Abstract:
A device ( 310, 320 ), in its receiver aspect, interconnects a high-rate terminal, e.g. DS 1 , link transmitting outgoing fractional DS 1 , e.g. DS 0 , channels destined for a customer site with autonomous first and second networks ( 301, 302 ) composed of high-rate links ( 112, 312 ) propagating incoming fractional DS 1  channels. Each network is adapted to insert a fault indication signal in any channel affected by a fault condition. The device monitors each incoming channel from each network for a fault indication signal and switches fractional DS 1  channels from the network manifesting a fault condition to the alternate network, thereby maintaining essentially uninterrupted digital service to the customer site. In its transmitter aspect, the device transmits replicated versions of channels supplied by the terminal link simultaneously onto both the first and second networks, thereby providing a substantially identical device connected at the other end of the networks with corresponding incoming high-rate link signals.

Description:
This application is a continuation of application Ser. No. 07/805,340, filed on Dec. 9, 1991, and now abandoned. 
     The invention relates to digital communication networks and, more specifically, to restoration circuitry and a concomitant methodology to provide high availability communication channels over a digital network. 
    
    
     BACKGROUND OF THE INVENTION 
     With modern telecommunications networks, a customer such as a business customer may select from an array of communication services aimed at providing cost-effective connections to geographically-dispersed sites maintained by the customer. The various available alternatives range, at one extreme, from ubiquitous direct dialing over the public network to, at the other extreme, specially provisioned private networks. Because of the vagaries of direct dialing, such as call blocking and connect-time cost, a customer with critical communication requirements most often selects the private network option. In particular, there has recently been a demand for large private data networks to connect, for example, numerous terminal devices such as reservation terminals to an arrangement of centrally located, fault-tolerant host computers so as to service consumer transactions. Data networks are generally implemented on private network facilities because, as exemplified above, computer applications necessitate continuous, on-line connections of terminals to centralized computers. 
     One common method of implementing a private line network is to interconnect various customer sites with DS 1  digital facilities wherein numerous channels are multiplexed to generate a DS 1  signal suitable for carriage over the high-rate links. A channel may have more than one DS 0  (i.e. N×DS 0 , where 1≦N≦24). Each channel embedded in the DS 1  will hereinafter be referred to as a fractional DS 1  channel. At the destination end (far end), the high-rate signals are demultiplexed to recover the fractional DS 1  channels. Oftentimes, the DS 1  facilities are provided by a common carrier over carrier-owned digital facilities on a long-term or semi-permanent basis. DS 1  facilities are provisioned by the common carrier through static cross-connect switches generally referred to as a Digital Access and Cross Connect System (DACCS). Unlike telephone-carrier switches which handle telephone call setups, a DACCS establishes routes which may be connected for years. 
     Efficient use of DS 1  facilities requires channel grooming. Because of channel grooming, the fractional DS 1  channels which are carried over each DS 1  link may not all be derived from the same customer site. Specifically, the fractional DS 1  channels that form an out-going DS 1  signal from a DACCS are usually composed of fractional DS 1  channels that originate from a plurality of in-coming DS 1  facilities that come from different customer sites. Typically, facility failures in private networks require restoration efforts that are correspondingly sophisticated and time-consuming. Accordingly, facility engineers have sought techniques to provide for efficient and automated restoration of facilities when deleterious service conditions are detected. 
     Recently, a service has been introduced by one inter-exchange carrier which provides switched digital data service at fractional DS 1  speeds. In using the service, a customer is able to establish a back up dial-up link to restore a failed private line fractional DS 1  channel. However, such a service has the disadvantage of blocking, that is, a link between the end points is not always available. Moreover, it takes at least a few seconds to establish a connection over such a link each time a dial-up is attempted. In the period of a few seconds, a significant loss of data can occur. 
     A need exists in the art for a relatively simple technique for efficiently and automatically restoring telecommunications service over a fractional DS 1  channel while providing essentially uninterrupted communications in the event that a DS 1  facility, which carries that channel in a private line network, fails. 
     SUMMARY OF THE INVENTION 
     These deficiencies as well as other shortcomings and limitations are obviated, in accordance with the present invention, by a device that couples to pairs of redundant digital networks and switches individual channels, e.g., bi-directional fractional DS 1  channels, to the alternative network whenever errors, such as failures of a DS 1  facility, are detected in the active network. 
     Broadly speaking, with respect to the circuitry aspect of the present invention, a pair of devices are situated at near and far ends of a digital link such as a fractional DS 1  channel. Each of these devices includes: detection circuitry to detect the presence of a fault indication signal, such as a digital access cross-connect system (DACCS)-generated trouble code, in any of the fractional DS 1  channels arriving over the network pair; and switching circuitry to transfer the corresponding fractional DS 1  channels having the trouble code from the active network to fractional DS 1  channels in the alternate network. The use of paired devices spanning both ends of the fractional DS 1  channel ensures that both directions of propagation are protected from fault conditions that could arise within either network and adversely affect transmission in either direction over either one of the links. In addition, each device also includes transmitter circuitry to transmit two identical versions of data generated by terminal equipment which then are transmitted to the far-end over primary and secondary networks. 
     In a particular embodiment, each device relies on detecting a so-called DACCS “trouble code” which is inserted by a digital access cross-connect system on all outgoing DSOs at the occurrence of a DS 1  facility failure and then, based upon the presence of this code, automatically switching a fractional DS 1  channel from an active route to a corresponding fractional DS 1  channel in the other DS 1  provided by the alternate network in order to restore service. It has been known for several years that a DACCS will automatically insert a pre-defined trouble code in each outgoing DSO which is affected by a facility failure. However, the art appears to be devoid of any teachings showing that these codes have ever been commercially used in switching between primary and alternate routes for two redundant facilities, each of which is carried through a separate network, in order to restore telecommunications service in the event the active route fails. 
     The invention further comprises a digital device for interconnecting a terminal link to both first and second communication networks. Both the terminal link, and the first and second communication networks are arranged to propagate a plurality of corresponding channels, wherein each of the communication networks is arranged to generate and then to transmit a fault indication signal over any of its corresponding channels upon detection of a fault condition affecting any of the corresponding channels in the first or second networks. The first network is initially the active network and the second network is initially the alternate network. The device comprises a means, responsive to the terminal link, for transmitting the channels carried by the terminal link simultaneously over both the first network and the second network. The device has a means, responsive to both the first network and the second network and coupled to the terminal link, for monitoring each channel in the active network to determine the presence of the fault indication signal. Upon detection of the fault indication signal, the responsive device replaces each channel from the active network having the fault indication signal with the corresponding channel from the alternate network so as to thereby supply the terminal link with high availability channels. 
     The invention further comprises a digital network having a plurality of channels for interconnecting a near end and far end customer transceivers, wherein one end of the channel is the near end and the other end is the far end. The transceivers transmit fractional DS 1  communication channels to and receive fractional DS 1  communication channels from the digital network. The digital network comprises autonomous primary and secondary digital communication networks each composed of DS 1  links arranged to propagate a plurality of the fractional DS 1  channels. Each of the communication networks being arranged to generate and then transmit a fault indication signal over any of the fractional DS 1  channels upon detection of a fault condition affecting said any of the fractional DS 1  channels. The primary network is active initially and the secondary network is in standby initially. A first circuit means is coupled to the near end customer transceiver and one end of both said primary and secondary networks, and a second circuit means is coupled to the far end customer transceiver and the other end of both said primary and secondary networks. Each of the circuit means comprise a means for transmitting simultaneous versions of the fractional DS 1  channels over the first and second communication networks, a means for sensing the fault indication signal on any of the fractional DS 1  channels on the active network, and a means for restoring any of the fractional DS 1  channels having the fault indication signal by selecting the corresponding fractional DS 1  channel from the standby network upon sensing of the fault indication signal. 
     In addition the invention further includes a communications network comprising a first and a second communication path one of which is an active communication path and the other being a stand-by path. The network comprises a means for simultaneously transmitting information over the first and the second communication paths and a means for receiving transmitted information from the active communication path. The network has a means for monitoring the active communication path to determine the presence of a fault indication signal, and upon detection of the fault indication signal, the network has a means for switching the active communication path having the fault indication signal with the stand-by path, such that the stand-by path is the active communication path until a fault indication signal is detected in the then active communication path. 
     Another embodiment of the invention is a method of transmitting information comprising the steps of simultaneously transmitting information over a first and a second communication path, one of which is an active communication path and the other being a stand-by path. The method further comprises monitoring the active communication path to determine the presence of a fault indication signal, and upon detection of the fault indication signal, switching the active communication path having the fault indication signal with the stand-by path, such that the stand-by path is the active communication path until a fault indication signal is detected in the then active communication path. 
     The invention also includes a method for interconnecting a terminal link to both first and second communication networks wherein both the terminal link and each of the communication networks are arranged to propagate a plurality of corresponding channels. Each of the communication networks are arranged to generate and then to transmit a fault indication signal over any of its corresponding channels upon detection of a fault indication condition affecting any of the corresponding channels in the first links or in the second links. The first network is initially the active network and the second network is initially the alternate network. The method comprising the steps of transmitting the channels carried by the terminal link simultaneously over both the first network and the second network, monitoring each channel in the active network to determine the presence of the fault indication signal, and upon detection of the fault indication signal, replacing each channel from the active network having the fault indication signal with the corresponding channel from the alternate network so as to thereby supply the terminal link with high availability channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention may be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a prior art digital network facility wherein a number of customer sites are interconnected by an arrangement of channel banks and digital access cross-connects systems (DACCSs) coupled by DS 1  facilities; 
     FIG. 2 depicts an exemplary prior art DACCS incorporating a DACCS “trouble code” generator to insert DACCS “trouble codes” into outgoing DS 0 &#39;s which are cross connected to a DS 1  port detecting a facility failure; 
     FIG. 3 illustrates a digital network in accordance with the present invention wherein two customer sites are interconnected by an arrangement of channels banks and DACCSs coupled by interposed DS 1  facilities, and including protection devices which interconnect primary and secondary facilities that serve as alternate propagation paths; and 
     FIG. 4 depicts, in block diagram form, one protection device, including transmit and receive circuit block diagrams for selecting the active channel signals from either the primary or secondary facility. 
     To facilitate understanding, identical reference numerals have been used, where possible, to denote identical elements that are common to various figures. 
    
    
     DETAILED DESCRIPTION 
     To place the detailed description of the present invention in perspective, it is instructive to first gain a basic understanding of the telecommunications environment in which the present invention operates. This approach has the advantage of introducing notation and terminology which will aid in elucidating the various detailed aspects of the present invention. Thus, the first part of the description focuses on a high-level discussion of the digital network hierarchy pertinent to the inventive subject matter; after which, the circuitry aspects of the present invention, as well as the concomitant operational methodology, are presented in detail. 
     A. Overview 
     With reference to prior art private network  100  of FIG. 1, the digital network facilities exemplified by links  111 - 117  are high-rate links which serve as backbone links to interconnect geographically-dispersed customers sites  121 - 124  (also referred to sites A, B, C, and D, respectively). In the digital network hierarchy,  2  such a high-rate link is commonly referred to as a DS 1  link. The DS 1  signal is formed by time-division multiplexing twenty-four DS 0 s that are to be carried over an associated DS 1  link. Each DS 0  signal is a 64 kbps channel. An exemplary DS 1  signal is a sequence of 193 bit frames, each frame being formed by juxtaposing the 8-bit patterns from twenty-four fractional DS 1  signals, plus one framing bit. It is also noted that a DS 0  may carry either digital data or digitized voice. 
     The various DS 1  links depicted in FIG. 1 are bi-directional and each has a different set of terminating equipment. For instance, link  111  has digital channel bank  131  as one termination and digital access cross-connect system (DACCS)  141  as a second termination; links  113 ,  115 , and  117  are terminated similarly. Broadly speaking, each DACCS is a semi-permanent switch which redistributes twenty-four individual DS 0 s carried over the DS 1  link received on one side of the DACCS to one or more outgoing DS 1  links on the other side of the DACCS, as discussed in more detail shortly. DACCS pair  141  and  142  terminate link  112 ; DACCS pairs  141  and  143 , and  141  and  144 , respectively, terminate links  114  and  116 . DACCS  141  also terminates one end of DS 1  link  111 , while DACCS  142 ,  143  and  144  also terminate one end of respective DS 1  links  113 ,  115  and  117 . 
     Each digital channel bank, of which four (banks  131 ,  132 ,  133  and  134  are specifically shown), interconnects up to twenty-four lower-level fractional DS 1  channels of each specific site to a DS 1  link in the following manner, depending on whether the specific site is considered to be functioning in its source mode or destination mode. By way of example, channel bank  131  and site A are considered as representative: (i) with site A as a source, a transceiver (not shown) at site A originates up to twenty-four fractional DS 1  channels which are multiplexed by channel bank  131  into a composite signal suitable for propagation over DS 1  link  111 ; (ii) with the transceiver at site A as a destination, channel bank  131  de-multiplexes the incoming DS 1  signal on link  111  to produce up to twenty-four independent fractional DS 1  channels. Thus, channel bank  131  is bi-directional, having incoming ports connected to site A and an outgoing port connected to DS 1  link  111  for channels originating at site A, whereas for channels terminating at site A, an incoming port of channel bank  131  is connected to DS 1  link  111  and outgoing ports are connected to site A. 
     In a similar manner, each DS 1  link has incoming and outgoing ends which are defined by the direction of signal propagation on the DS 1  link. For example, the incoming and outgoing ends of link  111  connect to channel bank  131  and DACCS  141 , respectively, for signals originating at site A. Accordingly, each link  111 - 117 , channel bank  131 - 134 , or DACCS  141 - 144  supports full-duplex transmission. In one implementation of a DS 1  link, the communication medium is paired wire cable; a DS 1  link utilizes two wire-pairs to support the bi-directional propagation—one pair for each direction. 
     In modern private DS 1  private networks, not all of the fractional DS 1  channels which originate at a near-end location terminate at the same location at the far end. This is exemplified in FIG. 1 wherein, at site A, illustratively five DS 0 s are destined for site B, ten DS 0 s for site C, with the remaining nine DS 0 s having site D as their destination. Routing is accomplished via interposed DACCS  141 - 144 . For DS 0 s originating at site A, the incoming port of DACCS  141  receives the full complement of twenty-four DS 0 s over link  111  and partitions this DS 1  into groups of five, ten, and nine DS 0 s for distribution over links  112 ,  114 , and  116 , respectively. At the far end of link  112 , the five DS 0 s destined for site B are switched through DACCS  142  and then propagated over link  113  and, after de-multiplexing by channel bank  132 , to site B. Similarly, at the far end of link  114 , the ten DS 0 s destined for site C are switched through DACCS  143  and then propagated over link  115  and, after de-multiplexing by channel bank  133 , to site C. Finally, at the far end of link  116 , the nine DS 0 s destined for site D are switched through DACCS  144  and then propagated over link  117  and, after demultiplexing by channel bank  134 , to site D. 
     Because of the interconnection arrangement of network  100 , a single, isolated failure in this network may affect only a fraction of the DS 0 s originating or terminating at a given site. For example, a break in DS 1  link  113  between DACCS  142  and site B affects only five DS 0 s at site A. In order to report a facility failure downstream, each DACCS  141 - 144  is implemented to insert a so-called DACCS “trouble code” in all outgoing DS 0 s which are cross-connected to DS 1  port detecting a facility failure. In this regard, reference is made to the Technical Reference TR-TSY-000170, Issue 1, “Digital Cross-Connect Requirements and Objectives”, Bell Communications Research (Bellcore) November, 1985; this document is incorporated herein by reference. This DACCS code is a pre-defined eight bit pattern. The user can select one of the two standard trouble codes, namely, TRB=11100100, or MUX=00011010. Also, DACCS allows selection of any 8-bit pattern for insertion into the outgoing DS 0 s during a failure. This “trouble code” generating aspect of each DACCS is shown with reference to FIG.  2 . 
     FIG. 2, which focuses on an arrangement for DACCS  141  shown in block diagram form, depicts that incoming DS 0 s, numbered  1  through  24  on line (i), are cross-connected in the following manner: (1) DS 0 s  1 - 5  are routed from port  1414  (Port A) to port  1415  (Port B) via cross-connect bus arrangement  1411 , as further depicted on line (ii); (2) DS 0 s  6 - 15  are routed from Port A to port  1416  (Port C) via cross-connect bus arrangement  1412 , as further shown by line (iii); and (3) DS 0 s  16 - 24  are routed from Port A to port  1417  (Port C) via cross-connect bus arrangement  1413 , as further illustrated by line (iv). In addition, DS 1  links  111 ,  112 ,  114 , and  116  are connected to Ports A-D, respectively. The bi-directional nature of each  0 of these links is shown by directional arrows on the two one-way links comprising each single bi-directional link. A full complement of DS 0 s in each DS 1  link is not shown for simplicity. It should also be noted that DS 0 s  1 - 5  in line (i) do not necessarily have to be placed into DS 0 s  1 - 5  in line (ii); they may be placed in any DS 0  slots, so what is shown is merely illustrative. 
     If a failure occurs in DS 1  link  111 , as shown by large dashed “X”  210 , then port A senses the loss of signal on the incoming DS 1  link and instructs the “trouble code” generator circuitry  1418  to place a “trouble code” in all the outgoing DS 0 s which are cross-connected to the DS 1  port reporting the failure, namely, Port A. Trouble code generator  1418  is connected to all DS 1  ports in the DACCS, including Ports A, B, C and D. DACCS may have a few hundred DS 1  ports; Ports A, B, C, and D are merely illustrative. The “trouble code” generator is connected to Ports A, B, C, and D via respective leads  1421 ,  1422 ,  1423  and  1424 . With this arrangement, the outgoing DS 1  link  112  has DACCS “trouble codes” in DS 0 s  1 - 5 , as shown cross-hatched on line (ii). Similarly, DS 0 s  6 - 15  of link  114  propagate DACCS “trouble codes” as depicted by the “wavy” line symbols on line (iii). Finally, DS 0 s  16 - 24  of DS 1  link  116  propagate DACCS “trouble codes” as depicted by the “asterisk” symbols on line (iv). Such DACCS “trouble codes” would also be transmitted if a hardware failure occurred in Port A of DACCS  141  that precluded the DACCS from passing information through Port A. The purpose of the DACCS-generated “trouble code” is to inform downstream terminal equipment, such as channels banks  132 - 134  in the example above, of a failure of a particular DS 0 . In the above example, upstream equipment, such as channel bank  131 , senses the loss of signal due to the break in DS 1  link  111 . 
     By way of an illustrative example of DACCS “trouble codes”, it is usually necessary to distinguish a DS 1  information frame from a signaling frame. Five out of every six DS 1  frames are information frames. In the information frames, each DS 0  time slot contains eight information bits; the DACCS “trouble code” is a pre-defined, though user changeable, eight-bit pattern (e.g. the DACCS TRB code is “11100100”) with the right-most bit being the least significant bit. In the signaling frames, each channel time slot contains seven information bits and one signaling bit; the same “trouble code” in this case is “1110010X”, where “X” indicates a “do not care” state. (The least significant bit is used for signaling.) 
     B. Specifics of the Present Invention 
     As shown in FIG. 3, digital network  300 , which is illustrative of an embodiment of the present invention, is arranged so that two customer sites  121  and  122  (sites A and B) communicate with each other utilizing a pair of protection devices  310  and  320  to protect against loss of communication due to a facility fault condition(s). Sites A and B are coupled to network  300  by digital channel banks  131  and  132 , respectively; consistent with the example of FIG. 2, five DS 0 s originating at site A are destined for site B and vice versa. Generally, each channel bank and associated device are both customer premises equipment, i.e., both are typically located at corresponding customer sites A and B. For example, channel bank  131  and device  310  are both co-located at site A. 
     Device  310  is coupled to channel bank  131  via interposed DS 1  link  1111 ; the connection of link  1111  to device  310  occurs on Terminal (TERM) port  317 . In turn, device  310 , through Network  1  and Network  2  (NET  1  and NET  2 ) ports  319  and  318 , connects to two independent DS 1  networks  301  and  302  via DS 1  links  1112  and  3112 , respectively. Networks  301  and  302  provide redundant communication paths for signal propagation between sites A and B. Network  301  is the primary or active network which is typically supplied by a primary inter-exchange carrier, whereas network  302  is secondary or backup network which is supplied by another, independent inter-exchange carrier. Network  301  is primary in the sense that it is the network of preference whenever network  300  is initially brought on-line. Network  301  in this illustrative embodiment is composed of DACCS pair  141  and  142  as well as DS 1  link  112  interconnecting this DACCS pair. Typically, DACCS  141  and  142  connect to other DACCS and customer sites (not shown). Similarly, network  302  is composed of DACCS pair  341  and  342  as well as DS 1  link  312  interconnecting this DACCS pair as well as other DACCS and sites (not shown). 
     Device  320 , through its NET  1  and NET  2  ports  329  and  328 , connects to networks  301  and  302  via interposed DS 1  links  1132  and  3132 , respectively. TERM port  327  of device  320  terminates one end of link  1131 , with the other end of link  1131  being connected to channel bank  132 . 
     Device  310  is arranged with transmit circuitry  315  which receives inputs from corresponding TERM port  317  and generates two identical copies of the DS 1  signal generated by channel bank  131 , including the five DS 0 s destined for site B. In turn, these two copies are then transmitted simultaneously from transmit circuitry  315 , via NET  1  and NET  2  ports  319  and  318  of device  310 , over primary and alternate (secondary) networks  301  and  302 . Similarly, device  320  is arranged with transmit circuitry  325  which receives inputs from associated TERM port  327  and generates two identical copies of the DS 1  signal generated by channel bank  132 , including the five DS 0 s destined for site A. In turn, these latter two copies are then transmitted simultaneously from transmit circuitry  325 , via NET  1  and NET  2  ports  329  and  328  of device  320 , over primary and secondary networks  301  and  302 . 
     To exemplify the restoration operation of devices  310  and  320  in order to preclude outages of channels connecting sites A and B (except for a momentary, transitory restoration interval), a failure in DS 1  link  112  (located within primary DS 1  network  301 ) is considered. Prior to the failure, it is presumed that network  301  is active, and network  302  operates in a backup mode. Thus, the actual propagation path of the five DS 0 s emanating from site A to site B, and vice versa, is over network  301  via the electronic coupling of each TERM port with each associated NET  1  port. Failure of link  112  causes a loss of the DS 1  signal normally arriving: (i) on Port A of DACCS  142  for propagation in the direction from site A to site B; and (ii) on Port B of DACCS  141  for propagation from site B to site A. Both DACCS  141  and  142  detect the failure of DS 1  link  112  and, in response, insert a DACCS “trouble codes” in each DS 0  cross-connected to Port A of DACCS  142  and Port B of DACCS  141  including the five DS 0 s propagating on links  1112  and  1132 . These DS 0 s with DACCS “trouble codes” arrive at the NET  1  ports of devices  310  and  320 . Device  310  is arranged with receive circuitry  316  to detect these manifestations of the failure, namely, the DACCS “trouble codes”, arriving at corresponding NET  1  port  319  from DACCS  141 . If a DACCS “trouble code” is present in any of the DS 0 s for a predetermined interval, typically 100 milliseconds, receive circuitry  316  first checks the information bits in the corresponding alternative DS 0  of the other DS 1 . If that DS 0  does not contain the DACCS “trouble code” pattern, the device initiates a switchover to the alternate path. Thus, in this particular case, receive circuitry  316  responds by selecting the five DS 0 s propagating on alternate network  302  as the active DS 0 s, that is, device  310  selects the five DS 0 s from its NET  2  port  318  rather than its NET  1  port  319 . Each device operates independently of the other. Since link  312  (the counterpart to link  112 ) is presumably operational, communication between site A and site B is re-established after the momentary switching transient. Independently, device  320  is arranged with receive circuitry  326  to detect these DACCS “trouble codes” arriving at corresponding NET  1  port  329  from DACCS  142  and respond in a manner similar to receive circuitry  316 . 
     As alluded to above, before any switchover occurs, each device  310  or  320 , through its respective receive circuitry  316  or  326 , ensures that a DACCS “trouble code” is present for 100 milliseconds. If so, then the receive circuitry checks to determine if the information bits in the corresponding DS 0 s in the backup network have been free of a DACCS “trouble code” in a preselected interval, such as the last 10 milliseconds preceding the end of the 100 millisecond interval. Again, if so, then each device initiates the switchover to the backup network. The channels with the DACCS “trouble codes” are declared as having an “unavailable” status. This status is subsequently removed if the DACCS “trouble code” is no longer present for a given time interval, such as 1 second. However, even though the “unavailable” status may be removed, the backup network now becomes the active network, whereas the previously active network now becomes the backup network. In this example, network  301  was originally active, but remains as the backup network even after link  112  is cleared of its fault condition. 
     As another example of the manner of system restoration, an outage in link  1132  is now considered. In this case, device  310  switches on the DACCS “trouble code” whereas device  320  switches all  24  DS 0 s to alternate network  302  based on the loss of the incoming DS 1  signal detected at device  320 . No DACCS “trouble code” is received at device  320 . 
     Also, it should be pointed out that if any of the DS 0 s has a DACCS “trouble code”, all DS 0 s in a fractional DS 1  channel shall switch. 
     Continuing with the description of an illustrative embodiment of the restoration circuitry and operational methodology, the details of a single protection device, namely, device  310 , are shown in block diagram form in FIG. 4; device  320  is realized with substantially-the same circuitry to effect substantially the same functionality. 
     In particular and to simplify FIG. 4, only those components that comprise receive circuitry  316  and operate to effect the required testing, selecting, and switching, if necessary, and thereby restore digital telecommunications between sites A and B are explicitly shown. The remaining components that are needed to implement circuitry  316  would be readily apparent to anyone skilled in the art. As depicted, NET  1  and NET  2  ports  319  and  318  supply incoming DS 1  signals from networks  301  and  302  to DS 0  detector  410  over lines  411  and  412 , respectively. In order to properly recover the fractional DS 1  channels from each of the incoming DS 1  signals, appropriate frame detectors  408  and  409  are required. DS 0  detector  410  then supplies two sequences of DS 0 s, one derived from primary network  301  and the other derived from secondary network  302 , to comparator  415  via lead  416 . Comparator  415  compares the DS 0  signal in the arriving sequences with the known DACCS “trouble codes”. Comparison results are passed to route selector  418  via lead  417 ; the results indicate for the i th  DS 0 , i=1, 2, . . . , 24, which route is to supply the DS 0  signal for the given slot. Comparator  415  also includes appropriate circuitry to monitor both the status of each DS 0  and each DS 1  facility and appropriately change the status of any such channel and facility from being unavailable to available, as set forth above. Incoming DS 1  signals serve as inputs to route selector  418 . Selector  418  establishes a sequence which specifies which route has been selected for the twenty-four distinct DS 0 s and then emits the sequence onto lead  420 . Frame generator  421  then juxtaposes these distinct DS 0 s to form each high-rate DS 1  frame. The output of frame generator  421 , appearing on lead  422 , forms the outgoing portion of DS 1  link  1111 . 
     Copy generator  3151  situated within transmit circuitry  315  generates the two identical copies of the DS 1  signal that is received over DS 1  link  1111  and appearing at TERM port  317  and subsequently transmits these copies to NET  1  and NET  2  ports  319  and  318  for carriage over both the primary and secondary networks. 
     By now those skilled in the art will clearly realize that although the inventive technique has been described in terms of use and incorporation within a private line network and particularly in conjunction with DS 1  links carrying fractional DS 1  channels, its use is not so limited. In fact, this technique can be used in conjunction with any of a wide variety of bi-directional multiplexed communication systems to provide redundant fault tolerant communication in the event of a failure of a multiplexed path in that system. For example, such a system could form part of a geographically dispersed telecommunications or other network or could be used within a multiplexed communication system that provides localized communication such as within a data processing facility or other digital system. 
     Furthermore, although one embodiment of the present invention has been shown and described in detail herein, many other varied embodiments that incorporate the teachings of the invention may be easily constructed by those skilled in the art. Accordingly, use and implementation of the invention is not limited to the specific illustrative embodiment shown herein, but rather by the scope of the appended claims.