disqualified for V.90
data transmission if test system controller 16, using the
appropriate hardware and software, determines: using any known
technique that the line circuit type equals Pair Gain (Step 601);
or using any known technique that the line circuit type equals
universal DLC (Step 602); or using any known technique that the
trunk to RAS path equals analog (Step 603); or using the technique
described below that the resistive imbalance is greater than some
threshold (step 604), which in a preferred embodiment is about
1,000 ohms or that the imbalance is unstable, meaning that the
measured imbalance changes more than some threshold amount. In a
preferred embodiment, that threshold is 5%. However, it is
contemplated that as the magnitude of the imbalance increases, a
lower percentage for instability will be tolerated. Thus, the
threshold for stability measurements might be a function of the
magnitude of the imbalance. If a line is disqualified for V.90
mode, the modem using that line operates at its slower fall back
speed which is commonly called V.34. Again, the operator may not
need to check for all of these conditions in every case since again
subsets are permissible in some situations.
[0055] It is noted that each of these methods (i.e.,
Disqualification, Pre-Qualification, and V.90 Disqualification)
include a measurement of resistance imbalance between the wires in
the transmission line. In particular, the stability of the
resistive imbalance is very important in qualifying a transmission
line for data signals. The lack of stability is particularly
harmful for signals in which information is encoded in the phase of
the signal. It is also harmful because a shifting imbalance can
cause adjacent pulses to smear together. One way to measure the
stability of an imbalance is to take multiple measurements over an
interval that would be on the order of a second. Changes in
imbalance could then be detected from changes in the measurement
over that window. A second and potentially faster way to measure
the stability of an imbalance is to make a plot of phase of the
test line versus frequency. If the resistive imbalance is unstable,
the curve will not be smooth (smooth is not used here in the
mathematical sense), rather there will be many ripples and possibly
discontinuities on the curve. The instable resistance could then be
detected though an automatic technique to recognize a curve with
these characteristics.
[0056] Referring now to FIG. 2, the measurement unit 18 measures
resistive imbalance in each of the wires, R and T of a twisted pair
14. This measurement unit 18 may be used to determine whether the
twisted pair 14 qualify for data transmission when such unit 18 is
connected to a subscriber's transmission loop, as described above.
Here, the measurement unit 18 contains a signal source 30, here a
voltage source which is adapted to have its frequency swept in
response to a signal fed thereto by the controller 16. Also
included in the measurement unit 18 are a pair of balanced (i.e.,
having equal resistances) resistors R1 and R2, and a pair of
voltmeters 22 and 24. The voltage source applies a common mode
voltage to the pair of wires T, R of the twisted pair transmission
line. More particularly, the voltage source has one terminal
reference to ground potential and the other terminal connected in
common to the pair of wires, T, R, here through the resistors R1
and R2, respectively, as indicated. Voltmeters 22 and 24, are
provided to measure both the magnitude and the phase of the voltage
at the node NT of tip wire T and the node NR of ring wire R,
respectively.
[0057] Alternately, measurement unit 18 in FIG. 2 is represented in
FIG. 3 as measurement unit 18'. Here, measurement unit 18' contains
a signal source 30' which consist of a digital voltage frequency
controller 31 coupled to a digital-to-analog converter 32, both
connected to a clock 33. The signal source 30' is designed to have
it frequency swept in response to a signal fed thereto by the
controller 16. Also included in the measurement unit 18' is a pair
of balanced resisters R1 and R2, and a pair of analog-to-digital
converters 22' and 24', also connected to clock 33.
Analog-to-digital converters 22' and 23', measure both the
magnitude and phase of the voltage at node NT of tip wire T and the
node RT of ring wire R, respectively.
[0058] The equivalent circuit for an exemplary one of the twisted
pair transmission lines 14 is shown in FIG. 2. It is noted that the
ring wire R and the tip wire T include series resistances RR and
RT, respectively and shunt capacitances CR, CT, respectively. There
is also a capacitance CTR between the tip and ring wires, T and R,
respectively, as shown. It is noted that a resistance imbalance AR
between the tip wire T and the ring wire R is represented here as
shown in ring wire R. To detect and isolate resistive imbalance on
a test twisted pair transmission line 14, the following steps are
performed under the control of the test system controller 16:
[0059] (1) Signal source 30 applies a swept frequency excitation
voltage common mode with respect to ground through balance
resisters R1 and R2 to each wire R and T which make up the
transmission line 14. This signal typically ranges from 0 to 10
volts peak, and is swept in frequency, f, under the control of the
controller 16, from 0 to 20 kilohertz.
[0060] (2) Voltmeters 22 and 24 measure the resulting magnitude and
phase of each signal at nodes T and R with respect to ground. These
voltages, as a function of frequency, f, are called Va(f) and
Vb(f). The phase imbalance is equal to the difference in phase
between: (1) the phase of a signal produced in one of the wires in
the twisted pair transmission line with respect to the applied
voltage; and, (2) the phase of a signal produced in the other one
of the wires in the twisted pair transmission line with respect to
the applied voltage.
[0061] (3) The phase of the voltage Va(f) is then compared with the
phase of the voltage Vb(f) as a function of frequency to determine
the phase imbalance between the wires T and R (i.e., the difference
in phase, .DELTA..phi., between the phase of Va(f), .phi.a(f), and
the phase of Vb(f), .phi.a(f), as functions of frequency, f, leaves
us the difference in phase, or in other words, the phase imbalance,
which equals to .DELTA..phi.(f)=.phi.a(f)-.phi.b(f)). If a line 14
is balanced, i.e. no resistive, inductive, or capacitive imbalances
exist, then the signals measured from wire R will equal the signal
measured from wire T in both magnitude, Va(f) and Vb(f), and phase,
.phi.a(f) and .phi.b(f), as functions of frequency. If a line is
unbalanced then the signals measured at T and R will not equal in
magnitude Va(f) and Vb(f), or phase, .phi.a(f) and .phi.b(f), or
both, as functions of frequency.
[0062] (4) If a phase imbalance exists (i.e., .DELTA..phi..noteq.
0), the controller 16 proceeds to measure the frequency at which
the largest phase imbalance occurs. This value is called the phase
peak frequency or Fpk.
[0063] Referring now to FIG. 7, to identify the type of imbalance
on a test line, should one exist, the controller 16 uses the
appropriate hardware and software to complete the following
analysis. The controller 16 first establishes two reference
frequencies, F1 and F2 (Step 701). F1 and F2 are computed based
upon the length of the selected paired transmission line, and aid
in determining the type of resistive imbalance present. The lower
reference frequency is called F1 and the upper reference frequency
is called F2. The frequency F1 is an empirically defined frequency
equal to about 0.4 times F2, i.e. F2 divided by 2.5. F2 is the
frequency at which a phase peak, Fpk, should occur a balanced wire
R or T of a test transmission line 14. For example, FIG. 8 shows
the phase of Va(f) versus frequency, .phi.a(f), for a 6 kilo-foot
length of 24 gauge balanced twisted pair transmission line 14. In
this example, F2 is equal to 3.86 kilohertz, and F1 equals roughly
1.5 kilohertz.
[0064] For a given line 14 under test, F2 is determined by using
one of the following three methods: First, F2 may be measured when
the line 14 is in a known good state and that measurement may be
stored in a system footprint as described in the above referenced
U.S. Pat. No. 5,699,402; second, F2 may be computed from line cable
records which identify line lengths and loop records; and third, F2
may be computed from measuring the line length and measuring the
loop resistance using any known technique.
[0065] Next, the controller 16 identifies the type of imbalance
present in the transmission line 14 by comparing the lines 14
measured Fpk, to its previously established F1, F2 frequencies
(Step 702). If the imbalance is capacitive, as would be caused by a
single leg disconnect, the imbalance can be identified as such by
the controller 16 if it finds that Fpk occurs before F1 (Step 703).
For example, FIG. 9 shows the computed phase imbalance
.DELTA..phi.(f) between .phi.a(f) and .phi.b(f) for a resistive
imbalance located 3 Kilo-feet from the measuring unit 18 on a 6
Kilo-foot length of 24 gauge twisted paired transmission line, i.e.
one leg disconnected, superimposed on the plot of F2 as described
above. Here, the Fpk of the tested transmission line occurred
before F1 indicating that the imbalance is capacitive and so is
identified as such by the controller 16.
[0066] Next, if the imbalance is inductive, as would be caused by a
miswired load coil, then the controller 16 identifies it as such by
determining whether there are two Fpks, one positive and one
negative, or by determining, should there is only one Fpk, that Fpk
occurs after F2, (Step 704). For example, FIG. 10 shows the
computed phase difference, .DELTA..phi.(f), between .phi.a(f) and
.phi.b(f) for an inductive imbalance at 3 Kilo-feet on a 6
Kilo-foot twisted pair transmission line superimposed on a plot of
F2. Here, the Fpks of the tested transmission line occurred twice,
at Fpk1 and Fpk2, indicating that the imbalance is inductive and so
is identified as such by the controller 16.
[0067] Furthermore, if the imbalance is resistive, as would be
caused by unequal series resistance, then the controller 16
identifies it as such by determining whether the Fpk occurs after
F1, but before F2 (Step 705). For example, FIG. 11 shows the
computed phase difference .DELTA..phi.(f) between .phi.a(f) and
.phi.(b(f) for a series resistive imbalance of 500 ohms located 3
Kilo-feet from the measuring unit superimposed on a plot of F2.
Here, the Fpk occurs after F1 and before F2 indicating that the
imbalance is resistive and so is identified as such by the
controller 16.
[0068] FIG. 12 shows all four conditions, the phase difference
.DELTA..phi.(f) between .phi.a(f) and .phi.b(f) for a capacitive
imbalance, an inductive imbalance or a series resistive imbalance
on a twisted pair transmission line 14, as well as .phi.a(f) or
.phi.b(f) for a balanced twisted pair transmission line 14
superimposed on the same graph.
[0069] Moreover, the controller 16 can determine when a very
unstable (i.e., time varying) series resistive imbalance is present
in a twisted pair transmission line by noting that the phase to
frequency measurement for line are also unstable (Step 706). Such
an unstable situation is seldom true for any other imbalance
condition.
[0070] After determining the type of imbalance on a test line, the
controller 16 may also provide a means to discover the location of
the imbalance on a twisted pair transmission line 14. There exists
many possible techniques for locating an imbalance on a test line.
Below are the two preferred techniques used by the controller 16 to
measure the distance to the imbalance from the measuring unit
18.
[0071] FIG. 13 provides a flow chart for a method used by the test
system controller 16 to discover the location of an imbalanced
resistance in a twisted pair transmission line 14. This method is
called the Measurement of Capacitance to ground method. First, the
capacitance to ground of each wire T and R of the twisted pair
transmission line 14 is measured using any known technique (Step
1301). Next, an swept (alternating) common mode voltage is applied
to the twisted pair transmission line 14 and the resulting
magnitude, Va(f), Vb(f), and phase, .phi.a(f), .phi.b(f), of the
voltage on wires R and T are measured by measurement unit 18 (Step
1302). Then, these values are used to compute the admittance of the
twisted pair transmission line 14 at those frequencies (Step 1303).
Next, capacitance for that test line 14 can be directly computed
for these frequencies and admittances, (Step 1304). Then the
controller 16 compares the capacitance measurements taken in step
1301 with the capacitance measurements derived from the admittance
measurements in step 1304, (Step 1305). If there is no series
resistive imbalances present on either of the wires then both of
the capacitances measured at the lower frequency will be slightly
smaller than those measured at the higher frequency. If, however, a
series resistance imbalance is present in transmission line 14, the
capacitance measured at 8 kilohertz will be significantly smaller
than the capacitance measured at 25 hertz. Finally, the controller
16 approximates the distance to the imbalance by dividing the 8
kilohertz derived capacitance by the per-unit-length capacitance to
ground for twisted paired transmission line 14 under test and
comparing the value found to reference data, (Step 1306).
[0072] FIG. 14 provides a flow chart for a second approach used by
the test system controller 16 to discover the location of an
imbalance on a twisted pair transmission line 14 called the Phase
Difference Peak approach. This approach may also be used to
determine the magnitude of a series resistive imbalance. First, the
controller 16 determines whether a series resistive imbalance is
present by comparing Fpk to F1 and F2 as described previously in
this patent application (Step 1401). Then, if such an imbalance
exists, the controller 16 applies an alternating common mode
voltage signal through measurement unit 18 to wires T and R of the
twisted paired transmission line 14, (Step 1402). Next, the
controller 16 compares the absolute magnitude of the voltage
signals, Va(f) and Vb(f), and Fpks for the two signals for wires T
and R, to a list of reference magnitudes for a given line
construction (Step 1403). Then the controller 16 determines the
location of the series resistive imbalance based on a comparison
between the measured data and the reference data (Step 1404). The
controller 16 compares the frequency at which phase .phi.a(f) and
.phi.b(f) occurs to a list of reference frequencies for a model of
a twisted pair transmission line 14 of like construction (Step
1405). Then the controller 16 estimates the magnitude of the series
resistive imbalance based on a comparison between the measured data
and the list of reference frequencies for a model twisted pair
transmission line 14 of like construction (Step 1406).
[0073] Other embodiments are within the spirit and scope of the
appended claims. For example, the invention is described in
conjunction with a twisted pair transmission line. The techniques
might be applied to any transmission line with at least two
legs.
[0074] Also, an important aspect of line qualification and line
disqualification using measurements taken from a single point is
the ability to detect imbalances, particularly series resistance
imbalances using single point measurements, and particularly single
point measurements that pass through a switch. Time domain
reflectometry (TDR) might also be used for such measurements.
However, to make TDR measurements through a switch , the pulse
widths must be chosen carefully. Appropriate pulse widths are
described in the above mentioned U.S. Pat. No. 5,699,402.
[0075] Another way to determine imbalance on a transmission line is
through the use of data generated in a modem training sequence.
When a data connection is established between two modems over a
transmission line, the modems undergo a training sequence. In the
course of training, the modems can compensate to some extent for
series imbalances on the transmission line. Currently, the
information that indicates the amount of compensation is not used
for testing. However, if the information on compensation needed for
imbalance were saved for each line, comparisons could be made to
determine whether the compensation has changed over time. If the
compensation changed, it would indicate an unstable imbalance.
While such data might not be available to pre-qualify a line, it
could be used to disqualify a line or to diagnose network
faults.
[0076] Also, it should be noted that the disclosed embodiment
illustrated detecting resistive imbalance from a single point,
which is the end of the line connected to a switch. It is not
necessary that the test equipment be connected to the network at
this point.
* * * * *