# Patent

## Title
Qualifying telephone lines for data transmission

## Abstract

The method assesses the suitability of customer telephone lines (12, 13, 14) for data transmission. The method includes selecting a telephone line having tip and ring wires by means of a computer (30) and switch (15), and electrically connecting the tip and ring wires together at a test access (29) adjacent one end of the selected line to produce a common mode configuration. Single-ended electrical measurements are performed on the wires in the common mode configuration by a measurement unit (27) connected to the test access (29) to determine an electrical property of the wires from the measurements.

## Background
BACKGROUND OF THE INVENTION
This invention relates generally to telephone lines, and more particularly, to qualifying telephone lines for data communications.
Public switched telephone networks, e.g., plain old telephone systems (POTS), were originally designed for voice communications having a limited frequency range. Today, the same POTS networks often carry data transmissions using higher frequencies. The difference in frequencies suggests that some POTS lines, which function well for voice, will function poorly for data. The risk of poor quality data transmissions has motivated telephone operating companies (TELCO's) to develop tests for predicting the quality of lines for data transmissions.
One quality test uses physical line length to determine a line's attenuation. The attenuation of a line whose length is less than about four kilometers (km) is usually low enough for data transmission. But, measuring the line length is typically more involved than measuring the straight line distance between a customer's address and a switching station. Typically, lines form branching structures rather than going radially from the switching station to the customer addresses. Thus, determining a line length usually entails manually mapping the actual branching structures connecting the customer to the switching station. Such complex manual techniques can be time intensive and may lead to errors.
Furthermore, determining that a line's length is less than a preselected limit, e.g., four km, may be insufficient to qualify the line for data transmission. The line's attenuation also depends on the physical properties of each branch segment making up the line, e.g., the gauge mixture of the line. In lines having segments with different properties, the above-described mapping technique generally should take into account the properties of each segment to determine the total attenuation of the line.
TELCO's have also used direct electrical tests to determine the quality of POTS lines for data transmissions. Typically, such tests are manual and two-ended. Two-ended tests involve sending one employee to a customer's address or final distribution point and another employee to a switching station. The two employees coordinate their activities to perform direct electrical measurements on the customer line using hand-held devices. These two ended measurements are substantially independent of the termination characteristics at the customer's address. An example of two-ended measurements is described in ROEHRKASTEN W: ‘MESSUNG VON XDSL-PARAMETERN’ NACHRICHTENTECNIK ELEKTRONIK, DE, VEB VERLAG TECHNIK. BERLIN, vol. 48, no. 2, 1 Mar. 1998 (1998-Mar.-01), pages 20–21, XP000752845 ISSN: 0323-4657.
Nevertheless, these two-ended tests need two separate employees, which makes them labour intensive. The labour requirements affect the cost of such tests. The two-ended tests cost about $150 per customer line. This cost is so high that a TELCO is often prohibited from using such tests for all customer lines.
HEDLUND, ERIC; CULLINAN, TOM: ‘DSL Loop Test’ TELEPHONY, vol. 235, no. 8, 24 Aug. 1998 (1998-Aug.-24), pages 48–52, XP002147002 USA, mentions single-ended testing but does not specify how such testing may be performed.
The present invention is directed to overcoming, or at least reducing, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In a first aspect, the invention provides a method of assessing the suitability of customer telephone lines for data transmission. The method includes selecting a telephone line via a test access of a switching station and electrically connecting the tip and ring wires adjacent one end of the selected line in a common mode configuration. The method also includes performing single-ended electrical measurements on the tip and ring wires with respect to ground by driving the tip and ring wires in the common mode.
The method includes determining an electrical property of the wires from the single-ended measurements.
In a second aspect, the invention provides a system for determining a signal attenuation of a customer line. Each customer line has tip and ring wires. The system includes a measurement unit having first and second input terminals to couple to a test access of a telephony switch. The measurement unit is capable of driving the input terminals in a common mode configuration with respect to ground to perform single-ended impedance measurements on the tip and ring wires of the customer lines in the common mode configuration.
In a third aspect, the invention provides a method of marketing customer telephone lines for selected data transmission services. Each line has associated tip and ring wires. The method includes automatically performing single-ended electrical measurements on the customer telephone lines and determining which of the customer lines qualify for a selected data transmission service from the measurements. The tip and ring wires are driven in a common mode configuration during at least a portion of the measurements upon the associated lines. The method includes sorting the lines by distribution point and qualification to transmit data. The method also includes offering the selected data service to a portion of the customers in response to lines determined to be qualified for the service being available.



BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features, and advantages of the invention will be apparent from the following description, taken together with the drawings in which:
FIG. 1 shows an embodiment of a system for testing the suitability of customer lines for data transmission;
FIG. 2 shows the segments of one customer line from FIG. 1;
FIG. 3 is a flow chart illustrating a method of testing telephone lines for data transmission;
FIG. 4 shows a portion of the measurement unit that performs impedance measurements on the lines of FIG. 1;
FIG. 5 is a flow chart for a method of qualifying customer lines using low frequency measurements on tip and ring wires driven in a common mode configuration with respect to ground;
FIG. 6 is a table comparing attenuations found with the methods of FIG. 5 to reference values; and
FIG. 7 is a flow chart illustrating a method of marketing data transmission services for customer lines.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a portion of a POTS network 8. The network 8 includes customer lines 12–14 connecting customer units 16–18, i.e., telephones and/or modems, to a telephony switch 15 located in a TELCO central office 10. The switch 15 may be a POTS switch or any other device for connecting the lines 12–14 to the telephone network 8, e.g., a digital subscriber loop access multiplexer (DSLAM) (not shown).
Each customer line 12–14 is a twisted copper two-wire pair adapted for telephone voice communications. The two wires of each line 12–14 are generally referred to as the ring and tip wires. The lines 12–14 are contained in one or in a series of standard telephony cables 20. The cable 20 may carry more than a dozen customer lines (not all shown) thereby creating an environment that changes electrical and transmission properties of the separate lines 12–14. The properties of the lines 12–14 may also depend on their segment structure.
FIG. 2 shows that the customer line 12 has several paired copper two-wire segments 21–23. The segments 21–23 are located in separate cables 20, 24–25 and couple serially through couplers 26. Each segment 21–23 may have a different length and/or gauge than the other segments 21–23. The segmented structure of the line 12 can affect electrical properties, e.g., the signal attenuation.
Referring again to FIG. 1, single-ended measurements on the lines 12–14 are performed with a measurement unit 27 located in the central office 10. The measurement unit 27 couples, via a line 28, to a standard voice test access 29 of the switch 15. The test access 29 provides electrical couplings to selected customer lines 12–14 in a voice frequency range of at least between 300 Hertz (Hz) and 4 kilo-Hz (KHz), i.e., a low frequency range. The measurement unit 27 uses the test access 29 to perform a single-ended measurements on the lines 12–14, e.g., impedance measurements.
The line testing is controlled by a computer 30. The computer 30 sends signals the switch 15, via line 31, e.g., to select the line 12–14 to be tested. The computer 30 sends signals to the measurement unit 27, via line 32, to select and control the test to be performed. The measurement unit 27 sends measurement results to the computer 30 via the same line 32.
The computer 30 includes a storage medium 33 encoding an executable software program for testing selected ones of the lines 12–14. The program includes instructions for one or more methods of controlling single-ended measurements on the lines 12–14. The program also includes instructions for methods of analyzing the measurements to qualify or disqualify the lines 12–14 for data transmissions. Both types of method are described below.
The line testing qualifies or disqualifies the individual lines 12–14 being tested. To qualify, the computer 30 must predict that the line 12–14, under test, will support data transmissions without remedial measures. To disqualify, the computer 30 must predict that the line 12–14, under test, will not support data transmissions without remedial measures. The computer 30 may perform tests before or after the line 12–14, under test, is in service for data transmissions.
Tests to qualify or disqualify a line 12–14 for data transmission involve several steps. For each step, the computer 30 signals the switch 15 to disconnect the particular line 12–14, selected for testing, from the line card (not shown) and reroute the line to the test access 29. When the switch 15 reroutes the line, the computer 30 signals the measurement unit 27 to perform preselected single-ended measurements on the selected line 12–14. The measurement unit 27 performs the measurements and returns results to the computer 30. After receiving the results of the measurements, the computer 30 signals the switch 15 to reroute the selected line 12–14 to the line card. Then, the switch 15 transfers connections for the selected line 12–14 to the line card enabling the associated customer unit 16–18 to again communicate with the rest of the network 8.
FIG. 3 is a flow chart illustrating a method 50 for determining the suitability of a selected one of the customer lines 12–14 for a preselected data transmission service. By way of example, the line 12 of FIG. 1 is selected, but any of the lines 12–14 can be evaluated by the method 50. Each step of the method 50 includes one or more single-ended measurements on the selected line 12 and an analysis of the measurements by the computer 30 as has been already described. In addition, the steps of the method 50 fall into two stages.
In the first stage, the computer 30 tests for traditional line faults by performing independent electrical measurements on the tip and ring wires T, R of the selected line 12. First, the computer 30 performs such measurements to determine whether the selected line 12 has any metallic faults (step 52). Metallic faults include shorts to ground, to a voltage source, or between the paired wires T, R, and/or capacitive imbalances between the paired wires T, R of the selected line 12. Second, the computer 30 performs such measurements to determine whether the selected line 12 has any speed inhibiting faults (step 54). Speed inhibiting faults include resistive imbalances between the paired wires T, R of the selected line 12, and split pair or load inductances. Speed inhibiting faults also include bridged taps that reflect signals resonantly, e.g., the spurious tap 55 shown in FIG. 2, and elevated line-noise levels.
The threshold values of single-ended measurements, which indicate the above-described faults, generally depend on the type of data transmissions. Methods for performing and analyzing such single-ended measurements are known in the art. For example, U.S. Application No. 60/106,845 ('845), filed Nov. 3, 1998, by Roger Faulkner et al, and U.S. Pat. Nos. 5,699,402 ('402) and 4,113,998 ('998) describe such methods and apparatus. The '845 application and '402 and '998 patents are incorporated by reference, in their entirety, in the present application. The '402 application and the '402 and '998 patents also describe apparatus 53, of the measurement unit 27 used for the single-ended measurements to detect the faults.
If the computer 30 to finds either a metallic or a speed-inhibiting fault, the computer 30 disqualifies the selected line 12 for data transmissions (block 56). If the computer 30 finds no such faults, the computer 30 determines whether the selected line 12 attenuates signals of a selected frequency by more than a threshold value for the preselected data transmission service (step 58). In the absence of faults, the signal attenuation at high frequencies is the primary measure for determining a line's ability to transmit data.
FIG. 4 shows portions of the measurement unit 27 for measuring the impedances subsequently used to determine the attenuation of the selected customer line 12. The measurement unit 27 includes an AC signal generator 36, which provides an AC driving voltage and current for measuring the impedances. During the measurements, the AC signal generator 36 drives two input terminals 40, 41 in a common mode configuration. The input terminals 40, 41 electrically connect internally at a point 43 to produce the common mode configuration. The terminals 40, 41 also couple, via the line 28, to the test access 29 of the switch 15. The measurement unit 27 also has a voltmeter 38 to measure the driving voltage with respect to ground, and an ammeter 40 to measure the driving current in the common mode configuration.
The test access 29 has internal connections 44, which electrically couple the tip and ring wires T, R of the line 12 under test to the terminal 40 and the terminal 41, respectively. Thus, the tip and ring wires T, R are electrically connected together, at the switch end, so that the signal generator 36 drives these wires T, R in common mode configuration during impedance measurements. Driving the wires T, R in common mode makes electrical measurements insensitive to termination characteristics of the customer unit 16.
Both the preselected threshold value for the signal attenuation and the preselected frequency depend on the type of data transmission. For ISDN data transmissions, the preselected threshold is about 45 deci-Bells (dB) at 100 KHz. For ASDL data transmissions, the preselected threshold is about 40 dB at 300 KHz depending on deployed terminal equipment.
If the signal attenuation at the preselected frequency is above threshold, the computer 30 disqualifies the selected line 12 for the corresponding type of data transmissions (block 56). If the signal attenuation is below threshold at the preselected frequency, the computer 30 qualifies the line 12 for the corresponding type of data transmissions (block 60) providing no faults were found at either step 52 or step 54.
FIG. 5 illustrates one method 70 of determining whether the signal attenuation for the selected line 12 is above the threshold in step 58 of FIG. 4. First, the measurement unit 27 performs single-ended common-mode measurements of the capacitance C and the impedance Z of the selected line 12 as described with relation to FIG. 3 (step 72). The measurements of C and Z are typically low frequency measurements, i.e., between about 300 Hz to 4 KHz, because the standard test access 29 of the switch 15 does not necessarily support high frequency measurements. If the test access 29 supports higher frequency measurements, such frequencies can be used to set a better resolution on the high frequency attenuation of the selected line 12.
The measurement unit 27 measures the capacitance C and then uses the value of C to determine the frequency for measuring the impedance Z. The capacitance C is a lumped value between the common mode tip and ring wires T, R and ground. The measurement unit 27 determines C at a low frequency, e.g., 80 Hertz (Hz). If the measured value of C is less than 400 nano-Farads (nF), the AC signal generator 27 drives the tip and ring wires T, R in common-mode at about 2.5 KHz to measure the impedance Z. If the value of C is greater than 400 nF, the AC signal generator 27 drives the tip and ring wires T, R, in common-mode, at a higher frequency between about 3 and 20 KHz, e.g., 3.0 KHz, to measure the impedance Z. The computer 30 uses the relation Z=V/I, where the voltage V is measured by the voltmeter 38 and the current I is measured by the ammeter 40, to find Z.
Next, the computer 30 determines the signal attenuation A(f) at high frequencies characteristic of data transmissions using the low frequency measurements of C and Z (step 74). The high frequencies are more than ten times the frequencies used for measuring Z and C. The value of “A(f)” at higher frequency “f” is known from an empirical formula (1) given by:

A(f)=K[Z2+(2πfC)−2]−1/2.  (1)

The value of K=5,000 dB-ohms provides good predictions of the attenuation A(f), in dB, for C and Z (in ohms) measured at low frequencies as described above. For this value of K, the frequency f, at which the attenuation is to be determined, should be between about 40 KHz and 300 KHz.

Next, the computer 30 determines whether the high frequency attenuation A(f) is above the preselected threshold for the selected type of data transmissions (step 76). If the attenuation A(f) is above the threshold, the computer 30 disqualifies the selected line 12. If the attenuation is below threshold, the computer 30 qualifies the selected line for the selected data transmissions.
FIG. 6 shows a table 80 comparing values of the signal attenuation A, in dB, at high frequencies, found using the method 70, to reference values, found by an independent method, i.e., simulators. Column 3 of table 80 shows the values of A(f) predicted from low frequency measurements of C and Z and the formula (1). Column 4 of table 80 shows the values of A(f) obtained from simulators of customer lines, i.e., the reference values. The values of attenuation A(f) of FIG. 6 are given in dB's at a frequency “f” of about 100 K Hz.
The values of the high frequency attenuation A(f) of the table 80 correspond to a variety of one and two segment structures for the selected customer line 12. Columns 1 and 2 list segment lengths and gauges, i.e., diameters in millimeters, for the copper tip and ring wires T, R of the selected line 12. For each one and two segment structure shown, the predicted and reference attenuations differ by less than about 2 dB. Generally, formula (1) gives values of the high frequency attenuation A, which differ by less than about 3 dB for various segment mixtures if the wire gauges are between about 0.4 mm and 0.7 mm and total line lengths are less than about 6.5 km.
FIG. 7 is a flow chart illustrating a method 90 of marketing preselected data transmission services for the customer lines 12–14 of FIG. 1. First, a TELCO performs common-mode single-ended electrical measurements on the selected group of lines 12–14 as described in relation to FIG. 3 and step 70 of FIG. 5 (step 92). Next, the TELCO determines which of the lines 12–14 qualify for the preselected data service from the measurements (step 94). This determination includes performing the steps 74 and 72 of the method 70 of FIG. 5 and may include the steps 52 and 54 of the method 50 of FIG. 4. The determination includes sorting the lines based on their final distribution points and qualification status for the preselected data transmission service. Next, the TELCO offers the preselected data transmission service to the portion of the customers to which the lines 12–14 qualified in step 94 are available, i.e., customers at final distribution points with qualified lines (step 96). The TELCO connects a portion of the qualified lines 12–14 to the customers who subsequently request the offered data services (step 98). The TELCO also bills usage for a portion of the lines 12–14 at prices that depend on whether the lines 12–14 qualify or disqualify for the data transmission services (step 100).
To provide the requested data services at step 98, the TELCO may swap customer lines to the same final distribution point. The swapping reassigns a qualified line to a customer requesting data service if the customer's own line is disqualified. The swap reassigns the customer's original disqualified line to another customer, who is at the same final distribution point and not demanding data service. The disqualified line can still provide voice services to the other customer. Thus, swapping can increase a TELCO's revenue by making more lines available for more expensive data services.
A TELCO can also use swapping in response to a request by the customer for data services. In response to such a request, the TELCO determines whether the customer's own line qualifies for the requested service by the above-described methods. If the line qualifies, the TELCO provides the customer data services over his own line. If the line disqualifies for the requested service, the TELCO performs additional qualification tests on other lines to the same final distribution point, which are not presently used for data transmission services. If one of those lines qualifies for the requested data service, the TELCO swaps the customer's line with the qualified line. Then, the qualified line provides data services to the customer requesting such services and the unqualified line provides normal voice service to the other customer.
Other embodiments are within the scope of the following claims.

## Claims

1. A method of assessing the suitability of customer telephone lines for data transmission, comprising:
selecting a telephone line having tip and ring wires via a test access of a switching station;
electrically connecting the tip and ring wires together adjacent one end of the selected line to form a common mode configuration;
performing single-ended electrical measurements by driving the wires in the common mode configuration with respect to ground; and
determining an electrical property of the wires from the single ended measurements.

2. The method of claim 1, wherein the determining comprises finding an impedance (Z) of the wires in the common mode configuration.

3. The method of claim 2, wherein the performing comprises driving the wires at low frequencies and the act of the determining finds a property at a high frequency, the high frequency being at least ten times the highest one of the low frequencies.

4. The method of claim 2, wherein the determining comprises calculating an attenuation from the impedance.

5. The method of claim 4, wherein the measuring comprises finding a capacitance (C) for the tip and ring wires in the common mode configuration.

6. The method of claim 5, wherein the calculating uses a formula to obtain the attenuation (A(f)), the formula being A(f)=K [Z2+(2πfC)−2]−1/2, the f being the frequency, and the K being a number.

7. The method of claim 2, further comprising:
determining whether the selected line has a line fault; and
disqualifying the line in response to finding the line fault.

8. The method of claim 2, wherein the fault is a speed inhibiting fault.

9. The method of claim 8, wherein the speed inhibiting fault includes one of a resistance imbalance, a bridged tap, a load coil, and a noise level above a preselected threshold.

10. The method of claim 8, wherein the line fault includes a metallic fault.

11. The method of claim 10, wherein the metallic fault includes one of a capacitance imbalance, a short to ground, a short to a voltage source, and an intermediate short between the tip and ring wires.

12. The method of claim 10, further comprising:
determining whether the selected line has a speed inhibiting fault; and
disqualifying the line in response to finding the speed inhibiting fault.

13. The method of claim 8, wherein the act of determining an electrical property includes calculating an attenuation for the line using the electrical measurements.

14. A system for determining signal attenuations of customer telephone lines, each line having tip and ring wires, comprising:
a measurement unit having first and second input terminals to couple to a test access of a telephony switch, the measurement unit capable of driving the input terminals in a common mode configuration to perform single-ended impedance measurements on the tip and ring wires of the customer lines.

15. The system of claim 14, wherein the measurement unit further comprises:
a voltmeter coupled to measure a voltage driving said input terminals in the common mode configuration; and
an ammeter coupled to measure a current going to said input terminals in the common mode configuration.

16. The system of claim 15, wherein the measurement unit further comprises:
a signal generator connected to the first and second terminals to drive said terminals in the common mode configuration.

17. The system of claim 15, wherein the measurement unit further comprises apparatus to perform single-ended measurements to detect one of metallic faults and speed inhibiting faults on the customer lines.

18. The system of claim 14, further comprising:
a processor coupled to the measurement unit and capable of coupling to the switch, the processor having a data storage medium encoding a program of instructions for a method, the method comprising:
ordering the measurement unit to perform the single-ended measurements; and
analyzing results of the ordered measurements to determine a signal attenuation of the one of the customer lines.

19. The system of claim 18, wherein the method further comprises:
determining whether the one of the lines is qualified to transmit data from the signal attenuation.

20. The system of claim 18, wherein the signal attenuation corresponds to a frequency at least ten times frequencies at which the measurement unit is capable of driving the one of the lines through the test access.

21. The system of claim 18, wherein the method further comprises:
ordering the switch to transfer connections for the one of the lines from the network to the test access prior to the act of ordering the measurement unit.

22. The system of claim 14, further comprising:
the switch having the test access, the switch being a central office switch.

23. The system of claim 17, wherein the test access is adapted to transmit electrical signals having voice-range frequencies.

24. A program storage device encoding an executable program of instructions for a method of determining the signal attenuation of customer telephone lines connected to a central switch, the method comprising:
ordering the switch to transfer connections for one of the lines from the network to a test access of the switch;
ordering a measurement unit to perform single-ended impedance measurements on tip and ring wires of one of the lines by driving the tip and ring wires in a common mode configuration using the test access; and
analyzing results of the ordered measurements to determine a signal attenuation of the one of the customer lines.

25. The device of claim 24, wherein the method further comprises:
determining whether the one of the lines is qualified to transmit data from the signal attenuation.

26. The device of claim 24, wherein the signal attenuation corresponds to a frequency (f) at least ten times signal frequencies of the single-ended measurements.

27. The device of claim 26, wherein the act of analyzing comprises:
calculating the attenuation (A) based on a formula, the formula being A(f)=K [Z2+(2πfC)−2]−1/2, and
wherein Z and C are the respective impedance and capacitance of the line in the common mode configuration.

28. The device of claim 24, the method further comprising:
determining whether the selected line has a line fault; and
disqualifying the line in response to determining that the line has a fault.