Abstract:
A method and apparatus for single-ended qualification of subscriber loops for xDSL services is described. The method involves first screening a subscriber loop database record for disqualifying devices or services on the subscriber loop. If none are found, a set of predetermined electrical characteristics of the subscriber loop are derived from information in the database, or directly measured using test equipment at a central office end of the subscriber loop. The electrical characteristics are used to compute an available bandwidth on the subscriber loop for xDSL services. The advantage is the rapid and inexpensive qualification of subscriber loops which reduces response time to potential customer queries and facilitates deployment of xDSL services.

Description:
TECHNICAL FIELD 
     The invention relates generally to the provision of data services over subscriber loops in the Public Switched Telephone Network and, in particular, to a method and apparatus for the single ended qualification of subscriber loops to determine the suitability of such loops for the provision of high-speed data services. 
     BACKGROUND OF THE INVENTION 
     The exponential increase in the popularity of the Internet and related data services has prompted service providers in the Public Switched Telephone Network (PSTN) to seek new technologies for delivering high-speed data services to their customers. One solution is provided by Digital Subscriber Line (DSL) technologies. Several DSL technologies offer high-speed services over existing copper facilities, commonly referred to as “subscriber loops”. Such technologies include Asymmetrical Digital Subscriber Line (ADSL); High-bit-rate Digital Subscriber Line (HDSL); Rate Adaptive Digital Subscriber Line (RDSL); Symmetric Digital Subscriber Line (SDSL); and, very High-speed Digital Subscriber Line (VDSL). These digital subscriber line technologies are known collectively as “xDSL” services. 
     A problem encountered in the provision of xDSL services is that the subscriber loops have been largely neglected from a technology upgrade perspective. Existing subscriber loops were designed for voice telephony as opposed to high-speed data services. Consequently, many subscriber loops include wire gage changes and bridged taps (unused extension lines) which limit the available bandwidth. Other equipment installed on subscriber loops may also render the loop unsuitable for the provision of xDSL service. For example, load coils, voice frequency repeaters, loop extenders, Private Branch Exchanges (PBXs), line intercepts and incompatible data services all render subscriber loops unsuitable for the provision of xDSL service. 
     Testing apparatus for determining the physical and/or electrical characteristics of subscriber loops is known. Such apparatus is taught, for example, in U.S. Pat. No. 4,105,995 which issued Aug. 8, 1978 to Bothof et al.; U.S. Pat. No. 4,870,675 which issued Sep. 26, 1989 to Fuller et al.; and, U.S. Pat. No. 5,881,130 which issued Mar. 9, 1999 to Zhang. While such apparatus enables the determination of certain physical and/or electrical characteristics of the subscriber loop, none enable single ended determination of the bandwidth capacity of the subscriber loop for the provision of xDSL service. 
     Consequently, it has been the practice of service providers in the PSTN to dispatch a skilled technician to the premises of a customer who has requested, or expressed an interest in an xDSL service. The technician coordinates testing with another technician at the service provider&#39;s Central Office (CO). The dispatch of the skilled technician contributes significantly to the service provider&#39;s operating overhead and delays service provision due to the scheduling of subscriber loop qualification. 
     In order to reduce the cost and improve the efficiency of subscriber loop qualification, several solutions have been tried without success. For example, basic metallic measurements of electrical characteristics such as voltage, resistance and capacitance readings of the subscriber loop have been tried. However, these metallic measurements do not assess the high bandwidth range of the subscriber loop and consequently fail to provide a consistently accurate assessment of bandwidth available for xDSL service. Time Domain Reflectrometry (TDR) readings have also been tried without success. The problem is that TDR readings do not take physical properties of the cable or network topology information into account. The prior art methods have attempted to use measurements alone to generate rate predictions. Consequently, those prior art methods have failed because they do not correct for the physical properties of the subscriber loop, or equipment on the subscriber loop. 
     In order to screen customer inquiries and limit the number of requests for subscriber loop qualification for xDSL service, many service providers in the PSTN have implemented postal code maps which permit a subscriber to retrieve an indication of whether their subscriber loop is suitable for xDSL. The indication is retrieved, for example, by inputting a ZIP code or a postal code into a query screen available on the Internet. While such tools effectively limit the requests for subscriber loop qualification, in many ways they work against both the subscriber and the service provider. The postal code maps are based solely on an approximation of a distance between the subscriber premises and a serving CO in the PSTN. Because of this limitation, they cannot take into account the factors which may affect the suitability of the subscriber loop for xDSL service. Besides, because of the granularity of the postal code system, many subscriber loops suitable for the provision of xDSL service may be disqualified even though they are capable of providing some level of service. 
     There therefore exists a need to significantly reduce the costs of qualifying subscriber loops for xDSL service. 
     There also exists a need for a method and apparatus to permit the single ended qualification of subscriber loops. 
     There further exists a need to provide a method and apparatus for the single-ended qualification of subscriber loops which reduces the skill level required by operators performing the qualification. 
     There further exists a need for a method and apparatus for single ended qualification of subscriber loops which enables the pre-qualification of subscriber loops to permit a service provider to inform any service subscriber of the qualification of their subscriber loop on request. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a simple and economical method and apparatus for the single ended qualification of subscriber loops for xDSL service. 
     Accordingly, an aspect of the invention provides a method of qualifying a subscriber loop for xDSL services. The subscriber loop is connected to a public switched telephone network (PSTN) via a switch at a central office (CO). The method comprises the steps of: determining, from a CO end of the subscriber loop, one or more electrical characteristics of the subscriber loop; and estimating, based on the determined electrical characteristics, an xDSL bandwidth available on the subscriber loop. 
     A further aspect of the present invention provides a system for qualifying a subscriber loop for xDSL services. The subscriber loop is connected to a public switched telephone network (PSTN) via a switch at a central office (CO). The system comprises a processor adapted to determine, from a CO end of the subscriber loop, one or more electrical characteristics of the subscriber loop, and to estimate an xDSL bandwidth of the subscriber loop based on the determined electrical characteristics. 
     Thus the present invention provides a method and apparatus for single ended qualification of subscriber loops for xDSL services. Electrical characteristics of the subscriber loop preferably include values for: resistance (R); capacitance (C); inductance (L); and conductance (G) for each cable segment forming the subscriber loop. Additional electrical characteristics can, for example, include a wide band noise (WBN) value on the subscriber loop, and the presence of any one or more of: bridged taps; load coils; loop extenders; and other devices in the subscriber loop. The electrical characteristics of the subscriber loop can be determined by any one or more of; probing the subscriber loop from the CO end; estimation based on physical characteristics of the subscriber loop, and use of default values. In an embodiment of the invention, values of R, L, G, and C are contained in a cable properties database and may be accessed on the basis of the physical characteristics of the subscriber loop. 
     The physical characteristics of the subscriber loop preferably include information of: length; conductor size; cable insulation type; and cable installation type of each cable segment forming the subscriber loop, as well as information of devices installed on the subscriber loop. These physical characteristics of the subscriber loop are preferably obtained from database queries, but at least some physical characteristics can be estimated from measured electrical properties of the subscriber loop, and/or approximated using default values. 
     In an embodiment of the invention, a carrier service database includes information of physical characteristics of the subscriber loop as well as devices and services installed on the subscriber loop. In this case, a subscriber loop record containing information specific to the loop is extracted from the data base and screened for devices and services known to be incompatible with xDSL services. If any such incompatible devices or services are found, the loop is disqualified. On the other hand, if no incompatible devices or services are found, the electrical characteristics of the subscriber loop are used to calculate estimates of the up-stream and down-stream bit-rates (band-width) of wide-band xDSL signals transmitted over the subscriber loop. 
     In a preferred embodiment of the invention, prior to band-width estimation, the subscriber loop is probed to detect the presence of any one or more of metallic faults, load coils, or line intercepts. If any of these conditions are detected, the subscriber loop is disqualified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained by way of example only and with reference to the accompanying drawings, in which: 
     FIG. 1 is a connection diagram showing a local area network of a telephone services provider connected to a public switch with a number of associated subscriber loops; 
     FIG. 2 is a schematic diagram showing the interconnection of line cards for providing telephone services, subscriber loops delivering telephone services and test equipment at a public switch in the PSTN; 
     FIG. 3 is a schematic diagram showing a subscriber loop to be qualified; 
     FIG. 4 is a schematic diagram showing a type of subscriber loop deployment in which a cable containing at least one subscriber loop is buried underground; 
     FIG. 5 is a schematic diagram showing another type of subscriber loop deployment in which a cable containing at least one subscriber loop is installed in an underground conduit; 
     FIG. 6 is a schematic diagram showing yet another type of subscriber loop deployment in which cables containing subscriber loops are installed on poles; 
     FIG. 7 is a schematic diagram showing a typical subscriber loop to be qualified using the methods and apparatus in accordance with the invention; 
     FIG. 8 is a schematic diagram showing a shielded subscriber loop; 
     FIG. 9 is a connection diagram showing a subscriber loop with a bridge tap; 
     FIG. 10 is a flow chart showing the principal steps of a method in accordance with the invention; and 
     FIG. 11 is a flow chart showing the principal steps in a process for computing a bit rate that can be supported by a subscriber loop, to determine if the subscriber loop is qualified to support an xDSL service. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides a method and apparatus for the single ended qualification of copper wire subscriber loops in a switched telephone network. 
     FIG. 1 is a schematic diagram showing a central office (CO)  110  in switched telephone network connected to a plurality of voice-grade subscriber loops. In accordance with the invention, a processor  100  may access the CO  110 , and various test equipment for qualifying subscriber loops. Access may be accomplished, for example, via a wide area network (WAN)  102  to which the processor  100  is connected by a data link  104 . Processor  100  executes an algorithm for subscriber loop qualification to determine the suitability of the subscriber loops for xDSL services. 
     A carrier service database  106  is connected to the WAN by a link  108 . The carrier service database  106 , which may either be located at the switch or (preferably) on a server attached to the WAN, contains subscriber equipment records indexed by subscriber directory numbers, for example. The carrier service database may include the physical characteristics of the subscriber loops, such as loop length, wire gauge, bridge taps, etc. Also, addressable on the local area network are other central offices (not shown) providing telephone services to other subscribers. 
     Telephone services are provided to a subscriber  114  via a subscriber loop divided in two segments  116  and  118 . This particular subscriber loop includes a load coil  120  installed between segments  116  and  118 . Load coils are the used to improve transmission of signals in the voice frequency band. 
     Telephone services are provided to subscriber  122  via a subscriber loop comprising two segments  124  and  126 . Installed between subscriber loop segment  124  and subscriber loop segment  126  is a voice frequency repeater  128 . Voice frequency repeaters are used to amplify and retransmit signals in the voice frequency band. Telephone services are provided to subscriber  130  via a subscriber loop divided into two segments  132  and  134 . Installed between loop segments  132  and  134  is a loop extender  136 . Loop extenders are used to amplify signals in the voice frequency band. Telephone services are provided to subscribers  138  and  140  connected to a key system  142  via subscriber loop  144 . Subscribers  138  and  140  are connected to the key system  142  via links  146  and  148 . Key systems are used to connect private telephone networks to the public switched telephone network. Intercepted telephone services are provided to subscriber  150  via subscriber loop  152 . Installed on subscriber loop  152  is a recording system  154  which records all the voice frequency payload carried by the subscriber loop  152 . Integrated Services Digital Network (ISDN) services are provided to subscriber  156  over subscriber loop  158 . Plain Old Telephone Service (POTS) voice-grade telephone service is provided to a subscriber  160  by a single segment subscriber loop  162 . Of the above described subscriber loop configurations, only the subscriber loop  162  is suitable for supporting xDSL services. All others of the subscriber loops contain devices or support services that are incompatible with xDSL services. 
     FIG. 2 it is a schematic diagram showing a portion of the public&#39;s witch  110  which serves subscriber loop  200  terminated on line card  202 . Test equipment  204  can be connected to individual subscriber loops through an access grid  206  which consists of an hierarchy of buses  208  and  210 . Subscriber loop  200  can be respectively connected to the access grid  206  by electrically activating a connection point  212 . This permits the probing of individual subscriber loops to determine electrical characteristics of each loop. 
     FIG. 3 shows the details of the connection point  212 . Each line card  202  provides a tip and ring pair of conductors  214  and  216 . During normal operation tip and ring pairs  214  and  216  are connected to the tip and ring pairs  218  and  220  of the subscriber loop  200 . This connection is provided at the connection point by relays  222 . During testing of the subscriber loop  200 , the tip and ring pair  218  and  220  of subscriber loop  200  is connected to an associated tip and ring pair  224  and  226  of a bus  210  in the access grid  206 . This interconnection permits the test equipment  204  to be connected directly to the subscriber loop  200 . 
     FIGS. 4,  5  and  6  illustrate different methods used to install cables carrying subscriber loops between the central office and subscriber premises. These methods of installation comprise: buried cable shown in FIG. 4 in which the cable is simply laid in a trench and carried with earth.; underground cable shown in FIG. 5 in which the cable is run through a conduit buried in the earth; and, aerial cable shown in FIG. 6 in which the cable is supported by poles above the ground. Each type of installation requires cable with particular properties. 
     FIG. 7 is a schematic diagram showing the deployment of a subscriber loop  200  connected to a public switch  110 . Subscriber loop  200  is made up of two segments. A first segment  300  includes a tip and ring pair  302  and  304  of a first wire gauge. This first segment  300  is characterized by having a resistance  306  and an electrical capacitance  308 . The second segment  310  is made up of tip and ring pairs  312  and  314  of a second gauge. This second segment is characterized by an electrical resistance  316  and an electrical capacitance  318 . 
     FIG. 8 shows another type of subscriber loop deployment in which the tip and ring pairs are shielded. Subscriber loops segment  320  is shielded by an outer sheath filled with a dielectric insulator  324 . This segment is characterized by an electrical resistance  326  and an electrical capacitance  328 . Subscriber loop segment  330  is shielded by a sheath  332  that is air filled. This segment is characterized by an electrical resistance  336  and an electrical capacitance  338 . 
     FIG. 9 shows a subscriber loop connected to a telephone  374 , the subscriber loop includes a bridged tap  360 . In this configuration subscriber loop segment  340  includes a tip and ring pair  342  and  344  having an electrical resistance  346  and an electrical capacitance  348 . Loop segment  350  includes a tip and ring pair  352  and  354  having an electrical resistance  356  and an electrical capacitance  358 . A bridged tap segment  360  includes a tip and ring pair  362  and  364  having an electrical resistance  366  and an electrical capacitance  368 , The bridged top segment  360  is connected to the loop segment  350  at connection points  370  and  372 . 
     According to the present invention, subscriber loops can be qualified for xDSL services on an individual basis, or in groups. For example, an individual subscriber loop could be qualified in response to a request for service by the subscriber. Alternatively, a carrier service provider can elect to qualify a group of subscriber loops (e.g. all of the subscriber loops connected to a particular switch) at a time convenient to the carrier service provider, such as, for example, following an upgrade of a switch to enable DSL services to be provided by the switch. 
     FIG. 10 illustrates a process for qualifying a group of subscriber loops, it being understood that the same process also applies when only a single subscriber loop is to be qualified. 
     To perform the subscriber loop qualification process, the processor  100  (FIG. 1) is instructed to qualify one or more loops in a start  400 . In step  404 , the processor  100  determines whether a last subscriber loop identified in a qualification request list has been qualified. If at least one subscriber loop remains to be qualified, the processor  100  queries the carrier service provider database  106 , and retrieves a subscriber loop record located using the subscriber directory number, for example. The subscriber loop record contains information respecting the physical characteristics of the subscriber loop and services deployed on the loop. As described above, the information regarding physical characteristics preferably includes the identity (type) of equipment installed for the subscriber loop. The information also preferably provides data describing the make-up of the loop including a length, gauge size, insulation type and installation type for each cable segment (see FIGS. 7-9) forming the subscriber loop. 
     At step  408 , the processor  100  screens the customer record to identify any equipment or services on the subscriber loop that are incompatible with xDSL (typically because they are known to reduce the available bandwidth above voice frequency to zero, or a negligible margin). Incompatible equipment and services include voice frequency (VF) repeaters, line intercepts, loop extenders, induction neutralizing transformers, added main line (AML) carriers, bridge lifters, and private branch exchange (PBX) services. FIG. 1 illustrates exemplary subscriber loops equipped with devices and services which preclude the provision of xDSL services. 
     If any such incompatible equipment or services are found (step  410 ), the processor  100  disqualifies the subscriber loop for xDSL services at step  411 . Disqualification means that xDSL services cannot be deployed on the subscriber loop until (or unless) the incompatible equipment and/or services are removed. Following disqualification of the subscriber loop, the processor  100  records the disqualification in the subscriber loop record, or one qualification report, or both. The processor selects a next subscriber loop at step  402 , and restarts the qualification process. 
     If no incompatible equipment and/or services are found at step  410 , the processor  100  determines (step  412 ) whether test equipment is available (either co-located with the switch or elsewhere on the network), that is capable of probing the subscriber line to enable discovery of the physical make-up of the subscriber loop. 
     If test equipment is not available (step  412 ), then the processor ends evaluation of the subscriber loop, because it lacks sufficient information to estimate available bandwidth. In this case, the processor  100  selects a new subscriber loop at step  402 , and restarts the qualification process. If test equipment is determined to be available (at step  412 ), then the processor proceeds to discovery of the physical characteristics of the subscriber loop (step  414 ). 
     Discovery of the physical characteristics of the subscriber loop can be conducted in any of a variety of ways known in the art, depending primarily on the type of test equipment available. For example, the subscriber loop can be probed using test signals to detect the presence of shorts, opens, grounds and load coils, as taught by U.S. Pat. No. 4,870,675 (Fuller et al.). The methods described by Fuller can be adapted to detect any of a variety of devices which have a detectable signature, such as, for example, line intercepts, and added main line (AML) carriers. Test equipment is also known for probing subscriber loops (through the switch), to measure values of , for example, AC and DC voltages, resistance and capacitance over the entire subscriber loop. Using these measurements in conjunction with known cable properties, it is possible to infer a physical make-up of the subscriber loop. It is also known to connect test equipment to the subscriber loop independently of the switch (i.e. on the analogue side of the loop) using a suitable loop test equipement, such as, for example, Telaccord™ manufactured by Tollgrade Communications, Inc. This equipment, which is illustrated in FIG. 2, permits measurement of wide-band noise on the subscriber loop, in addition to other electrical characteristics. Wide band noise on the subscriber loop cannot be measured through the switch. Accordingly, if test equipment capable of measuring wide band noise is not available, a default value (e.g. −140 dBm/Hz) can be used to compute a Signal to Noise Ratio (SNR), as will be explained below in more detail. 
     Some communication devices (e.g. modems) available on the market generate high levels of noise in the wide-band region. Where such devices are installed on the subscriber loop (i.e. at the customer premises), it is possible that a measured value of wide band noise will be significantly higher than the default value. In such cases, it is preferable to adjust the default value of the wide band noise to a higher value. 
     Typically, the subscriber loop record will not contain information of metallic faults or load coils. Accordingly, at step  416 , the processor controls the test equipment to probe the subscriber loop to test for metallic faults (e.g. shorts or grounds), line intercepts, and load coils (as well as any other devices which preclude transmission of wide-band xDSL signals). If any such conditions are found, the processor disqualifies the subscriber loop (step  411 ) because the subscriber loop cannot support xDSL services until these conditions are resolved. Otherwise, the processor completes the discovery of the physical characteristics of the subscriber loop, and prepares for band-width estimation (step  418 ). 
     Upon completion of discovery of the physical characteristics of the subscriber loop (steps  414 - 416 ), the processor preferably updates the customer record with the discovered characteristics, and then proceeds with bandwidth estimation (step  418 ). 
     As mentioned above, at step  418 , the processor  100  estimates the total bandwidth available for wide-band xDSL signals transmitted over the subscriber loop. The total bandwidth is simply a total bit-rate at which data can be transmitted in each direction (e.g. up-stream and down-stream) over the subscriber loop. In some cases, the up-stream and down-stream bandwidths may be similar, in which case it may be possible to qualify the subscriber loop by calculation of bandwidth in only one direction. However, up-stream and down-stream bandwidths typically differ significantly, and it is normally preferable to estimate values for both the up-stream and downstream bandwidths. 
     FIG. 11 illustrates in greater detail the process of estimating bandwidth (step  418  of FIG. 10) in accordance with the present invention. The band width (or bit-rate of data transmission) available on a subscriber loop (or any portion thereof) at a particular frequency can be calculated from the signal-to-noise ratio (SNR) of the subscriber loop at that frequency. The SNR at a particular frequency can, in turn, be calculated from values of resistance (R), inductance (L), conductance (G) and capacitance (C) of the loop. Additionally, the values of R, L, G, and C are primarily functions of physical properties of the cable (e.g. conductor gauge (size), insulation type, and temperature) and may also vary with frequency. Accordingly, for the purpose of the estimating the total bandwidth, the wide-band xDSL signal is considered to be divided into a plurality of subchannels (e.g. of 4312 Hz width), with each subchannel having a predetermined center-frequency. Additionally, the subscriber loop is considered to be divided into one or more cable segments having a respective combination of length, conductor gauge, insulation type, and installation type. This permits values of R, L, G, and C to be found for each cable segment, which can be aggregated to calculate a SNR for each sub-channel. The SNR for each sub-channel is then used to determine a bit-rate (bandwidth) for each subchannel. The subchannel bit-rates are then summed to find the total bandwidth available for wide-band xDSL signal over the subscriber loop. 
     With reference now to FIG. 11, at step  500 , the processor  100  selects a subchannel having a center frequency (f), and, at step  502 , a cable segment (s). At step  504 , the processor  100  finds values of R(f,s), L(f,s), G(f,s), and C(f,s) for the selected center frequency (f) and cable segment (S). These values can conveniently be found by performing a look-up function in a cable properties database (not shown), which provides representative values of R, L, G, and C for each combination of conductor gauge and insulation type, measured at specific temperatures. An exemplary table of the cable properties database is as follows: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 gauge 
                 26AWG 
                   
                   
                   
               
               
                   
                 Insulation 
                 PIC 
               
               
                   
                 temp. 
                 70° F. 
               
               
                   
                 Center 
                 R 
                 L 
                 G 
                 C 
               
               
                   
                 Frequency 
               
               
                   
                 ... 
                 ... 
                 ... 
                 ... 
                 ... 
               
               
                   
                 20000 
                 83.48 
                 0.1868 
                 0.295 
                 15.72 
               
               
                   
                 ... 
                 ... 
                 ... 
                 ... 
                 ... 
               
               
                   
                 30000 
                 83.8 
                 0.1854 
                 0.295 
                 15.72 
               
               
                   
                 ... 
                 ... 
                 ... 
                 ... 
                 ... 
               
               
                   
                   
               
             
          
         
       
     
     The data stored in the cable properties database may be supplied by a cable manufacturer and/or obtained from reference texts, such as, for example the Digital Subscriber Loop Signal and Transmission Handbook, Whitman B. Reeve, IEEE Telecommunications Handbook Series, 1995. In order to extract the appropriate data from the cable properties database, the processor  100  uses the installation type (e.g. aerial, buried, or underground) from the customer record to determine a temperature parameter applicable to the selected cable segment (s). Exemplary temperature parameters are as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Installation 
                   
               
               
                   
                 type 
                 temperature parameter 
               
               
                   
                   
               
             
             
               
                   
                 Aerial 
                 T(s) = Maximum temperature at CO + 30° F. 
               
               
                   
                 Buried 
                 T(s) = (Maximum temperature at CO) − 10° F. 
               
               
                   
                 Underground 
                 T(s) = 68° F. 
               
               
                   
                   
               
             
          
         
       
     
     Using the temperature parameter, in combination with the conductor gauge, and insulation type of the selected cable segment (s), values of R, L, G, and C can be extracted from the cable properties database for temperatures bracketing (i.e. above and below) the temperature parameter. Values of R(f,s), L(f,s), G(f,s), and C(f,s) for the selected cable segment (s) can then be approximated from the extracted values by using a known interpolation technique. 
     At step  506 , the processor determines whether values of R(f,s) , L(f,s), G(f,s), and C(f,s) have been found for all of the cable segments (s) forming the subscriber loop. If the result of this determination is “NO”, then the processor selects the next cable segment (at step  508 ) and repeats steps  504  and  506 . 
     When values of R(f,s), L(f,s), G(f,s), and C(f,s) have been found for all of the cable segments forming the subscriber loop, the processor  100  calculates (at step  510 ) a signal to noise ratio (SNR) for the subscriber loop at the center frequency of the selected subchannel (f). 
     Calculation of the signal to noise ratio (SNR) for the subscriber loop at the center frequency of the selected subchannel (f) can be performed using known techniques, such as, for example, as described in  ADSL/VDSL Principles: A Practical And Precise Study of Asymmetric Digital Subscriber Lines and Very High Speed Digital Subscriber Lines , by Denis J. Rauschmayer, Macmillan Technical Publishing, 1999. Thus, as an intermediate step, the values of R(f,s), L(f,s), G(f,s), and C(f,s) can be used to calculate values of A(s), B(s), C(s), and D(s) for each cable segment (s) at the center frequency of the selected subchannel (f). For a cable segment, values of A(s), B(s), C(s), and D(s) are given by:                A        (   s   )       =     Cosh        (     P   ×   l     )                     B        (   s   )       =       Sinh        (     P   ×   l     )       ×   I                   C        (   s   )       =       Sinh        (     P   ×   l     )       I                   D        (   s   )       =     A        (   s   )                   where   :               P   =           (     R   +     j                 ω                 L       )     ×     (     G   +     j                 ω                 C       )                         (     the                 Propagation                 Constant     )                     I   =           R   +     j                 ω                 L         G   +     j                 ω                 C                           (     the                 Characteristic                 Impedance     )         ,   and               l                 is                 the                 cable                 segment                   length   .                                  
     A bridged tap can be treated, for the purposes of the present calculation as a virtual cable segment disposed between adjacent cable segments. In the case of a bridged tap, values of A(s), B(s), C(s), and D(s) are given by:                A        (   bt   )       =   1                 B        (   bt   )       =   0                 C        (   bt   )       =       1           R   +     j                 ω                 L         G   +     j                 ω                 C                      ×     Coth        (     P   ×   L     )           =     1     I   ×     Coth        (     P   ×   L     )                           D        (   bt   )       =   1                                
     These values of A(s), B(s), C(s), and D(s) for each cable section (s) are then combined to find values of A, B, C and D for the entire subscriber loop at the center frequency of the selected subchannel (f). Thus:          [         A       B           C       D         ]     =       [         A1       B1           C1       D1         ]     ×     [         A2       B2           C2       D2         ]        …                            
     It should be noted that, in the above calculation of A, B, C and D for the entire subscriber loop, the order of calculation of the segment matrices preferably follows the order in which the cable segments are arranged on the subscriber loop (in a direction moving away from the switch. Thus where the subscriber loop includes a bridged tap, the matrix of A(bt), B(bt), C(bt) and D(bt) values will be arranged between the corresponding matrices of the adjacent cable segments. 
     From the values of A, B, C and D for the entire subscriber loop, the loop attenuation (preferably based on an assumed 100 ohm termination) can be found, again as described by Rauschmayer Finally, the processor  100  calculates the signal to noise ratio SNR for the subscriber loop at the center frequency of the selected subchannel (f) as follows: 
     SNR=(Transmission Power)−(Loop Attenuation)−(noise)−(margin) 
     where: 
     (Transmission Power) is a default value equal to −36.5 dBm/Hz for down-stream signals and equal to −38 dBm/Hz for up-stream signals; 
     (noise) is a value derived from the wide band noise of the subscriber loop. For up-stream signals, noise=wide band noise. For down-stream signals, noise=wide band noise—loop attenuation. In order to arrive at a more conservative estimate of SNR for down-stream signals, it is preferable to set (noise) equal to the greater of the default wide-band noise value and (wide band noise—loop attenuation); and 
     (margin) is a default value of 6 dB. 
     The above calculation of SNR will yield two values of SNR: one each for up-stream and down-stream xDSL signals. It is thus possible to calculate, for example as described in  DSL Simulation Techniques and Standards Development for Digital Subscriber Line Systems  by Dr. W. Y. Chen, MacMillan Technical Publishing, 1998, respective up-stream and downstream transmission bit-rates (bandwidth) at the center frequency of the selected subchannel(f). 
     At step  514 , the processor  100  determines whether up-stream and downstream transmission bit-rates have been calculated for all of the subchannels. If the result of this determination is “NO”, then the processor selects the next subchannel (f) at step  516 , and then repeats steps  502  through  514 , until up-stream and downstream transmission bit-rates have been calculated for all of the subchannels. 
     When up-stream and downstream transmission bit-rates (bandwidth) for all of the subchannels have been computed, the processor  100  proceeds to step  518  to calculate the total up-stream and downstream transmission bit-rates (bandwidth) for the entire wide-band xDSL signal. This final calculation is a simple summation of all of the respective up-stream and down-stream subchannel bit-rates, as all of the subchannels operate to transport parallel data streams. 
     Upon completion of step  518 , the estimated total up-stream and downstream transmission bit-rates (bandwidth) of the subscriber loop can be displayed on a monitor (not shown), or stored in the subscriber loop record for future use or output in a suitable report format. If a single subscriber loop is being evaluated, the processes of qualifying the subscriber loop can terminate at this point. If a group of subscriber records are being qualified, the process returns to step  402  (FIG. 10) for selection of the next subscriber loop. 
     Field tests have been conducted to determine the accuracy of the methods and apparatus in accordance with the invention. The field tests were conducted by dispatching a skilled technician to subscriber premises of subscriber loops qualified using the method and apparatus. The field tests have substantiated that the methods and apparatus in accordance with the invention consistently predict with acceptable accuracy the bandwidth available on a subscriber loop for xDSL services. The methods and apparatus can therefore be relied on for single-ended subscriber loop qualification. 
     The embodiments of the invention described above are intended to be exemplary only, the scope of the invention being limited solely by the scope of the appended claims.