Abstract:
Splitting circuitry receives external telecommunications lines and splits them into corresponding primary and secondary lines for implementing an N:l sparing arrangement in an associated telecommunications system. Sparing circuitry receives the secondary lines and includes selection circuitry and a sparing bus, where the selection circuitry selectively connects a desired group of secondary lines to the sparing bus in response to a selection control signal. Preferably, the telecommunications system routes communications traffic between subscriber equipment connected via the external telecommunications lines and a larger communications network using N primary line interfaces coupled with the primary lines. When desired, the telecommunications system controls the sparing circuitry such that secondary lines corresponding to a group of primary lines associated with a given primary line interface connect to the spare line interface. Thus, communications traffic that would otherwise be routed through the given primary line interface is routed through the spare line interface. Because sparing functions associated with the secondary signals do not interfere with primary signals, maintenance and service operations may be performed on elements of the sparing circuitry without interrupting data traffic being carried on the primary lines. This is particularly advantageous for telecommunications systems having primary line interfaces that support a large number of telecommunication lines, where removing even a single primary line interface from service for sparing equipment maintenance is unacceptable.

Description:
FIELD OF THE INVENTION 
     The present invention relates to using backup or spare communications circuitry in a communications system as needed, and particularly relates to using separate communications signal paths in circuit sparing operations. 
     BACKGROUND OF THE INVENTION 
     Modern telecommunication systems are extraordinarily complicated, and yet they are oftentimes taken for granted by the consumers who use them. This tendency for people to under appreciate their underlying complexity is perhaps the greatest testament to the reliability of modern telecommunication systems. Achieving this level of reliability in a complex network of interconnected equipment requires substantial effort, ingenuity, and a strong belief in contingency planning. 
     Failure at some point is inevitable for all electronic circuits. One may assume that any electronic circuit is subject to failure at some time. This basic assumption finds its fullest refinement in the art of reliability engineering, where complex prediction techniques allow engineers to estimate values for parameters such as mean-time-to-failure (MTTF) for various circuits and systems. While the use of reliable components and careful engineering results in telecommunication circuits with low rates of failure, the possibility of failure is inescapable. Some telecommunication circuits are simply too critical to leave this possibility of failure unmitigated. In such cases, it is common to provide spare or backup communication circuits that are used to substitute for primary circuits as needed. 
     In 1:1 sparing schemes, each critical primary circuit has a corresponding spare circuit. However because of constraints on equipment size and cost, 1:1 sparing is reserved for the most critical circuits. As an alternative, a piece of telecommunications equipment may include a single spare circuit that may be substituted for any one of a number of primary communication circuits. The sparing of this type is referred to as N:1 sparing. A common problem with existing sparing techniques is that the communication signal paths running through the primary circuits are also routed through the sparing circuits. For example, switches or their electronic equivalents are commonly used to connect communication lines to either primary communications circuits or to a spare communications circuit. While this does allow switching between primary and backup communications circuits as needed, it makes the primary communications signal paths directly dependent upon the switches themselves. Thus, switch failures or switch maintenance activities undesirably disrupt communication signals from the primary communications circuits. 
     Conventional sparing techniques are particularly problematic in telecommunications line interface equipment. In a high-density system, the telecommunications line interface equipment uses a number of line cards to interface a large number of telecommunication lines with associated transmission or network equipment. Each line card carries interface circuits for a substantial number of telecommunication lines. Because of the large number of telecommunication lines serviced by any one line card, removing even a single line card from service would result in a loss of service to a greater number of customers than controlling telecommunications standards allow. With conventional sparing techniques applied to an N:1 arrangement, each of the telecommunication lines is routed through some type of switching circuitry that selectively switches the telecommunication lines from any primary line card to a spare line card. While this solves the problem of providing a spare line card when needed, it also means that any problems or maintenance associated with the switching circuitry can remove at least as many telecommunication lines from service as would the failure of one of the primary line cards. 
     Accordingly, there remains a need for a telecommunications line interface architecture that includes sparing circuitry that may be used as a backup to one or more primary communication circuits, without failures or maintenance of the sparing circuitry compromising the availability of the primary communication circuits. 
     SUMMARY OF THE INVENTION 
     The present invention splits telecommunication lines, such as DS1 subscriber lines, into primary and secondary signal paths within a piece of telecommunications line interface equipment. The primary signal paths connect the telecommunication lines to primary interface circuits, while the secondary signal paths are used to selectively connect desired ones of the telecommunication lines to spare interface circuits as needed. Preferably, the present invention includes a switching network that operates only on the secondary signal paths to connect selected ones of the telecommunication lines to the spare interface circuits. As such, the telecommunication lines remain connected to the primary line interfaces through the primary signal paths, which are independent of the switched secondary signal paths used with the spare line interface circuits. This allows maintenance to be performed on the switching network or on the spare line interface circuits without disrupting services on any of the telecommunication lines supported by the primary interface circuits. 
     Preferably, the present invention includes circuitry for splitting incoming telecommunication lines into corresponding primary and secondary lines, and operating on the secondary lines to implement sparing functions. The primary lines couple directly to the primary line interfaces through the primary signal paths and are not routed through the sparing circuitry operating on the secondary lines. The sparing circuitry may include sparing switches (switching network) and associated control circuitry for selectively connecting desired ones of the secondary signal paths to a spare line interface. Preferably, the sparing circuitry is configured as a backplane, sparing switch cards, and a sparing switch control card. The backplane connects a group of secondary signals to each sparing switch card, and each sparing switch card includes switched connections for selectively connecting its associated group of secondary signals to a sparing bus carried on the backplane. A sparing bus interface on the backplane allows connection of the sparing bus with the spare line interface. When a given primary line interface fails or requires maintenance, the telecommunications line interface equipment uses the sparing circuitry of the present invention to connect the group of secondary lines corresponding to the primary lines associated with the given primary line interface to the spare line interface by controlling the appropriate switches on the sparing switch cards. 
     Typical telecommunications line interface equipment organizes line interface circuitry into one or more line cards. In high-density systems, a single line card may interface with a substantial number of telecommunications lines. Telecommunication standards dictate a maximum number of such lines that may be out of service at any one time due to maintenance activities or circuit failure. Thus, even the loss of one such line card may represent an unacceptable loss of service. The present invention may be advantageously used in such a system to provide sparing functions for any one of the primary line cards using a single backup line card. Unlike conventional approaches to sparing critical telecommunication circuits, the present invention does not use a common signal path for both sparing functions and primary communications. With a common signal path, failures or maintenance of sparing-related circuits disrupts primary communications, which is unacceptable in the high-density systems described above. The split secondary and primary signal paths provided by the present invention allow sparing functions to use secondary signal paths that do not interfere with primary communications. Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of exemplary embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an existing approach to providing spare circuitry in a telecommunications line interface environment. 
     FIG. 2 further illustrates problems associated with the existing sparing approach of FIG.  1 . 
     FIG. 3 illustrates an exemplary embodiment for the sparing technique of the present invention. 
     FIG. 4 illustrates exemplary telecommunication line splitting details for the present invention. 
     FIG. 5 illustrates an exemplary passive network for performing the line splitting used by the sparing technique of the present invention. 
     FIG. 6 illustrates an exemplary architecture for the sparing functions of the present invention as applied to a type of telecommunications line interface equipment. 
     FIG. 7 illustrates exemplary card architecture for implementing the sparing circuitry of the present invention. 
     FIG. 8 illustrates an exemplary embodiment for a telecommunications line interface card adapted for the sparing technique of the present invention. 
     FIG. 9 illustrates an exemplary embodiment for a sparing switch card adapted for the sparing technique of the present invention. 
     FIG. 10 illustrates an exemplary embodiment for an I/O card adapted for the sparing technique of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a conventional approach to providing spare circuitry in a typical telecommunications environment  2 . Telecommunications line interface equipment  10  includes a switch  16  (or equivalent electronics), a switched primary signal path selectively connected to a primary line card  14 A, a switched secondary signal path selectively connected to a spare line card  14 B, and a controller  12 . In operation, the equipment  10  connects the telecommunications line (subscriber line) with either the primary line card  14 A or the spare line card  14 B, as desired or needed. Using either the primary or spare line cards  14 , the equipment  10  transfers communications signals between subscriber equipment connected to a remote end of the telecommunication line and a network  4 , which may, for example, be transmission equipment. As illustrated, the equipment  10  switches a single subscriber line between the primary line card  14 A and the spare line card  14 B, but it should be understood that many subscriber lines might be switched in similar fashion between a plurality of primary line cards  14 A and the spare line card  14 B. 
     In normal operation, the equipment  10  routes communications traffic to and from the subscriber line through switch  16  using the primary line card  14 A. Upon detecting a failure (or for maintenance of the primary line card), the controller  12  causes switch  16  to connect the subscriber line to the spare line card  14 B. FIG. 2 depicts switch  16  removed and illustrates a fundamental problem with this conventional approach to providing spare communications circuitry. Assume that instead of a problem with the primary line card  14 A, there is instead a failure of the switch  16  (or associated switch  16  electronics). As shown, when switch  16  is faulty or otherwise removed from service, both the primary and secondary signal paths are disrupted, leaving the controller  12  unable to route communications signals through either the primary line card  14 A or the spare line card  14 B. 
     In a more complex illustration, the switch  16  would be shown as a group of switches or equivalent electronic circuitry adapted to switch a large number of telecommunication lines into corresponding primary line cards  14 A, or switch a selected group of these telecommunication lines into the spare line card  14 B. Because the switch  16  really serves no other purpose than to enable the use of the spare line card  14 B when desired, it is undesirable that failures or maintenance activities associated with switch  16  should disrupt normal communications using the primary line card  14 A. 
     Telecommunications standards, such as GR-512-Core, LSSGR Reliability, Section 12 (Issue 2, January 1998), set strict limits on permissible service outages. For example, a reportable Partial Outage is defined as an event including a loss of origination or termination capability in more than 64 terminations (e.g., telecommunication line interfaces) for fewer than all terminations on the switch (line interface) for greater than 30 seconds, or the loss of access to one or more services (e.g., 911, voicemail services, operator services) because of failure conditions in the line interface that affect more than 64 terminations for greater than 30 seconds. The requirement for Multitermination Downtime is set at 1.2 minutes/year or less, where Multitermination Downtime is the expected long-term average annual time spent in failure modes (due to hardware failures, operations, administration, and maintenance activities). This is a generic requirement for telecommunications switching in transmission equipment that dictates maximum hardware failures and interruptions. In light of these requirements, it is impractical to apply conventional sparing techniques in high-density telecommunication line interface equipment, because of the number of telecommunication circuits that are taken out of service due to failures or maintenance of the conventional sparing circuitry. 
     FIG. 3 illustrates an exemplary embodiment of the present invention included in telecommunications line interface equipment  100 . As illustrated, the equipment  100  includes communications and control circuitry  120  for handling communications signal routing and system-level operations, primary lines cards  140 A, and a spare line card  140 B. The equipment  100  further includes splitting circuitry  190 , switches  160 , selection control circuitry  180 , and a sparing bus  106 . The splitting circuitry  190  splits the connected telecommunication lines into corresponding primary lines  102  and secondary lines  104 . 
     Note that the primary lines  102  couple to primary line cards  140 A without passing through the sparing switches  160 . This allows communications traffic to pass through the primary signal lines  102  independent of whether circuitry associated with sparing operations and secondary lines  104  is installed or functional. The sparing switches  160 , which operate under control of selection control circuitry  180 , selectively connect desired ones of the secondary lines  104  with a sparing bus  106 . In turn, the sparing bus connects with a spare line interface card  140 B. 
     In one embodiment, the switches  160  (including associated switch drive circuitry not illustrated for simplicity), the sparing bus  106 , and the selection control circuitry  180  cooperate to form sparing circuitry. Such sparing circuitry allows the communications and control circuitry  120  to route the communications traffic normally handled by any one of the primary line cards  140 A through the spare line card  140 B as needed. 
     In more detail, the splitter circuitry  190  splits a number of external telecommunication lines into corresponding numbers of primary lines  102  and secondary lines  104 . The primary lines  102  couple to a plurality of primary line cards  140 A, with each primary line card  14 A 0  receiving a different group of primary lines  102  corresponding to a number of the external telecommunication lines. The secondary lines  104  are coupled to selection circuitry, shown here in simplified form as a collection of switches  160 . 
     Primary lines  102  are preferably organized into equal numbered groups (P 1 , P 2 , . . . , P L ), with each group being assigned to a particular primary line card  140 A. Similarly, the secondary lines  104  split off from the primary lines  102  are organized into corresponding, like-numbered groups (P 1 , P 2 , . . . , P L ), with each group being connected to a group of switches  160 . With this arrangement, the switches  160  are controlled by selection control circuitry  180  to connect a desired group (P x ) of secondary lines  104  to the sparing bus  106 . In turn, the sparing bus  106  connects with the spare line card  140 B. 
     In operation, the communications controller  120  may cooperate with the selection control circuitry  180  to connect a group of secondary lines  104  to the spare line card  140 B through the sparing bus  106 , when a given primary line interface card  140 A fails. This action provides an alternate connection between the telecommunication lines affected by the failed primary line card  140 A and the network  4 . Notably, all sparing functions operate on the secondary signal lines  104 , allowing the switches  160 , the spare line card  140 B, and the selection control circuitry  180  to fail or be taken offline for service, without interfering with the telecommunication signals being transferred to and from the telecommunication lines over the primary lines  102  via the primary line cards  140 A. 
     FIG. 4 illustrates an exemplary arrangement of the splitter circuitry  190  for use with telecommunication lines comprising typical wire line subscriber loops, such as North American DS1 or European E1 digital subscriber lines. Commonly, each telecommunication line comprises a receive loop and a transmit loop, with each loop comprising a differential conductor pair. These conductor pairs are often implemented as unshielded twisted pairs (UTP). Thus, for any one telecommunications line, the splitter circuitry  190  comprises an individual splitter circuit  190  for each conductor used in the receive and transmit loops (RX and TX). 
     FIG. 4 illustrates the interface between the transmit conductors for a number M telecommunication lines configured as UTP wire lines. Each conductor in each transmit loop is split into a primary and secondary signal by a splitter circuit  190 . While not shown, the associated receive loops would be similarly split into primary and secondary signals. With this configuration, primary line differential RX and TX pairs are coupled to primary line cards  140 A and secondary line differential RX and TX pairs are coupled to the switches  160 . 
     Signal characteristics vary widely depending upon the particular type of telecommunication line in use. The present invention may be adapted to interface with a wide range of telecommunication or network line types. When applied to DS1 or E1 lines, the splitter circuits  190  may be advantageously implemented as passive splitting networks. 
     FIG. 5 illustrates an exemplary embodiment of a resistor-based passive splitting network. Note that when referred to as splitter circuitry  190  herein, such circuitry will include splitter circuits for each telecommunication line (or individual conductors comprising such lines.) The splitter network  190  includes a common terminal  192  that is interconnected with a primary terminal  194  and a secondary terminal  196 . Circuit arrangements used to interconnect these terminals may be varied as needed for a particular type of telecommunication line. 
     Those skilled in the art will readily appreciate that the arrangement shown in FIG. 5 represents an advantageous embodiment for splitting certain types of telecommunication lines. Other types of communication lines may favor different splitting circuitry arrangements, which may advantageously incorporate other passive circuit elements, such as capacitors and inductors. Further variations of the splitter circuits  190  may incorporate active elements, such as active buffers or amplifiers to provide signal isolation between primary and secondary lines, or to alleviate loading problems and impedance matching issues associated with the telecommunication lines. 
     The splitter circuit  190  illustrated in FIG. 5 provides an interface impedance that is designed to be proportional to the expected or nominal line impedance of the telecommunication line circuit. Most DS1 telecommunication lines are configured as balanced conductor pairs ranging from 100 to 130 Ohms. Short, intra-building DS1 runs within a telecommunications system Central Office may use unbalanced, 75-Ohm coax cable. The present invention is readily adaptable to these and other variations. A preferred method of matching the passive splitter network illustrated in the splitter circuit  190  of FIG. 5 for a balanced, symmetrical telecommunication line impedance in an N-way splitting arrangement is given as follows,        R   =     z     (     2        (     N   +   1     )       )                              
     Where R=the value of all resistors R 1 , R 2 , and R 3 . Note that a balanced conductor pair (e.g., UTP) will use two such splitter circuits  190 , one for each conductor. Thus, the above equation computes R for the six resistors comprising these two splitter circuits  190 . Further, z=the telecommunication line design impedance, N=the desired number of splits, which is “2” for a primary and secondary line split. Thus, for a telecommunication line impedance of 100 Ohms and a two-way split,          R   =       100     (     2        (     2   +   1     )       )       =       100   6     =     16        2   3        Ω           ,                          
     In manufacturing, the closest available standard resistor value is chosen. For the above configuration, a value of 17.8 Ohms may be chosen. This value represents only a 6.8% error from the calculated resistance value and provides for approximately 30 dB of return loss on the telecommunication line. This represents approximately a 10 dB margin above a desired minimum 20 dB return loss value. 
     FIG. 6 presents an exemplary arrangement for the telecommunications line interface equipment  100  introduced in FIG.  3 . Preferably, the equipment  100  is arranged as a rack system, with its contained circuitry arranged on a series of cards and backplanes organized by associated functionality. With this in mind, the controller  120  may comprise a plurality of associated communications controllers arranged on a number of circuit cards that, collectively, control the communications signal routing between the network  4  and the connected telecommunication lines. A number of I/O module cards  250  provide physical interfaces for the plurality of telecommunication lines connected to the telecommunication line interface equipment  100 . In an exemplary rack system arrangement for the telecommunication line equipment  100 , each I/O card  250  connects with up to 32 external telecommunication lines comprising RX and TX loop pairs. Thus, each I/O card  250  includes a corresponding number of splitter circuits  190  adapted to split the receive and transmit pairs (RX/TX) of each telecommunication line into corresponding primary and secondary TX and RX line pairs. Exemplary primary line cards  140 A are each adapted to receive primary RX and TX lines split from 16 of the external telecommunication lines. Thus, in this exemplary arrangement, there are two primary line cards  140 A for each I/O card  250 . A backplane  200  is adapted to receive secondary RX and TX line pairs split from the external telecommunication lines connected to all of the I/O cards  250 . A number of sparing switch cards  230 , also referred to as Protection Switch Relay (PSR) cards, are connected to the backplane  200 . Preferably, each sparing switch card  230  includes 16 secondary TX line pair switches  160  and 16 secondary RX line pair switches  160 , along with associated switch control and interface circuitry. In this arrangement, there is a one-to-one correspondence between the sparing switch cards  230  and the primary line cards  140 A. That is, each sparing switch card  230  receives a group of secondary TX and RX lines corresponding to a group of primary TX and RX lines attached to one of the primary line cards  140 A. The backplane  200  also carries a switch selection control card  180  that is adapted to control the sparing switch cards  230 , such that selected groups of secondary TX and RX lines may be connected to the sparing bus  106 . 
     FIG. 7 illustrates a basic complement of circuit cards in this exemplary architecture. A basic card arrangement includes one I/O card  250 , two primary line cards  140 A, a spare line card  140 B, a sparing backplane  200 , two sparing switch cards  230 , one switch selection control card  180 , and a number of controller cards comprising the communications controller  120 . Each I/O card  250  interfaces with up to  32  telecommunications lines and includes splitter circuitry  190  splitting these lines into corresponding primary and secondary lines  102  and  104 , respectively. Each I/O card  250  provides  16  primary RX and TX lines  102  to each of the two primary line cards  140 A and provides  16  corresponding secondary RX and TX lines  104  to the backplane  200 . The backplane  200  routes these secondary lines  104  to two sparing switch cards  230  in groups corresponding to the manner in which the primary lines  102  are grouped for connection to the primary line cards  140 A. 
     The switch selection control card  180  provides selection control signals to the two sparing switch cards  230 . In an exemplary architecture, the telecommunications line interface equipment  100  accommodates up to four basic card sets, and thus holds four I/O cards  250 , eight primary line cards  140 A, eight sparing switch cards  230 , one switch selection control card  180 , and one spare line card  140 B. In this configuration, the telecommunications line interface equipment  100  interfaces up to 128 telecommunication lines with the network  4 . 
     This exemplary architecture contemplates a rack system comprising the telecommunications line interface equipment  100  in which the sparing circuitry (e.g., backplane  200 , and associated cards and circuitry) is organized on a “sparing shelf” that may be optionally included in the rack system, such that the telecommunications line interface equipment  100  can be configured with or without sparing capability. With this arrangement, the telecommunication lines are coupled to the primary line cards  140 A via the primary lines  102  and split off using the splitting circuitry  190 . These primary lines  102  are routed independently of the secondary lines  104 . Thus, the sparing circuitry  190  may be installed or not installed, without interfering with communication operations through the primary line cards  140 A. 
     FIG. 8 illustrates an exemplary line card  140 , which may be used as either a primary or spare line card,  140 A and  140 B respectively. Note that other configurations of line card  140  may be implemented as desired. For example, there may be economic advantage in developing a line card  140 A for use in primary communications that omits the interface  146  used to communicate with the switch selection control card  180 . When used as a spare line card  140 B, the line card  140 B communicates with the switch selection card  180  via the switch control interface  146  under control of the communications controller  120 . This communications link allows the spare line card  140 B to direct the switch selection card  180  to control one of the sparing switch cards  230  to connect a desired group of secondary lines  104  through the sparing bus  106 . 
     The line card  140 A is adapted to receive a number of primary lines  102  from the splitter circuitry  190  of the I/O cards  250 . In exemplary arrangements, each line card  140 A receives primary lines  102  for 16 telecommunication lines, which may be implemented as 16 TX and 16 RX differential line pairs. Using line interface circuitry  142 , the line interface card  140 A transfers telecommunication signals between the communications controller  120  and the associated telecommunication lines via the primary line connections. Depending upon the logical and electrical characteristics of the signaling scheme implemented for the external telecommunication lines, the line interface  142  may provide differential-to single-ended signal conversion, level shifting, and buffering functions. 
     The communications and control interface  144  may communicate directly with the communications controller  120 , or may communicate in a multiplexed fashion using the signaling or connections between the line interface  142  and the communications controller  120 . The communications controller  120  monitors communications information or communications signal characteristics for the line interface card  140 A in order to detect possible failure states of the line card  140 A. 
     When the communications controller detects a failure state in one of the primary line cards  140 A, it communicates with the spare line card  140 B. In response, the spare line card  140 B uses its switch control interface  146  to communicate with the selection control card  180 . In turn, the selection control card  180  causes the desired group of switches  160  on one of the sparing switch cards  230  to connect their associated secondary lines  104  with the sparing bus  106 . This scheme may be easily adapted to include alternate command and control interconnections. In some embodiments, the selection control card  180  may receive communications directly from the communications controller  120 . 
     FIG. 9 illustrates an exemplary sparing switch card  230 . The sparing switch card  230  preferably includes physical connections adapted to receive a group of secondary TX and RX lines  104  from the backplane  200 , a group of switches  160  selectively connecting secondary lines  104  with the sparing bus  106 , and switch control and interface circuitry  232 . Preferably, the switch control and interface circuitry  232  includes a number of driver circuits and monitoring circuits (not shown for clarity). The structure and design of such circuits will depend upon the type of switches used, as will be readily appreciated by those skilled in the art. 
     Therefore, the switch control and interface circuitry  232  may be used to selectively operate associated switches  160  under control of the selection control card  180 . Further, the switch control and interface circuitry  232  may be used to monitor the operation or state of individual switches  160  (i.e., open or closed). Optionally, the switch control and interface circuitry may provide a “presence” indicating output, which may simply be a discrete signal line (e.g., a resistive pull-down line). Such a signal line can provide a cost-effective way for the selection control card  180  to determine the number and location of sparing switch cards  230  installed in the backplane  200 . Preferably, the sparing switch cards  230  are physically adapted to plug into the backplane  200  in a removable fashion to aid maintenance and service operations. 
     FIG. 10 illustrates an exemplary I/O card  250 , which includes physical interfaces  252  and  254 , each adapted to connect with the external telecommunication lines. Preferably, the physical interfaces  252  and  254  provide different connection options and may, for example, comprise a multi-conductor male or female connector, or an array of wiring terminals. Splitter circuits  190  connect with the physical interface  252  and  254  to split each telecommunications line into a primary line  102  and a secondary line  104 . As explained above, the splitter circuitry  190  may comprise individual splitter circuits  190  adapted to split the individual conductors in each telecommunications line into a primary and secondary line. This splitting establishes primary and secondary signal paths for the signals carried on each telecommunications line conductor. Primary signal paths are coupled to the primary signal lines  102  to establish the connection between the primary lines  102  and the primary line cards  140 A. Secondary signal paths are coupled to the secondary signal lines  104  for connection with the sparing switch cards  230  through the backplane  200 . 
     The present invention allows substantial variation in implementation and circuit organization. For example, circuitry supporting the functionality of the present invention may or may not be organized into individual circuit cards and backplanes. The structure and implementation in the foregoing descriptions are exemplary rather than limiting. Those skilled in the art will readily appreciate that the present invention can be adapted to a broad range of circuit arrangements. 
     Further, the splitter circuitry  190  may be adapted to the characteristics of specific types of telecommunication lines. While the foregoing description describes an advantageous resistor-based passive network for individual splitter circuits  190  for use with digital subscriber lines, such as DS1 or E1 lines, the splitter circuitry  190  may be implemented in active or passive configurations depending upon the communication signals involved. Additional variation may be found with the configuration of the supporting telecommunications line interface equipment  100 , both in its physical configuration and in the number of telecommunication lines supported. 
     These variations, along with other variations understood by those skilled in the art, are all considered to be within the scope of the present invention. Exemplary embodiments discussed above should not be considered as limiting the scope of the present invention. Indeed, the present invention is limited only by the scope of the claims included herein, and the range of reasonable equivalents thereof.