Abstract:
Methods and apparatus for protecting quality of service during a process involving bridge tap in a communications system are described. Bridge tap may occur during system construction, central office re-concentration, replacement projects, upgrade projects, expansion projects, and installation of back-up cables, etc. Cables, with one end connected to a point in the system, and the other end left unterminated, result in bridge tap. In the bridge tap moderator of the present invention, a plurality of termination networks are attached to a modular connector such that a separate termination load is placed across each connector terminal pair for each corresponding wire pair of the cable. Communication lines can be easily attached to the moderator thereby easily, quickly, and efficiently eliminating bridge tap on a large number of wire pairs. The bridge tap modulator apparatus and method of use also minimize the amount of time bridge tap exists during cable and/or communication device installation. The use of bridge tap moderator results in improved overall system performance reflected in an improvement in attainable sync rate and increase in allowable loop length.

Description:
RELATED APPLICATIONS 
   The present application claims the benefit of U.S. Provisional Application Ser. No. 60/346,450 filed Jan. 7, 2002 now abandoned which is hereby expressly incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention is directed to communication networks, and more particularly, to methods and apparatus for protecting the quality of service during a process involving a bridge tap. 
   BACKGROUND OF THE INVENTION 
   Cable construction and central office re-concentration or replacement projects often require half-tapped or double-tapped placement of wire or cabling to facilitate the conversion or construction process. As part of the process of installing new cables and/or equipment, bridge taps often occur. A bridge tap is a length of wire or cable attached to normal endpoints of a circuit that introduces unwanted impedance imbalances that can interfere with data transmission. 
   In some instances, cables may be placed weeks or months in advance of the actual conversion or cable use. When pre-run, the cable ends are often laid in place, with one end of the new cable being connected to an existing cable and the other end left unterminated resulting in a bridge tap. 
   Bridge tap causes a wire to reflect signals from the unterminated end back to the source. Data signals, e.g., DSL signals, operate at high frequencies and can be severely impacted by the presence of bridge tap reflections. Circuits may operate at lowered speeds or data rates as a result of the bridge tap interference. Marginal circuits could exceed the operational limits of a design, thus entirely preventing the circuit from operating. 
   Accordingly, bridge taps can cause problems in operating systems. As the quality of service and attainable data rates degrade due to bridge taps, the supplier may be forced to move a customer to a lower service tier or deny services to a customer. The customer may be unsatisfied because the system no longer meets his needs. In addition, the overall system capacity loss experienced due to the bridge taps, the moving of a customer to a lower service tier, or the denial of services to a customer, may result in financial losses for both the communications service provider and the customer. 
   Given the negative effects of bridge tap, there exists a need for mitigating the effects of bridge tap. There is also a need for methods and apparatus for minimizing the amount of time bridge tap exists during cable and/or communications device installation which may occur during, e.g., system construction, central office re-concentration, replacement projects, upgrade projects, expansion projects, and installation of back-up cables/systems to provide reserve capacity or redundancy. At least some new methods of reducing the effects of bridge tap should be suitable for use with cables which include a large number of wires commonly used in many modern applications. 
   SUMMARY OF THE INVENTION 
   Methods and apparatus of the present invention can be used to mitigate the effects of bridge tap and the duration of bridge tap. 
   The present invention provides an apparatus, a bridge tap moderator, having a separate termination impedance for each one of, e.g., a plurality of wires to be terminated as part of a cable or communication installation. Pairs of wires forming a communication loop are terminated using a separate impedance network placed across each pair of wires used to form a communications loop. The impedance used to terminate each pair of wires can be matched to the type of circuitry to be protected and/or the length of wire ending in a bridge tap. To form the bridge tap moderator, a plurality of impedance circuits are mounted on a single connector, one for each utilized pair of connector terminals. The plurality of impedance circuits may each have the same impedance or, in some embodiments, circuits with different impedances are mounted on the same connector so that different pairs of wires are terminated with different impedances. 
   In various embodiments the connector is any one of a plurality of different standard connectors, e.g., standard connectors with multiple terminals to which wires of a cable can be attached or coupled, e.g., to a first side and to which termination networks can be attached to a second side. 
   The termination of each wire pair provided by the bridge tap moderator of the invention works to absorb and reduce the reflection of signals back into each of the wire loops connected to the moderator, resulting in improved quality of service for the cable and/or lines to which the moderator is connected. 
   In one embodiment, the multi-terminal connector is part of a punchdown block which has mounted with it a separate impedance network across each pair of connector terminals. Such an arrangement provides for quick wire connections/disconnections and bridge tap moderator portability. The ability to quickly connect and disconnect wires helps minimize the time period where bridge tap is introduced in the system during testing, connection and disconnection operations. 
   The method of the present invention for utilizing the bridge tap moderator provides an economical and efficient means to mitigate the effects of bridge tap even in cases where cables include large numbers of wire pairs to be terminated. 
   Additional features, benefits and details of the methods and apparatus of the present invention are described in the detailed description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary communications system in accordance with the present invention. 
       FIG. 2  illustrates a bridge tap moderator implemented using a punch down block. 
       FIG. 3  shows a blown up rear view of an exemplary bridge tap moderator in accordance with the present invention with a single terminator network shown for purposes of explaining the invention. 
       FIG. 4  illustrates a schematic view of an exemplary terminator network used on the exemplary bridge tap moderator in accordance with the present invention. 
       FIG. 5  illustrates an exemplary method for utilizing the bridge tap moderator in accordance with the present invention. 
       FIGS. 6–8  illustrate various test results relating to the use of an exemplary bridge tap moderator of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary communications system  100  in accordance with the present invention. The exemplary system  100  includes a first location  110 , for example, a customer premise, a second location  120 , for example, a telephone central office, and a tap point  130 . The tap point  130  represents a point where an original cable, comprising cable portions A  140  and A′  141 , old cross connect frame A  123  and central office cable A  122  has been tapped to connect to a new cable B  150  which, ultimately, will terminate at the communications device A  121 . As a result of the tap at tap point  130 , the original cable has been divided into two sections A  140  and A′  141 . Cable A  140  includes multiple pairs of wires, e.g., dozens in some cases. Individual conductors, wires, in cable A  140  are coupled at tap point  130  to corresponding individual wires in cable A′  141  and cable B  150 . 
   The first location  110  includes a communication device  111 , such as, for example, a Digital Subscriber Line (DSL) modem. The second location  120  includes an communication device A  121 , for example, a Digital Subscriber Line Access Multiplexer (DSLAM), an original central office cable A  122 , and an original cross connect frame A  123 , which provides a connection point for cables A  122  and A  141 . The second location  120  also includes a bridge tap moderator A  124  implemented in accordance with the invention, a new central office cable A  125 , and a new cross connect frame A  126  which provides a connection point for cables A  125  and A  150 . Thus in the exemplary system original cable segment  140  is coupled to communication device A  121  by Tap Point  130 , cable segment  141 , original frame  123 , and cable segment  122 . The bridge tap moderator  124  is also coupled to original segment  140 , and thus also cable  141 , by Tap Point  130 , new cable  150 , new cross connect frame  126  and cable  125 . 
   The first communication device  111  is coupled to tap point  130  by the first segment A  140  of the original cable. The second segment A′  141  of the original cable couples communication device  121  to tap point  130  through cable A  122  and cross connect point A  123  and thus to the first communication device  111  which is also coupled thereto. The bridge tap moderator  124  is used in accordance with the invention to terminate new cable B  150  through cross connect B  126  and central office cable B  125  until it is connected to communications device  121 . The bridge tap moderator  124  may be coupled to the new central office cable B  125  by a punch down block to which the wires of new cable B  125  are connected at the second location. 
   The DSLAM  121  is rate adaptive, meaning it will attempt to sync with the modem  111  at the maximum speed allowed. The maximum allowable speed is determined by, e.g., line conditions as determined by signal to noise measurements subject to various constraints. Such constraints include, for example, a customer selected subscription rate, service tier, and other factors. Line conditions include such things as distance between the DSLAM and modem, line noise, signal reflections on a line, etc. A significant factor in lowering service rate is near end bridge tap, i.e., a bridge tap near, e.g., at or within 300 feet of, the DSLAM or modem. The effect of a short bridge tap near the DSLAM tends to be highly detrimental to a DSL signal. 
   In accordance with the present invention, the bridge tap moderator  124  is used to terminate the bridge tap with an appropriate load. In various embodiments the bridge tap moderator  124  is implemented as a passive device that electrically terminates open-ended wires. Each bridge tap moderator  124  includes circuitry, e.g., resistors, capacitors and/or inductors, for moderating the effects of bridge tap. 
   The bridge tap moderator  124  may be a block mountable device such as a punch down block incorporating terminating networks or a plate mounted connector to which terminating networks are connected.  FIG. 2  illustrates a bridge tap moderator  124  implemented using a punch down block  202  which is visible from the front perspective of  FIG. 2 . The block  202  provides a standard twenty-five pair Category 3 cable interface. In one exemplary embodiment of the invention, the bridge tap moderator terminating networks,  25  in the  FIG. 2  embodiment, are mounted within the interior of the punch-down block  202 . 
     FIG. 3  shows a blown up rear view of the bridge tap moderator  124 . The exemplary bridge tap moderator  124  includes a connector assembly  201  which is part of the punch down block  202 .  FIG. 3  is intended to be descriptive in nature with the actual rear view of block  202  appearing somewhat different but with 25 pairs of terminals  208  being present. The moderator  124  includes plurality of incorporated wire terminal pairs  208 , and a terminator network  300  for each wire terminal pair  208 . For purposes of illustration,  FIG. 3  shows a single terminator network  300  but it is to be understood that the bridge tap moderator normally includes one such network for each of the 25 wire pairs  208 . Thus, a single bridge tap moderator  124  can be used to service 25 pairs of wires, i.e., 25 bridge taps. In various other embodiments, the type of connector  201  and number of wire terminal pairs  208  per connector  201  on the bridge tap moderator  124  of the present invention vary according to the particular application and/or number of tapped wire pairs. 
   The terminator network  300  on each wire terminal pair  208  may vary according to the requirements of the system, including the type of circuit being protected and the length of wire subjected to bridge tap. 
   In one embodiment of the present invention, the terminator networks  300  may be soldered to the wire terminal pairs  208  interface on the connector  201  of the bridge tap moderator  124 . 
     FIG. 4  shows a schematic diagram of an exemplary embodiment of a terminator network  300  suitable for use in the bridge tap moderator  124 . Each terminator network  300  includes a first terminal (T 1 )  301 , a second terminal (T 2 )  302 , one or more capacitors C 1   303 , C 2   304 , C 3   305 , C 4   306 , a resistor R 1   307 , and an inductor L 1   308 . When placed across a connector terminal pair  208 , the first terminal T 1   301  will couple the network  300  to one side of a wire pair  208  while the second terminal T 2   302  will couple the network  300  to the other side of the wire pair resulting in a load across the previously unterminated wire pair  208 . The R, L and C components of the network  300  form a series loop between the first and second terminals, T 1   301  and T 2   302 , as shown in  FIG. 4 . In the exemplary embodiment, the series loop comprises a plurality of capacitors  303 ,  304 ,  305 ,  306  arranged in series with the resistor R 1   307  and indictor L 1   308  which are arranged in parallel. It is to be understood that while a plurality of capacitors are shown, a single equivalent capacitor could be used in their place. Similarly, multiple resistors and/or inductors could be used in place of the individual resistor R 1   307  and individual inductor L 1   308 , respectively. Network  300  serves as a termination at the end of a wire loop, for example one pair of wires in new (replacement) cable B  125  which is coupled at its other end to a corresponding pair of wires in the original cable at tap point  130 . 
   In one exemplary embodiment of the present invention, the following values were chosen for the elements of the terminator network  300 : C 1   303 =0.1 micro-farad, C 2   304 =0.1 micro-farad, C 3   305 =0.12 micro-farad, C 4   306 =0.12 micro-farad, R 1   307 =100 ohm, and L 1   308 =0.47 milli Henries. 
     FIG. 5  illustrates an exemplary method for utilizing the bridge tap moderator  124  in accordance with the present invention. The process starts at START node  701  with the first operation occurring in step  703 . In step  703  a determination is made as to the type of service to be protected, for example, DSL, the type of circuitry equipment to be protected, and/or the length of the bridge tap to be terminated. In the next step, step  705 , an appropriate bridge tap moderator  124  having termination networks  300  suitable for the particular application are selected, e.g., based on the information determined in step  703 . The bridge tap moderator  124  may be custom built for the specific application. Alternatively, in common situations such as bridge taps associated with DSL services, a bridge tap moderator  124  may be selected as a best fit from a set of pre-built re-usable bridge tap moderators  124  based on the characteristics determined in step  703 . The pre-built bridge tap moderators  124  may be designed to correspond to different bridge tap lengths and connector types providing a wide range of different moderators  124  from which a selection may be made for various applications. Thus, in some cases, step  705  involves selecting a bridge tap moderator  124  having a plurality of preinstalled networks  300  from a variety of moderators at least some of which have different network component values and thus different termination loads. 
   Proceeding to step  707 , the selected bridge tap moderator  124  is mounted on a rack near the communication device  121  to receive the new (replacement) cable B  125 . Next, in step  709 , a new (replacement) cable B  125  is run to the new cross connect frame B  126 . In the following step, step  711 , the new (replacement) cable B  125  is terminated on the bridge tap moderator  124 . 
   After attachment of cable B  125  to the bridge tap moderator  124 , and any optional testing, in step  713 , cross connections are made to new cross connect frame B  126  and cable B  150  thereby coupling cable  150  to the bridge tap moderator  124 . 
   When the system is ready to begin conversion to the new equipment/cable, the method resumes and proceeds with step  715 . In step  715 , the new (replacement) cable B  125  is removed from the bridge tap moderator  124 . At this point, unterminated bridge tap is introduced into the system. Following removal of the cable from the moderator  124 , operation rapidly proceeds to step  717  where the old cable A  122  is removed from the communication device  121 . In step  719 , the new cable B  125  is connected to the communications device  121 . The process should proceed as rapidly as possible to step  721 . In step  721 , the original cable A′  122  is connected to the bridge tap moderator  124 . This involves inserting the wire of original cable A′  122  into the connectors of bridge tap moderator  124  to which the new (replacement) cable B  125  wires had previously been connected. This eliminates the unterminated bridge tap in the system. In the following step  723 , a decision is made as to whether all cables involved in the system conversion process have been transferred. If another cable is required to be transferred, flow is directed back to step  715 . If all the cables in the conversion process have been transferred, flow proceeds forward to step  725 , where the half tap or double tap is removed at the tap point  130 . Now, the original (old) cables A′  122  no longer create a bridge tap condition. Next, in step  727 , the original (old) cables A′  122  are removed from the bridge tap moderator  124 . 
   Finally, in step  729 , the bridge tap moderator  124  is removed form the punch down block  202  to which cable segment A′  122  was connected. The process stops with step  731  at which point the removed bridge tap moderator may be stored for use on another project. 
   Computer testing was done to predict the effect of the use of the bridge tap moderator  124  of the present invention in a simulated DSL system where a DSL modem was connected to a DSLAM by a cable which was to be tapped. For simulation purposes, a loop distance factor based on 100 foot increments was used to predict the effect for loop lengths between 6,500 feet and 15,000 feet in the case of a bridge tap from a 300 foot length of cable with the tap point located near the DSLAM. 
   For purposes of the test, the DSL subscription rate was set to 7168 Kbps. In ideal conditions the DSL modem would always sync with the DSLAM at 7168 Kbps. However, as discussed above other factors including loop distance and interference due to near end bridge can interfere with signals making it difficult or impossible to obtain this maximum rate. The graphs illustrated in  FIGS. 6–8  were obtained through simulated tests and depict the difference in sync rate that may be obtained with a 300 foot bridge tap with and without use of the bridge tap moderator of the present invention. 
     FIG. 6  is a graph displaying sync rate with and without the bridge tap moderator  124  present.  FIG. 7  illustrates the average reach gain in the simulated system achieved using the bridge tap moderator  124  while  FIG. 8  illustrates the average gain in terms of rate for the same simulated system. 
   From the  FIG. 6  through  FIG. 8  charts, it can clearly be seen that use of the bridge tap moderator  124  can be beneficial to DSL system performance as compared to the case where bridge tap is left unmoderated. 
   It is to be understood that numerous variations on the above described methods and apparatus are possible without departing from the scope of the invention.