Abstract:
A subscriber loop test apparatus relates to the testing of a subscriber loop by the use of a touchtone telephone. The touchstone buttons on a telephone provide Dual-Tone, Multiple Frequencies for each key pressed. For example, referring to FIG.  1,  pressing the ‘3’ key causes tones at 697 and 1477 Hz to be sent across the telephone line. Pressing a sequence of keys provides discrete coverage of the frequency band from 697 to 1477 Hz, through the use of the touchtone telephone. The frequency band from 697 to 1477 Hz can be characterized in this fashion. Information about the line: loop loss (loop length), presence of bridge taps (i.e., excessively long bridge taps in particular) or loading coils, and the presence of Digital Loop Carriers (DLCs), et cetera. The use of the switch-hook flash provides the ability to determine the channel (impulse) response of the loop, including any noise and hum that may be on the loop due to imbalance or coupled interference.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/297,070 filed on Jun. 8, 2001 and entitled “Method and System For Testing Broadband Capability Of A Subscriber Loop Using touchtone Telephone Signals.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates to determining suitability of an existing subscriber loop for supporting broadband services (i.e., DSL). It specifically concerns remote testing of such a subscriber loop and in particular performing tests without access to the subscriber loop and without requiring equipment and/or craft personal to be present at a customer end of the subscriber loop. In a specific embodiment, it involves using existing voiceband and touchtone keys of a standard telephone to determine suitability of an access line to provide broadband/DSL service. 
     BACKGROUND OF THE INVENTION 
     To provide broadband service over conventional subscriber loops (i.e., herein that circuitry connecting a network&#39;s central office to a subscriber premises; commonly a pair of wires) conventionally requires access to the subscriber premises to perform tests that assure that the loop is capable of providing broadband (i.e. DSL) service. The effects of loop network components such as loading coils, bridge taps, loop distance, and digital loop carriers (DLCs) severely adversely impact the ability to provide broadband service to an unacceptable extent. 
     Testing of subscriber loops (physical circuitry connecting a customer&#39;s premises to a central office) has become an important process for broadband service providers. All loop lines are not capable of providing broadband DSL service. These lines were never originally designed for this purpose. Unfortunately, the lines that are incapable of supplying broadband service are not known ahead of time; so, the lines must be tested for their ability to send/receive broadband signals. Several methods exist to perform this function. If you are the service provider who owns the loop (Incumbent Local Exchange Carrier or ILEC who runs the CO, typically), then you can test the wire directly, over any frequency band, from the CO to the customer premises. However, if you are a Competitive Local Exchange Carrier (CLEC), then the digitization of received signals that occurs at the CO limits an ability to test the loop to the voice band of the line, typically 300 to 3300 Hz. This limitation is due to a digital signal sampling property (Nyquist Criterion) of 4000 Hz maximum since the signal is typically sampled at 8000 Hz with 8 bit precision. 
     It would be advantageous to be able to test the subscriber loop using data obtained only from the voice band and extrapolate the results to the broader DSL frequency band (typically to 1 MHz). Such a system has been disclosed in U.S. Pat. No. 6,091,713, but the technique described therein requires a subscriber to be connected to test equipment through a voice band modem located at the customer premises. Their process uses data stored in registers that provide data obtained during the handshake or negotiation process; this voice band information is extrapolated for use over the greater DSL band. There may be times, however, where it is necessary or desirable to test the line without the use of a modem or computer (logic device) at the customer premises. One particular situation is where the test provider does not have direct access to the subscriber loop. In this instance, sending a craft person to the premise or providing special equipment at a customer site is prohibitively expensive. 
     It is desirable to test a subscriber loop for broadband service suitability without access to the customer premises and without the use of modems, personal computers and special logic devices on the customer premises. 
     A SUMMARY OF THE INVENTION 
     A subscriber loop test apparatus relates to the testing of a subscriber loop by the use of a touchtone telephone. The touchtones on a telephone provide Dual-Tone, Multiple Frequencies for each key pressed. For example, referring to FIG. 1, pressing the ‘3’ key causes tones at 697 and 1477 Hz to be sent across the telephone line. Pressing a sequence of keys provides discrete coverage of the frequency band from 697 to 1477 Hz, through the use of the touchtone telephone. The frequency band from 697 to 1477 Hz can be characterized in this fashion. Information about the line: loop loss (loop length), presence of bridge taps (i.e., excessively long bridge taps in particular) or loading coils, and the presence of Digital Loop Carriers (DLCs), et cetera. The use of the switch-hook flash provides the ability to determine the channel (impulse) response of the loop, including any noise and hum that may be on the loop due to imbalance or coupled interference. Both of these techniques shall be termed ‘user telephony input (UTI).’ 
     In one specific embodiment, the testing technique uses existing voiceband and touchtone keys on a standard telephone to determine the suitability of a subscriber loop to provide broadband (DSL) service. The technique is advantageously integrated with an interactive voice response (IVR) system that provides inquiring subscribers instructions as to how to proceed and interact with a test server to determine broadband service suitability. With this arrangement a potential DSL user can be tested, prequalified and provisioned using a single automated process. This allows prompt and inexpensive service to the customer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a network including a subscriber loop, a central office, a subscriber device, a test facility and connecting lines; 
     FIG. 2 is a block schematic of an illustrative architecture for providing DSL loop prequalification; and 
     FIG. 3 is a schematic of an illustrative process architecture used in prequalifying a line for suitability in providing broadband service to a customer. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the invention requires a user, using a normal touchtone telephone  110 , to dial a call center  101 . The call is routed via local subscriber loop lines  111 , as an analog signal to a central office  102  that digitizes the analog signal. This resulting digital signal makes it impossible to remotely test the subscriber loop over the full DSL frequency range. Through the call connection to the call center  101 , the user is prompted to press a sequence of digits and to execute switch hook flashes from the touchtone telephone. This sequence of digits is the stimulus for the digital signal analyzer to test the line over the frequency band from 697 to 1477 Hz. Additional voice band characterization may be performed by instructing the user to quickly depress and release the switch hook of the telephone: this action provides a near-impulse response to the digital signal analyzer and allows the full voice band to be characterized. At the call center, these touchtone signals are digitally processed to analyze such variables as: 
     a) Touchtone amplitude slope versus frequency; 
     b) Residual hum and noise within the voice band; and 
     c) Impulse response through the switch hook flash. 
     None of these actions require modems, computers or logic devices to be present or used at the customer&#39;s premises. 
     The testing of suitability for DSL service through this automated process may in one illustrative aspect consist of up to four different parts. The results of these individual parts can be analyzed either independently, or taken as a whole. When looked at as a whole, an even better determination of DSL suitability can be made. The four aspects to the prequalification process include the items discussed below. 
     An address lookup based on the calling user&#39;s phone number (delivered through Automatic Number Identification,) is made in coordination with the incumbent local exchange carrier (ILEC), and can provide an estimate of the geographical distance from the serving central office (CO) and the user&#39;s home site. Since service suitability for DSL is strongly linked to distance between the customer premise and the CO, this measurement is a primary critical element used in the decision process. 
     A second aspect in determining loop suitability is made by querying a database of existing users who reside in the same neighborhood as the user seeking prequalification. This existing user database would be able to provide a broadband loop measurement result on existing users, as well as the current and maximum data rates possible on these users lines. Users close to the person being prequalified will most likely share the same wire bundle back to the central office, and similar statistics with regards to the use of load coils and bridge taps. Therefore, the availability of this information would be very helpful in predicting DSL suitability. 
     A third aspect in determining DSL suitability involves a direct electrical measurement of the user&#39;s line. An IVR system would prompt the user to press several keys on his telephone keypad. By doing so, the user generates dual tone multi-frequency (DTMF) signals back to the IVR system for measurement. These DTMF signals consist of a low tone (indicating keypad row), and a high tone (indicating keypad column). The DTMF frequency pairs are as follows (the last column is not used in standard telephones): 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 1209 Hz 
                 1336 Hz 
                 1477 Hz 
                 1633 Hz 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 697 Hz 
                 1 
                 2 
                 3 
                 A 
               
               
                 770 Hz 
                 4 
                 5 
                 6 
                 B 
               
               
                 852 Hz 
                 7 
                 8 
                 9 
                 C 
               
               
                 941 Hz 
                 * 
                 0 
                 # 
                 D 
               
               
                   
               
             
          
         
       
     
     The amplitude of the tones when transmitted from the user&#39;s telephone is relatively well dictated by existing specifications, though minor variability will exist. By comparing the amplitude of each of the received tones with the expected amplitude of the tones, the following information about the user&#39;s line can be determined: 
     Since the transmission line characteristics of analog telephone loops are well understood, a mathematical loop model can be created based on loop length and wire gauge. By measuring the individual received tone amplitudes and fitting this data to the model, an electrical (as opposed to geographical) estimate of loop length can be made. 
     This electrical estimate of loop length can be compared to the geographical estimate made earlier. If the electrical estimate indicates a significantly shorter loop than indicated by geography, then there is a high likelihood that the user&#39;s loop is attached to a digital loop carrier (DLC). Since a DLC terminates the user&#39;s loop remote from the CO, such loops are considered unsuitable for DSL service. 
     If the electrical measurements indicate there is attenuation in the loop consistent with its geographical length, but the frequency response is flatter than predicted, then there is a high likelihood that this loop has a load coil attached to it. Load coils are devices commonly used on very long loops to improve voice quality by boosting high frequency response (which is heavily attenuated on long loops). While these devices improve voice response, they attenuate all frequencies above the voice band. Since this is precisely where the DSL signal lies, load coils are a strong indication that this line is not suitable for DSL service. 
     A third form of loop impairment that can affect DSL service is bridge taps. A bridge tap is an unterminated wire pair that is attached to the user&#39;s loop somewhere along its length. The effect of bridge taps is to cause a series of dropouts in the frequency response of the loop, which affects the available data rate to/from the user&#39;s home. Since bridge taps are more likely to impact frequency above the voice band rather than in the voice band, they are not as easy to detect with a simple voice band measurement. However, it is possible that one of these dropouts could fall in the range of the DTMF tones and be detected. If so, this detected bridge tap could also be used in conjunction with the distance estimates to determine DSL suitability. A bridge tap is not a direct indication that the line is unsuitable, but when combined with length, it may be determined that the user&#39;s loop cannot support the expected data rate offered. 
     In addition to the measurement of DTMF tones from the user&#39;s phone, an active measurement of the loop could be performed. This measurement would involve the following phases: (1) The IVR system informs the user that a brief test is to be performed and that a series of tones and noise will be heard. (2) The IVR system then activates a 2100 Hz tone with 180° phase reversals to deactivate the network echo cancellation. By deactivating the echo cancellation, the IVR system will be able to measure both the near-end (CO) and far-end (CPE) echo on the user&#39;s loop. (3) Once the echo cancellers are deactivated, the IVR system transmits a pseudo-random noise sequence to the user&#39;s phone and collects measurements. (4) The received measurements are correlated with the transmitted noise to determine the impulse response of the loop. (5) The frequency response (determined by taking a Fourier transform of the impulse response) can be used to detect load coil presence, and whether a bridge tap that affects the voice band is present. As above, this data can be combined with the DTMF measurement to improve the decision process. 
     An illustrative DSL prequalification architecture executing these steps embodies an interactive voice response system as shown in the FIG. 2 to inform customers that an active test of the loop is to be performed. This system interacts with a customer loop  201  joining a subscriber device  203  to a central office  205  to permit interactive evaluation of the loop  201 . An interactive voice response (IVR) system  209  is connected to the central office via a digital interconnect  207 . System  209  includes an interactive response voice subsystem  211  to respond to voice commands and to generate voice messages and responses. Such interactive voice subsystems are known in the art and further discussion is not believed necessary. 
     The IVR system includes a coordination processor  210  which through sub processors  213 ,  215   217  and  219  interact with the digital interconnect  207 . Sub processor  213  is a tone detection processor, which deciphers the received multitones generated by a subscriber in assisting loop evaluation. Correlation processor  215  correlates the received dual tones with noise generated and transmitted by a noise generation processor  217 , the noise being transmitted to determine an impulse response of the loop. A tone generator  219  is provided to generate a series of tones for transmission to the loop under test. 
     A prequalification process is illustrated by the architecture diagram of FIG. 3 in which an evaluation is initiated by a customer representative  301  in response to either an inbound telemarketing system or an outbound telemarketing system (i.e., on line customer initiated or network initiated). A direct marketing initiation is also permitted to initiate the process. The process is initiated by step  303 , which evaluates tone signals provided by a testing sequence such as indicated by the steps described above. The process proceeds, in step  305 , to investigate records to determine an IVR loop length estimate based on amplitude of tones received from a customer. This is derived from the existing DSL customer database whereby the known length of nearby DSL customers is determined. Step  307  investigates if a caller ID is available. If available, the process proceeds to step  311  where the caller&#39;s number recorded for an address lookup in step  313  and the caller&#39;s address is recorded in step  315 . A geographic database uses the address information for evaluation in step  317  to provide a geographic loop length estimate in step  319 . 
     The data determined by steps  305  through  319  and test system  303  are transmitted to a DSL Serviceability determination  321  which in conjunction with stored data in the DSL customer data base  351  and the DSL equipped Central Office (CO) database  353  (i.e., identifies which CO can provide DSL service) assembles all the information to make a decision in the decision step  323 . If DSL is not available in a nearby CO as in step  339 , a decision is made in step  341  as whether to notify the customer that DSL is not available. In step  343 , the availability is periodically revisited. In step  345 , the test results are stored and the system test is disconnected. 
     If the loop supports DSL service in the determination of step  323  the availability is noted in step  325  and a determination is made by the customer (in decision step  327 ) as to whether to order DSL service now. Service is ordered immediately through the IVR system in step  329 . If a service representative is contacted as per step  331 , the order may be processed by a customer representative with prequalification status in step  333 . In an alternate qualification, results are stored for further use in step  335  and the customer is disconnected in step  337 . 
     It is also readily apparent that this testing method and system does not require ownership or access to the subscriber loop, nor is special equipment (i.e., modem, PC, logic devices etc.) required to be located at a customer premise This permits CLECs to readily determine their ability to provide broadband services over a normally voice band subscriber loop. 
     It is readily apparent that this system is closely integrated with customer service and art various points in the process a potential customer has an ability to interact with a customer service representative. These points occur at the beginning of the process and at several steps within the process. Accordingly, the potential customer may start with a customer representative switch into the automated process and return to a customer representative and subsequently proceed with the automated process. 
     In one illustrative process, a customer may call into AT&amp;T asking about DSL service with a representative. The representative can transfer them to the DSL prequalification system, and then get the customer transferred back along with the results. At that time, if service is available, the customer could immediately enter the provisioning process. 
     In contrast, most DSL competitors must do their DSL prequalification separate from their provisioning—the customer must either do a prequalification using their modem (requiring a completely different phone call), must enter address information into a website, or must speak with a company representative who manually processes the prequalification. All of these methods are more expensive and less efficient. 
     Another option for measuring the user&#39;s line would be to have the IVR (FIG. 2) ask the potential customer whether they have a facsimile (FAX) machine. If so, the user could be requested to fax a document (any document, content is unimportant) to the IVR system. The information about the channel transmitted during the fax modem handshake could be used to determine the frequency response and attenuation of the channel. As before, this information could be used to supplement information gained from the geographical measurement, the DTMF measurement, and the impulse response measurement to further improve the estimate of the user&#39;s loop length and serviceability.