Abstract:
A method for testing a digital loop carrier is provided. The method includes initiating at least one test with a test request, communicating the test request to a remote terminal of the digital loop carrier over a digital communication channel, conducting the at least one test at the remote terminal and reporting the results of the at least one test over the digital communication channel.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to the field of telecommunications and, in particular, to testing a digital loop carrier.  
         BACKGROUND  
         [0002]    Telecommunication systems provide communication between subscriber equipment, e.g., telephones, computer modems, etc. The telephone system developed around the use of narrow-band analog communications to carry voice traffic between subscribers. Classically, telephones were connected to centralized switches, often referred to as the “central office,” over a pair of copper wires, colloquially referred to as a “twisted pair,” “loop” or “customer drop.” These twisted pairs have an inherent limitation in that signals can be transmitted and received with acceptable levels of fidelity only over a limited distance. This limitation imposes a maximum allowable distance between subscribers and the central office.  
           [0003]    Over time, digital technology for aggregating and transporting communication signals over longer distances was developed. Through the use of digital transport technology, the reach of central office equipment was extended. One development in this area included the digital loop carrier. A conventional digital loop carrier includes a central office terminal (COT) and a remote terminal (RT). The central office terminal connects to or is integrated with a digital switch at a central office. The remote terminal is located a distance from the central office and terminates a plurality of twisted pairs or customer drops. The central office terminal is coupled to the remote terminal over a digital link such as a T1, E1 or other appropriate digital communication line that aggregates a number of communications channels using, for example, a time division multiplex (TDM) technique.  
           [0004]    As with conventional customer drops, the twisted pairs connected to the remote terminal are tested periodically to assure proper operation. To accomplish this, an additional twisted pair, referred to as a “bypass pair,” is provided between the central office terminal and the remote terminal. Test equipment at the central office connects to the digital switch as well as to the bypass pair at the central office terminal. When a customer loop is to be tested, the test equipment applies a start voltage to the line through a metallic connection through the digital switch. A line card in the central office terminal detects the start voltage. The line card signals the remote terminal that a test is requested. The remote terminal also connects the customer drop to the bypass pair. This creates a metallic connection between the test equipment in the central office through the COT to the RT and eventually to the customer drop. The test equipment then tests the customer drop over the bypass pair. The RT provides a resistive signature to the test equipment indicating the results of the test when the tests are completed. The test equipment removes the starting voltage from the line being tested and the normal service is restored.  
           [0005]    Unfortunately, the requirement to test the customer drop of a digital loop carrier system limits the distance between the COT and the RT due to the use of the bypass pair. Since signals must be transmitted over the bypass pair, the RT cannot be located beyond the range imposed by signaling requirements for the bypass pair. This range is typically much shorter than the range of the digital communication medium, e.g., T1 or E1 lines, coupled between the COT and RT.  
           [0006]    Therefore, there is a need in the art for a digital loop carrier system that allows for testing of the customer drop with improved distance between the central office terminal and the remote terminal.  
         SUMMARY  
         [0007]    Embodiments of the present invention accomplish testing of customer drops of a digital loop carrier system with increased distance between the central office terminal and remote terminal by eliminating the bypass pair. For example, in one embodiment, signaling normally carried over the bypass pair is presented to the central office terminal over a digital communication channel between the central office terminal and the remote terminal. In one embodiment, the digital communication channel is accomplished using the Concentrator Field Bits of the TR-08 standard. In other embodiments, other fields are used to carry the data in the digital communication channel. Advantageously, through the use of the digital communication channel, the need for the bypass pair is removed.  
           [0008]    In one embodiment, a method for testing a digital loop carrier is provided. The method includes initiating at least one test with a test request, communicating the test request to a remote terminal of the digital loop carrier over a digital communication channel, conducting the at least one test at the remote terminal and reporting the results of the at least one test over the digital communication channel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a block diagram of one embodiment of a digital loop carrier with improved testing capabilities according to the teachings of the present invention.  
         [0010]    [0010]FIG. 2 is a block diagram of another embodiment of a digital loop carrier with improved testing capabilities according to the teachings of the present invention.  
         [0011]    [0011]FIG. 3 is a flow chart of one embodiment of a process for testing a digital loop carrier according to the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.  
         [0013]    Embodiments of the present invention provide for testing of customer drops in a digital loop carrier system without the use of a bypass line between a central office terminal and a remote terminal. Embodiments of the present invention accomplish control of testing and communication of test results over a digital communication link instead of a bypass line. This provides the advantage of allowing the remote terminal and the central office terminal to be located further apart than in existing systems with bypass lines.  
         [0014]    In Technical Reference TR-TSY-000008, “Digital Interface Between the SLC96 Digital Loop Carrier System And A Local Digital Switch” (commonly referred to as TR-08), a data link frame structure is defined for use in the digital loop carrier system. Part of this definition includes 11 bits that are used as Concentrator Field Bits. Some digital loop carrier systems, such as the PG-Flex Plus  system commercially available from ADC Telecommunications Systems, Inc. of Eden Prairie, Minn., do not use line concentration. Thus, these bits of the data link frame are not used. Therefore, in one embodiment, the Concentrator Field Bits are used as the digital communication channel or message channel from the Central Office Terminal (COT) to the Remote Terminal (RT) in the digital loop carrier system. In other embodiments, other reserved and un-used fields are used as the message channel. Using a message channel, mechanized line tests are completed on the digital loop carrier system without a bypass pair and with no limitation on the distance between the COT and RT. In one embodiment, the message channel also carries short messages between the COT and RT.  
         [0015]    [0015]FIG. 1 is a block diagram of a digital loop carrier system, indicated generally at  100 , according to the teachings of the present invention. System  100  has an improved testing capability compared to existing digital loop carrier systems.  
         [0016]    System  100  communicates subscriber traffic and other information between central office terminal  106  and remote terminal  108 . Central office terminal  106  is coupled to remote terminal  108  over one or more digital communication links  110 . In one embodiment, the digital communication links  110  are time division multiplexed (TDM) links such as T1 or E1 links. In one embodiment, digital communication links  110  comprise four T1 (DS1) links.  
         [0017]    Central office terminal  106  has a number of interfaces. First, central office terminal  106  has a link interface coupled to digital communication link  110 . Central office terminal  106  also includes a switch interface  107 . Switch interface  107  couples traffic between local digital switch  104 , e.g., a class 5 switch, and central office terminal  106  over a plurality of voice frequency lines  105 . Finally, central office terminal  106  includes a test interface  109 . Test interface  109  connects to test equipment  102  over test bus  111 . Test equipment  102  is also coupled to local digital switch  104 .  
         [0018]    Remote terminal  108  also includes a number of interfaces. First, remote terminal  108  includes a link interface that is coupled to digital communication link  110 . Further, remote terminal  108  also includes subscriber interface  112 . Subscriber interface  112  is coupled to a plurality of customer drops  114 , e.g., a plurality of twisted pairs that provide service to a plurality of subscribers. Further, remote terminal  108  also includes a loop test function that is operable to perform one or more tests on customer drops  114 .  
         [0019]    In operation, test equipment  102  is operable to test customer drops of remote terminal  108  without the use of a bypass pair between central office terminal  106  and remote terminal  108 . Test equipment  102  initiates the test by applying a voltage through switch  104  to a selected voice frequency line  105 . When this voltage is applied, the central office terminal  106  sends a message to the remote terminal over a digital communication channel that a test is requested. In one embodiment, the digital communication channel uses an otherwise unused field in the data link frames sent over digital communication link  110 . For example, with a digital loop carrier system that does not support concentration, the concentration field bits of a TR-08 frame are used to communicate messages between the remote terminal  108  and central office terminal  106  regarding the test of customer drops  114 .  
         [0020]    Remote terminal  108  conducts the requested test or tests on the selected customer drop  114 . The results of the test or tests are reported to central office terminal  106  over the digital communication channel. At central office terminal  106 , the test results are provided to test equipment  102  over test bus  111 . Further, a test complete signal is sent from remote terminal  108  to central office terminal  106 . When this is received, an appropriate resistive signature is applied to test bus  111 . When received, test equipment  102  removes the test request voltage from the voice frequency pair  105  at digital switch  104  and normal service is restored.  
         [0021]    [0021]FIG. 2 is a block diagram of another digital loop carrier system, indicated generally at  200 , according to the teachings of the present invention. System  200  has an improved testing capability compared to existing digital loop carrier systems.  
         [0022]    System  200  communicates subscriber traffic and other information between central office terminal  206  and remote terminal  208 . Central office terminal  206  is coupled to remote terminal  208  over one or more digital communication links  210 . In one embodiment, the digital communication links  210  are time division multiplexed (TDM) links such as T1 or E1 links. In one embodiment, digital communication links  210  comprise four T1 (DS1) links.  
         [0023]    Central office terminal  206  has a number of interfaces. First, central office terminal  206  has a link interface coupled to digital communication link  210 . The link interface is coupled to multiplexer (MUX)  212 . Central office terminal  206  also includes a switch interface. The switch interface couples traffic between local digital switch  204 , e.g., a class 5 switch, and line cards  216 - 1  to  216 -N of central office terminal  206  over a plurality of voice frequency lines  205 . Finally, central office terminal  206  includes a test interface  209 . Test interface  209  connects to test equipment  202  over test bus  211 . Test equipment  202  is also coupled to local digital switch  204 . Central office terminal  206  also includes a management unit  214 . Management unit  214 , line cards  216 - 1  to  216 -N, and MUX  212  are coupled together over a backplane bus  230 .  
         [0024]    Remote terminal  208  also includes a number of interfaces. First, remote terminal  208  includes a link interface that is coupled to digital communication link  210 . The link interface is coupled to multiplexer (MUX)  218 . Further, remote terminal  208  also includes a subscriber interface. The subscriber interface is coupled to a plurality of customer drops  214 , e.g., a plurality of twisted pairs through line cards  222 - 1  to  222 -N. Customer drops  214  provide service to a plurality of subscribers. MUX  218 , management unit  220 , and line cards  222 - 1  to  222 -N are coupled together over backplane bus  232 .  
         [0025]    MUX  212  and MUX  218  are set to a “channel bank” mode. This mode establishes the digital communication channel or message channel between central office terminal  206  and remote terminal  208  using, for example, the otherwise unused concentrator field bits. The operation of the embodiment of FIG. 2 is described below with respect to the method of FIG. 3.  
         [0026]    [0026]FIG. 3 is a flow chart of one embodiment of a method for testing a digital loop carrier according to the teachings of the present invention. The method begins at block  300 . At block  302 , the method initiates the testing of a customer drop or loop. This testing comprises one or more appropriate tests to evaluate selected aspects of the operation of the customer drop. In one embodiment, test equipment, e.g., test equipment  202 , applies a test request voltage to a voice frequency (VF) pair at the COT, e.g., COT  206 . The COT detects this voltage such as by line card  216 - 1  of COT  206 . In one embodiment, several additional events happen at this point. First, the signaling being sent to the remote terminal, e.g., signaling sent to line card  222 - 1  of remote terminal  208 , changes to the channel test signaling. Further, the line card at the COT, e.g., line card  216 - 1 , sends a message to management unit  214  indicating the test has begun. The management unit, in turn, forwards this information to the multiplexer, e.g., MUX  212 , at the COT. The multiplexer then forwards the test request as a message to the remote terminal over the message channel. This message is received, for example, at a multiplexer in the RT, e.g., MUX  218 . This multiplexer then sends a message to its associated management unit, e.g., management unit  220  of remote terminal  208  over shelf backplane  232 .  
         [0027]    When test signaling is detected at the line card, e.g., line card  222 - 1  of remote terminal  208 , the line card starts testing of the line at block  306 . This includes one or more tests that are accomplished using existing or later developed techniques for testing the operation of the customer drop of the digital loop carrier.  
         [0028]    In one embodiment, when the one or more tests are complete, the results are sent to the COT over the digital communication channel or message channel at block  308 . In one embodiment, the results are first sent to the management unit of the RT, e.g., management unit  220  of remote terminal  208 . The management unit of the remote terminal, e.g., RT  208 , forwards the results to the multiplexer, e.g., multiplexer  218 . The multiplexer sends these results back to the multiplexer at the COT, e.g., MUX  212 , over the message channel. The multiplexer forwards the message to the management unit at the COT, e.g., management unit  214 .  
         [0029]    To indicate that the test is complete, the management unit at the COT applies the appropriate resistive signature to the test bus at block  310 . This resistive signature is the result that the test equipment, e.g., test equipment  202 , is looking for. When the test equipment reads the results over the digital communication channel, the test equipment then removes the test request voltage and ends the test.  
         [0030]    Advantageously, this testing technique eliminates the bypass pair and removes the limitation on the distance between the COT and RT. This can give a significant increase in the customer service area for digital loop carriers.