Abstract:
A system and method for enhancing reliability in a communication system. In the system and method, redundant signal paths are continuously tested to determine if any faults are present while the operating signal paths are in use. If any faults are present in the redundant signal paths, they can be addressed while the operating circuits are still in use. This minimizes the chance that a switch from an operating signal path to a redundant signal path because of a fault in the operating circuit will result in downtime.

Description:
This Application claims the benefit of application Ser. No. 60/136,445 filed May 28, 1999 for SPARE LINE SWITCHING APPARATUS AND METHOD. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention Field of the Invention 
     This invention relates generally to the field of telecommunications and, more particularly, to a system and method of enhancing reliability while providing communication services to multiple subscribers. 
     2. Description of Related Art 
     Communication technology has had steady progress in functionality and speed, especially since the advent of the global Internet. A typical architecture includes a so called central office that transfers data between multiple servers and multiple subscribers. Hardware failure in a central office, however, may interrupt service to one or more subscribers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system and method for enhancing reliability in a communication center, such as a central office. 
     To achieve this and other objects of the present invention, there is a method for a system including a plurality of encoders each for receiving a first signal and encoding to generate a respective second signal for sending to a respective subscriber. The method comprises receiving, in each encoder, the first signal from a first source, while testing a signal path between the second source and the encoders; and subsequently a second receiving step of receiving the first signal from a second source. 
     According to another aspect of the present invention a system for operating with a network, the system comprises a first assembly for receiving from the network to generate a first signal; a second assembly for receiving from the network to generate the first signal; a plurality of third assemblies; a first signal path, the first signal path being for sending from the third assemblies to the first assembly; a second signal path, the second signal path being for sending from the third assemblies to the second assembly, wherein each third assembly includes a multiplexor that generates a multiplexor output responsive either to the first signal from the first assembly or the first signal from the second assembly, an encoder that encodes the multiplexor output to generate a respective second signal for sending to a respective subscriber, a sender that sends on the second signal path, at a time when the multiplexor is responsive to the first signal from the first assembly, thereby testing the second signal path. 
     According to yet another aspect of the present invention a system comprises a plurality of encoders each for receiving a first signal and encoding to generate a respective second signal for sending to a respective subscriber; means for receiving, in each encoder, the first signal from a first source, while testing a signal path between the second source and the encoders; and means for receiving the first signal from a second source. 
     According to yet another aspect of the present invention, a method comprises receiving from a network port to generate a first signal and a second signal; sending the first signal to a first set of assemblies via a first signal path; sending the second signal to a second set of assemblies via a second signal pat; sending third signals from the first set of assemblies to the network port via a third signal path; and sending fourth signals from the second set of assemblies to the network port via the third signal path. 
     According to yet another aspect of the present invention a method comprises receiving from a network port to generate a first signal and a second signal; sending the first signal to a first set of assemblies via a first signal path; encoding, in one of the first set of assemblies, a portion of the first signal using a first protocol to send a first encoded signal to effect a first data rate for a first subscriber; sending the second signal to a second set on assemblies via a second signal path; and encoding, in one of the second set of assemblies, a portion of the second signal using a second protocol to send a second encoded signal to effect a second data rate for a second subscriber. 
     According to yet another aspect of the present invention a method comprises receiving from a network port on a first assembly to generate a first signal; sending the first signal to a set of second assemblies via a first signal path; sending second signals from the set of second assemblies to the first assembly via a second signal path; communicating between the first assembly and the second assemblies via third signal paths, each third signal path being electrically insulated from the other third signal paths; subsequently, receiving from a network port on a third assembly to generate the first signal; sending the first signal to the set of second assemblies via a fourth signal path; sending second signals from the set of second assemblies to the third assembly via a fifth signal path; communicating between the third assembly and the second assemblies via a sixth signal paths, each sixth signal path being electrically insulated from the other sixth signal paths. 
     According to yet another aspect of the present invention, a system comprises a housing with a plurality of signal busses; a plurality of assemblies, each assembly including a first connector with a plurality of conductors for sending signals between the assembly and the signal busses, an encoder that generates subscriber signals responsive to signals on the signal busses; and a plurality of second connectors, each located to receive subscriber signals from 2 adjacent assemblies. 
     According to yet another aspect of the present invention, a system comprises a housing with a plurality of slots and a plurality of signal busses; a plurality of first assemblies removably connected to slots in the housing; a second assembly, removably connected to a slot in the housing, the second assembly having circuitry for receiving signals from a network port, to send a signal on a selected one of a first plurality of signal paths, depending on an association between routing signals and first assemblies; a third assembly, removably connected to a slot in the housing, the third assembly having circuitry for receiving signals from a network port, to send a signal on a selected one of a second plurality of signal paths, depending on the association; and a fourth assembly, removably connected to a slot in the housing, having a memory for storing the association. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is diagram of a communication system in accordance with a first preferred embodiment of the present invention. 
     FIG. 2 is a diagram showing a connection of shelves in the first preferred system. 
     FIG. 3 is a view of a shelf with plug-in circuit cards in the preferred system. 
     FIG. 4 is a diagram showing a backplane connector for plugging a circuit card into the backplane of a shelf. 
     FIG. 5 is a block diagram showing some circuitry in the shelf shown in FIG.  2 . 
     FIG. 6 is a diagram emphasizing some circuitry shown in FIG.  5 . 
     FIG. 7 is a diagram emphasizing other circuitry shown in FIG.  5 . 
     FIG. 8 is a diagram emphasizing other circuitry shown in FIG.  5 . 
     FIGS. 9A and 9B are a flow chart showing a process performed by the first preferred system. 
     FIG. 10 is a diagram describing a test signal generated in the preferred system. 
     FIG. 11 is a timing diagram for enabling the signal described in FIG.  10 . 
     FIG. 12 is a diagram emphasizing other circuitry shown in FIG.  5 . 
     FIG. 13 is block diagram showing some circuitry in a communication system in accordance with a second preferred embodiment of the present invention. 
    
    
     The accompanying drawings which are incorporated in and which constitute a part of this specification, illustrate embodiments of the invention and, together with the description, explain the principles of the invention, and additional advantages thereof. Throughout the drawings, corresponding parts are labeled with corresponding reference numbers. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows system  1  in accordance with a preferred embodiment of the present invention. System  1  includes central office  5  managed by a telephone company or other type of communication provider. Central office  5  provides communication services to a plurality of subscribers, in office buildings  5 ,  8 ,  10 , and  14 ; and homes  12  and  16 . Central office  5  provides communication services to the subscribers via respective subscriber lines  9 ,  11 ,  13 ,  15 , and  17 . Each subscriber line is a tip and ring twisted pair, including 2 copper wires constituting 2 contiguous current paths between central office  5  and the building of a subscriber. 
     Central office  5  includes access circuitry  25 , telephone switch  22 , and interface  27  to a wide area network (WAN) communication link  28  to service provider networks  20  in the global Internet. In this Patent Application, the word circuitry encompasses both dedicated hardware and programmable hardware, such as a CPU or reconfigurable logic array, in combination with programming data, such as sequentially fetched CPU instructions or programming data for a reconfigurable array. 
     Access circuitry  25  acts to combine data from networks  20  with an analog, voice band, signal from telephone switch  22 , to send a composite signal to subscribers via the subscriber lines. For example, circuitry  25  receives and encodes data from networks  20  to generate a discrete multitone technology (DMT) signal, combines the DMT signal with an analog signal from telephone switch  22 , and sends the composite signal over line  11  to a subscriber in office building  10 . Conversely, circuitry  25  receives a composite signal from the subscriber in building  10  via line  11 , filters the composite signal to send a digital signal to networks  20 , and filters the composite signal to send an analog signal to telephone switch  22 . 
     The exemplary system  1  ADSL (Asymmetric Digital Subscriber Line) and is optimized for SONET (Synchronous Optical NETwork) OC3 technologies and standards. Those skilled in the art will understand that the basic architecture of system  1  is applicable to many other technologies and standards. 
     FIG. 2 shows a plurality of a first shelf  30  connected to a plurality of shelves  30 ′ via daisy chain cables  42  and  46 . Shelves  30  and  30 ′ house access circuitry  25  in central office  5 . Network Termination (NT) card  37  includes a SONET OC3 port  21 . NT extender cards  37 ′ include all of the circuitry of NT card  37 , described later in the Patent Application, except for OC3 port  21 . 
     Redundant NT card  36  includes a SONET OC3 port  21 . Redundant NT extender cards  36 ′ include all of the circuitry of NT card  37 ′, described later in the Patent Application, except for OC3 port  21 . 
     Signal buffers  48  and ATM switching circuitry sends ATM cells to other circuitry in shelves  30  and  30 ′, via downstream busses  35  and downstream busses  31 . Upstream multiplexors  49  receive ATM cells from other circuitry in shelves  30  and  30 ′, via upstream busses  38  and upstream busses  39 . 
     FIG. 3 shows high density shelf  30  supporting access circuitry  25  in central office  5 . Shelf  30  is a rear access module with 2 tiers of card slots. Upper tier  32  houses upper tier cards (UTs)  70 - 87 . Lower tier  33  houses line termination cards  50 - 67  (LTs) for communication with subscribers. Network termination cards  36  and  37  (NTs) interface with circuitry  27  leading to WAN line  28 . Alarm-craft interface card  45  collects alarm information from circuitry  25 , displays the alarm information locally, and sends the alarm information to other systems. Shelf  30  can accommodate either 1 or 2 NTs, depending on whether redundancy is required, and up to 18 LTs. Each LT includes 12 subscriber lines. Thus, with 18 LTs×12 lines/LT, shelf  30  interfaces to 216 subscriber lines. 
     Shelf  30  is essentially a mechanical backplane mechanically supporting signal busses  35 ,  31 ,  38 , and  39 ; and supporting point-to-point connections  150 . Each of busses  35 ,  31 ,  38 , and  39  includes a plurality of parallel data lines and a plurality of control lines. 
     Each of cards  36 ,  37 ,  45 ,  50 - 67 , and  70 - 87  connects to the mechanical backplane via a respective backplane connector  18 , such as connector  18  of card  50  shown in FIGS. 3 and 4. Each backplane connector  18  includes a plastic, insulating housing  93  enclosing and supporting a plurality of parallel conductors  94  for sending signals between a card and the backplane. For each of cards  36 ,  37 , and  50 - 67 , the conductors are for sending signals between the card and busses  35 ,  31 ,  38 , and  39 . For example, the conductors inside connector  18  of NT card  37  allow card  37  to sends signals to downstream busses  35  and receive signals from upstream busses  38 . The conductors in connector  18  of LT card  51  allow LT card  51  to receive signals from busses  35  and busses  31 , and to send signals to busses  38  and busses  39 . 
     Each of cards  36 ,  37 ,  45 ,  50 - 67 , and  70 - 87  is removably connected to the mechanical backplane. 
     Point-to-point connections  150  include a pair of current paths in each connection. Point-to-point connections  150  include a connection between NT  36  and each of LTs  50 - 67  Point-to-point connections  150  also include a connection between NT  37  and each of LTs  50 - 67 . Point-to-point connections  150  are arranged in parallel in the backplane of shelf  30 . Each of point-to-point connections  150  has a higher bandwidth than that of any one of busses  35 ,  31 ,  38  or  39 . 
     Since certain services inherently interfere with other services, due to incompatibility of spectrum, and since a variety of services may be provided on the same shelf, shelf  30  is structured to keep the services and cabling of those services H, separate and shielded. Adjacent pairs of LT card slots are cabled to a respective cable connector, represented by reference number  29  in FIG.  3 . Each cable connector  29  includes 50 pins, thereby providing for 24 subscriber lines serviceable by the two LTs in the slot pair. For example, the slots for LT  50  and LT  51  share a common cable connector  29 . Thus, interfering services are kept on a 2 adjacent slot basis, with cables capable of being shielded through frame ground connections (backplane to connector housing). 
     For some types of LTs, such as DS1 type (for connections to a remote access multiplexor, for example), the adjacent slot backplane wiring is such that the transmit and receive pairs are routed to separate connectors via the applique cards, allowing one set of pairs (transmit or receive) to be routed to the line connector, while the second set is routed to the conventional telephony network connector, allowing alternate use of these connectors. 
     Some types of LTs, such as DS3 type, can be provided in a redundant or non-redundant configuration via the use of applique card variants, with coax cable exiting from the applique inset faceplates, then exiting the shelf via cable notches in the shelf top plate. Other than these coax for this special application, all wiring is rear access; only fiber comes off the front of any card. This structure facilitates the EMI requirements and the flammablity requirements imposed by GR1089. 
     FIG. 5 is a block diagram emphasizing some signal paths in the preferred system. In the example immediately following, NT  37  is a SONET OC3 in an active mode and NT  36  is a SONET OC3 in a standby mode. Referring FIGS. 3 and 5, each LT has an associated upper tier card (UT) in the slot directly above the LT. For example, bus  88  includes 12 pairs of conductors, a pair for each subscriber, between LT  50  and UT  70 . Bus  89  includes 12 pairs of conductors between LT  51  and UT  71 . Bus  90  includes 12 pairs of conductors between LT  52  and UT  72 . Bus  91  includes 12 pairs of conductors between LT  53  and UT  73 . 
     A UT includes any filtering circuitry provided to the subscriber lines. For example, each upper tier card (UT) includes a respective low pass filters (LPF)  92  between the subscriber lines and telephone switch  22 . 
     NT  37  receives Asynchronous Transfer Mode (ATM) cells from interface  27  and sends the cells over downstream busses  35 . Each ATM cell includes a pair of identifiers: a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). Each LT recognizes a set of VPI/VCI pairs (addresses) as identifying a cell destined for one or more subscribers connected to the LT. For example, LT  52  recognizes a set of 1 or more VPI/VCI addresses as identifying a cell destined for a subscriber in building  14 . Upon recognizing such a cell, LT  52  generates a DMT signal encoding the cell, and sends the signal to UT  72 . UT  72  combines the DMT signal with an analog signal from telephone switch  22 , to send a composite signal to the subscriber in building  14 , via line  15 . 
     When a subscriber wishes to send data to service provider networks  20 , the subscriber modem encodes the data in a DMT signal and sends the DMT signal over a subscriber line. This DMT signal passes from one of the Uts, to a high pass filter in an LT card, to send a digital signal to NT  37  via one of upstream busses  38 . 
     Thus, NT card  37 , downstream busses  35 , and upstream busses  38  act to provide the subscribers with access to service provider networks  20 . During this time, NT card  36 , downstream busses  31 , and upstream busses  39  are redundant. In other words, NT card  36 , downstream busses  31 , and upstream busses  39  are in a standby mode in case NT  37 , busses  35 , or busses  38  should malfunction. During this time, circuitry  25  acts to test downstream busses  31  and upstream busses  39  for redundant bus integrity, as discussed below in connection with FIG.  4 . 
     FIG. 6 emphasizes some of the circuitry shown in FIG.  3 . Downstream busses  35  include downstream bus  351 , downstream bus  352 , downstream bus  353 , and downstream bus  354 . Upstream busses  38  include upstream bus  381 , and upstream bus  382 . A set of the LTs share upstream bus  381  using a priority-based, cell grant multiplexing scheme, such as described in U.S. patent application Ser. No. 09/084,750 by PHILIPPE GUILLAUME DOBBELAERE and PASCAL LEFEBVRE, filed May 26, 1998 for a method of prioritized data transmission and data transmission arrangement. The contents of U.S. application Ser. No. 09/084,750 are herein incorporated by reference. 
     A priority-based, cell grant multiplexing scheme, is also described in U.S. Pat. application Ser. No. 09/022,177 by PHILIPPE GUILLAUME DOBBELAERE and GEERT ARTHUR EDITH VAN WONTERGHEM, filed Feb. 11, 1998 for a priority-based access control method and arrangement. The contents of U.S. application Ser. No. 09/022,177 are herein incorporated by reference. 
     The priority-based, cell grant multiplexing scheme, cited in the previous paragraph, is also described in European Patent Application No. 97400303.0 by PHILIPPE GUILLAUME DOBBELAERE and GEERT ARTHUR EDITH VAN WONTERGHEM, filed Feb. 11, 1997 for a Priority-based access control method and arrangement. The contents of European Patent Application No. 97400303.0 are herein incorporated by reference. 
     Another set of the LTs share upstream bus  382  using the priority-based, cell grant multiplexing scheme. 
     This configuration of multiple busses allows a bandwidth of 622 Mbps downstream and 300 Mbps upstream. Downstream busses  31  include downstream bus  311 , downstream bus  312 , downstream bus  313 , and downstream bus  314 . Upstream busses  39  include upstream bus  391 , and upstream bus  392 . When no malfunction exists in circuitry  25 , downstream busses  31 , upstream busses  39 , and NT  36  are redundant. 
     The architecture of system  1  allows services to be managed and refined, and allows failure recovery without manual intervention. Upon activating an LT, such as LT  51 , NT  37  assigns LT 51  to one of busses  351 ,  352 ,  353 , or  354 , and assigns LT  51  to one of busses  381  or  382 . For example, NT  37  may initially assign LT  51  to bus  351  to bus  381 . Subsequently, depending on bandwidth needs and congestion, NT  37  may reassign LT  51  to a more appropriate bus set. 
     NT  37  also instructs LT  51  to recognize the set of VPI/VCI addresses for the subscribers connected to UT  71 , including the subscribers on lines  11  and  13 . More specifically, NT  37  sends the set of VPI/VCI addresses, to be recognized by LT  51 , via an operations channel on downstream bus  351 . This operations channel is a stream of ATM cells having a VPI/VCI address assigned to LT  51  itself. When LT  51  sees an ATM cell with the VPI/VCI of LT  51  itself, LT  51  interprets the remainder of the cell as a command from NT  37 . One such command is to recognize a new VPI/VCI as belonging to the subscriber(s) to be associated with LT  51 . Thus, NT  37  routes the proper subscriber ADSL traffic to LT  51 . 
     Referring back to FIG. 5, Non-volatile memory  47  in alarm-craft unit  45  stores a cross connect database, which is effectively a respective list of VPI/VCI pairs for each subscriber line associated with each LT. Non-volatile memory  47  also stores a table of data rates for each subscriber. 
     Each of downstream busses  351 ,  352 ,  353 , and  354  includes 8 parallel data bit signal lines, and signal lines for cell synchronization, idle cell indication, upstream access start, and grant. Each of upstream busses  381  and  382  includes 8 parallel data bit signal lines, and signal lines to indicate upstream cell synchronization; upstream access arbitration, on which the LTs write respective priority codes via open drain buffers (pull line low); upstream access allowed, which identifies the shelf that won the access in the multiplexing scheme described in application Ser. No. 09/022,177 cited above; the winning priority code in the multiplexing scheme; and upstream output enable, asserted low by an LT (open drain) when it sends a cell. 
     NT  37  generates common clock synchronization signals for the LTs. NT  37  generates a maximum of 1 clock synchronization signal for every 2 LTs. 
     A signal called “extender change status” is common to busses  35  and  38 , and indicates if busses  35  and  38  are in active mode or standby mode. A signal called “error indication” is common to busses  35  and  38 , and is asserted low by an LT (open drain) if and when the LT detects an error on busses  35  or  38 . 
     Each of downstream busses  311 ,  312 ,  313 , and  314  includes 8 parallel data bit signal lines, and signal lines for cell synchronization, idle cell indication, upstream access start, and grant. Each of upstream busses  391  and  392  includes 8 parallel data bit signal lines, and signal lines to indicate upstream cell synchronization; upstream access arbitration, on which the LTs write respective priority codes via open drain buffers (pull line low); upstream access allowed, which identifies the shelf that won the access in the multiplexing scheme described in application Ser. No. 09/022,177 cited above; the winning priority code in the multiplexing scheme; and upstream output enable, asserted low by an LT (open drain) when it sends a cell. 
     NT  36  generates common clock synchronization signals for the LTs. NT  36  generates a maximum of 1 clock synchronization signal for every 2 LTs. 
     A respective “extender change status” signal is common to busses  31  and  39 , and indicates if busses  31  and  39  are in active mode or standby mode. A respective “error indication” signal is common to busses  31  and  39 , and is asserted low by an LT (open drain) if and when the LT detects an error on busses  31  or  39 . 
     FIG. 7 is a diagram emphasizing signal paths used to test backup downstream busses  31  and backup upstream busses  39 . The purpose or this testing is to prevent silent failures of busses  31  and  39 , while busses  35  and  38  are being employed for subscriber signal traffic. This error detection process relies on transmit activity of each active LT, in turn. In the example shown in FIG. 5, downstream bus  351  and upstream bus  381  act with NT  37  to provide network access to LT  50 . Concurrently, transmit activity of LT  50  tests the signal paths in upstream bus  391 , NT  36 , and downstream bus  311 , which are in a standby mode. When LT  50  is enabled to transmit a cell on upstream bus  381 , test circuitry in LT  50  receives data from downstream bus  311 , to transmit on upstream bus  391 . This error detection process may detect LT backplane connector faults, such as open pins, at locations represented at reference number  126  in FIG.  7 . This process also may detect backplane faults, such as shorts and opens, at locations represented at number  127 . This process may also detect backplane connector faults, such as open pins, at the standby NT  36  at locations represented by the reference number  128 . This process may also detect faults on the NT  36  between the IC-resident bus controller and the backplane connector, such as faults including IC pin faults, printed board assembly track faults, and driver faults, as represented by reference number  129 . 
     FIG. 8 shows the circuitry of FIG. 7 with more emphasis on the signal paths between LT  50  and NT  36 . Downstream busses  311  includes data signals IQDD 0 , IQDD 1 , IQDD 2 , IQDD 3 , IQDD 4 , IQDD 5 , IQDD 6 , IQDD 7 . Downstream busses  311  also includes control signals called IQDCLK, IQDCS, IQUCS, IQULAA, IQUWP 7 , IQECS, and “extender chain active.” 
     Upstream busses  391  includes data signals IQUD 0 , IQUD 1 , IQUD 2 , IQUD 3 , IQUD 4 , IQUD 5 , IQUD 6 , IQUD 7 . 
     As represented in FIG. 8, an LT, such as LT  50 , generates bus test signals when upstream output enable (IQUOEZ) is true and the extender chain active signal is false. In other words, because of this dependence on upstream output enable, in essence each LT generates test signal in synchronism with the upstream, priority-based, cell grant multiplexing scheme, cited above. 
     Generating test signals in an LT includes generating IQUD 0  through IQUD 6  by looping back the signals IQDD 0  through IQDD 6 , respectively. Generating test signals in an LT also includes generating an odd parity signal on the combination of IQDCS, IQUCS, IQULAA, IQUWPZ, IQECS, and IQDD 7 . The LT then sends the parity signal on IQUD 7 . 
     As represented in FIG. 8, the standby NT generates bus test signals when upstream output enable (IUOEZ) is true and the extender chain active signal is false. Generating test signals in the standby NT includes comparing IQDD 0  to IQDD 6  with IQUD 0  through IQUD 6 , respectively, and setting “standby bus error” to be true if there is a mismatch. Generating test signals in an NT also includes generating an odd parity signal on the combination of IQDCS, IQUCS, IQULAA, IQUWPZ, IQECS, and IQDD 7 , comparing this odd parity signal with IQUD 7 , and setting “standby bus error” to be true if there is a mismatch. Generating test signals in an NT also includes sending a standby bus test pattern on IQDCS, IQUCS, IQULAA, IQUWPZ, IQECS, and IQDD 7 , as discussed below in connection with FIG.  7 . 
     FIGS. 9A and 9B show a process performed by system  1 . NT  37  receives the cross point inter-connect, allowing NT  37  to route ATM cells from Network  20  to the appropriate one of downstream busses  351 ,  352 ,  353 , or  354 . (step  5 ). System  1  selects one of the LT cards assigned to upstream bus  381 , to determine which of these bus- 381 -assigned LT cards is eligible to send the next cell on bus  381 . System  1  selects this LT card using the priority-based, cell grant multiplexing scheme cited above. (step  10 ). The selected LT card sends an upstream ATM cell on bus  381 , while the selected LT card sends a signal on a back-up upstream bus, such as upstream bus  391 , to test a signal path between the selected card, bus  391 , and one of backup downstream busses  31 , such as bus  311  (step  15 ). If step  15  detects an error in the back-up circuitry (step  16 ), alarm craft unit  45  generates an alarm signal (step  17 ), to allow personnel to service the back-up circuitry. 
     While NT  37 , busses  35  and busses  38  continue to operate without fault (step  20 ), system  1  performs steps  10  and  15  for upstream transmission, and also and sends ATM cells from NT  37  onto downstream busses  35 . 
     If system  1  detects a fault in NT  37 , busses  35 , or busses  38  (step  20 ), alarm craft unit  45  sends the VPI/VCI pairs of the cross point inter-connect database from non volatile memory  47  to NT  36 , to allow NT  36  to assume the function of routing ATM cells from Networks  20  to one of downstream busses  311 ,  312 ,  313 , or  314  (step  25 ). 
     FIG. 10 shows the standby bus test pattern. The standby NT sets a bus test pattern on IQDCS, IQUCS, IQULAA, IQUWPZ, and IQDD 7  as shown in FIG. 10, wherein N, N+1, etc indicate consecutive cells. 
     FIG. 11 is a timing diagram for the enablement of the standby bus test. To avoid problems with different clock domains, the standby bus test is only enabled during ATM cell header bytes  3  and  4  (H 3  and H 4  in FIG.  7 ), the header error correction byte (HEC in FIG.  8 ), and payload byte  1  through  46  (P 1  through P 46  in FIG.  11 ). 
     Thus, standby busses are constantly monitored via a test pattern to help ensure that the standby busses are operational; to reduce the chance of a silent failure. 
     In summary, each LT includes circuitry to receive a common ATM cell stream signal from an active NT via a downstream bus to which the LT is assigned. The cell stream is common in the sense that other LTs may be assigned to the same downstream bus. LT assigned to a particular down stream bus examine a common ATM cell stream from the active NT. Each LT encodes selected parts of the common cell stream signal to generate a respective DMT signal for sending to a subscriber. In other words, a particular LT will only send a DMT signal for cells having a VPI/VCI address corresponding to a subscriber on one of the subscriber lines connected to the LT. 
     Either NT  37  or NT  36  is a potential source of a downstream cell stream signal. For example, a set of LTs may receive a common cell stream signal from NT  37 . Concurrently, the system  1  may test the standby busses between each upstream enabled LT and NT  36 . Subsequently, if a problem is detected with downstream busses  35 , NT  37 , or upstream busses  38 , NT  36  is enabled so that the set of LTs will receive the common cell stream signal from NT  36  and one of downstream busses  31 . 
     More specifically, while the set of LTs are receiving the common cell stream signal from NT  37  via one of busses  35 , NT  36  sends a downstream test signal on busses  31 . The downstream test signal includes IQDCS, EQECS, IQDD 7 , IQDD 0 -IQDD 6 . When an LT becomes upstream enabled, the LT generates an upstream test signal, in response to the downstream test signal. The upstream test signal includes a plurality of digit positions (IQUD 0 -IQUD 6 ), each corresponding to a respective IQDD 0 -IQDD 6 . Generating the upstream test signal includes generating parity, which is a type of a redundancy signal. Comparators in NT  36  act to examine the upstream test signal received from the LT. 
     FIG. 12 emphasizes another aspect of system  1 . System  1  includes a plurality of respective point-to-point connections  150  between each LT and each NT as shown in FIG.  12 . Point-to-point connections allow high speed data to be sent/received, in various formats. Point-to-point connections  150  may be configured for various services via other ports, such services including IP, frame relay, or frame relay to ATM. 
     Each point-to-point connection  150  is a pair of current paths. Each point-to-point  150  connection is insulated from the other point-to-point connections  150 . 
     Second Embodiment 
     FIG. 13 shows system  2  in accordance of a second preferred embodiment of the present invention. NT  6  and NT  7  are the same as NT  36  and NT  37  of the first preferred embodiment except that NT  6  and NT  7  each contain a DS3 port, instead of an OC 3 port, and are each connected to common DS3 I/O circuitry  8 . System  2  also includes a server  161  residing in shelf  30 , the first shelf of the daisy-chained series of shelves  30  and  30 ′. An ATM switch function in the NT  7 , allows data to be routed to the normal ATM network of the LT cards, or routed to server  161 , which acts as a server or gateway function. In the example shown, server  161  acts as a translator between ethernet  163  and the ATM network upstream. Other types of applications may employ this function-including an interworking unit that takes ATM cells containing voice, and converts them into a TDM interface to a class 5 telephony switch. 
     Respective downstream busses may be used for respective services, having respective protocols and data rates. 
     The NT may be DS1 (or E1), HDSL2, DS3 (or E3), OC3 or OC12 based. Upgrades to higher bandwidth NTs may be effected without interrupting subscriber service. Upgrades to higher bandwidth NTs includes removing the inactive NT (in a redundant configuration), replacing with a higher bandwidth NT, switching over service to the new NT, then replacing the original NT with the higher bandwidth redundant unit. 
     To provide for extended bandwidth when upgrading to 622 Mbps service, an extra set of daisy chains cables  42  and  46  may be installed between the NT and NT′ extender cards. 
     Alarm craft unit  45  communicates with whichever NT is designated active. An ethernet port may be provided between unit  45  and an external OS. The ethernet port may be accessible from the front panel of unit  45 , or via a rear access connector on the backplane for more permanent connections. Similarly, craft interfaces are provided by unit  45  on either front panel, or rear access. 
     The remainder of the extra I/O between the NTs and ACU consist of high speed communication interfaces (fire wire) to allow a source of common database memory for the redundant NTs, implemented on the ACU with reprogrammable, high-density, flash memory. This allows quick recovery of service during NT failures in redundant configurations. 
     LTs may be for ADSL, HDSL2, IDSL, DS1, E1, DS3, E3, OC3, or other xDSL service. While ADSL, HDSL2, and IDSL are primarily used for subscriber interfaces, HDSL2 may be used in hubbing arrangements to communicate with remote access multiplexors, as is the DS1, E1. Multiplexors are disclosed in U.S. patent application Ser. No. 08/891,145 by RICHARD M. CZERWIEC, JOSEPH E. SUTHERLAND, PETER M. L. SCHEPERS, GEERT A. E. VAN WONTERGHEM, MARLIN V. SIMMERING, EDUARD C. M. BOEYKENS, CHRIS VAN DER AUWERA, PETER A. R. VAN ROMPU, KURT PYNAERT, DANIEL A. C. VERLY, GILBERT A. F. VAN CAMPENHOUT, RICHARD H. BAILEY, ROBERT N. L. PESCHI, DIRK M. J. VAN AKEN, EMMANUEL F. BOROWSKI, PETER P. F. REUSENS, HERMAN L. R. VERBUEKEN, FRANK RYCKEBUSCH, KOEN A. G. DE WULF filed Jul. 10, 1997 for TELECOMMUNICATIONS SYSTEM FOR PROVIDING BOTH NARROWBAND AND BROADBAND SERVICES TO SUBSCRIBERS; SUBSCRIBER EQUIPMENT; A SHELF THEREFOR; A REPLACEABLE LOWPASS FILTER UNIT; LINE TERMINATION EQUIPMENT; NETWORK TERMINATION EQUIPMENT; AND A TELECOMMUNICATIONS RACK WITH A PLURALITY, the contents of which is herein incorporated by reference. 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or the scope of Applicants&#39; general inventive concept. The invention is defined in the following claims.