Abstract:
Telecommunications Network In a telecommunications network, a level of redundancy is often provided so that in the event of a circuit failure a back-up route may be used. A telecommunications network in which automatic switching of telecommunications traffic to back-up circuits is disclosed herein when faults are detected on a traffic carrying circuit. In particular, when dedicated private circuits develop faults, a network automatically switches traffic to a dedicated back-up circuit. Particular points on incoming and outgoing circuits of a switch are monitored. Faults at particular points result in autonomous switching of traffic to a dedicated back-up circuit. Communication is facilitated between switches by using predetermined signals on the dedicated back-up circuit.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to a telecommunications network and in particular to such a network including back-up circuits. In a telecommunications network a level of redundancy is often provided so that in the event of a circuit failure a back-up route may be used. This invention relates to a telecommunications network in which automatic switching of telecommunications traffic to backup circuits is provided when faults are detected on a traffic carrying primary circuit. In particular, when dedicated private circuits develop faults, this invention provides a network in which automatic switching of traffic to a dedicated back-up circuit takes place. This invention also relates to methods of automatically switching traffic to a back-up route in the event of a circuit failure. 
     2. Description of Related Art 
     In a network requiring protection using back-up circuits, each circuit is monitored within an exchange using a conventional test apparatus, sometimes referred to as an In Service Monitoring Network Element (ISMNE), which is connected to various monitor points on a circuit. Each test apparatus generates an alarm when certain error conditions are detected. Each test apparatus is monitored centrally by a Transmission Network Surveillance system (TNS). When alarms indicate that a traffic carrying circuit has failed, a central restoration co-ordinator determines that errors detected by two test apparatus correspond to a particular traffic bearing circuit and the traffic is switched onto a back-up circuit. 
     The problem with using such a centralized restoration process is that it can take some time for the alarms to be sent to the centralized processor, the alarms to be interpreted and commands to be sent to the exchanges to switch traffic to a backup circuit. Clearly, once a circuit has failed, a customer using that circuit has no service until traffic is switched to a back-up circuit. If a technique can be used which allows each exchange to autonomously switch traffic from a failed primary circuit to a back-up circuit, then the time for which the customer loses service can be considerably reduced. 
     BRIEF SUMMARY OF ASPECTS OF THE INVENTION 
     According to a first aspect of the present invention there is provided a telecommunications network comprising a plurality of switching nodes interconnected by a plurality of circuits, each circuit comprising
         a respective forward transmission path and   a respective reverse transmission path;
 
telecommunications traffic signals being transmitted by a switching node from an incoming circuit to another of said switching nodes on a primary circuit of said plurality of circuits;
 
each of said switching nodes including
   monitoring means for monitoring signals on said circuits, the monitoring means being responsive to detection of a fault on said primary circuit to cause said telecommunications traffic to be transmitted on a second of said circuits;   signal generating means for generating a predetermined signal pattern on the forward transmission path of the second of said plurality of circuits; and   means to inhibit switching of traffic between said primary circuit and said second of said circuits if the predetermined signal pattern is not detected on the reverse transmission path of the second circuit.       

     Using this invention traffic is switched from a primary circuit to a back-up circuit autonomously depending on the status of locally monitored signals. The test apparatus at one end of a traffic bearing circuit cannot directly communicate with the test apparatus at the other end of a traffic bearing circuit as they are not directly connected to each other. In this invention such communication effectively takes place via signals on the back-up circuit. Switching takes place in dependence upon the status of local monitor points including the status of a monitor point on the back-up circuit. Switching of traffic to the back-up circuit is inhibited if the status of the local monitor points indicates that a fault is somewhere other than on the traffic bearing circuit. 
     In one embodiment of the invention the predetermined pattern is not transmitted if fault conditions are detected on the forward transmission path of the incoming circuit. 
     In another embodiment of the invention the predetermined pattern is not transmitted if fault conditions are detected on the reverse transmission path of the second circuit. 
     The inhibiting means may be further arranged to inhibit such switching if the predetermined pattern is not being transmitted. 
     The monitoring means may be further arranged to be responsive to detection of telecommunications traffic on said second circuit, to cause telecommunications traffic to be transmitted on said second of said circuits. 
     The inhibiting means may be further arranged to inhibit switching of telecommunications traffic unless detected telecommunications traffic is preceded by said predetermined pattern signal. 
     Additionally the monitoring means may be further arranged to be responsive to detection of said predetermined pattern signal followed by telecommunications traffic on said second circuit, to cause telecommunications traffic to be transmitted on said second of said circuits. 
     According to a second aspect of the present invention, there is provided a method of restoration control in a communications network in which at least some primary circuits are associated with respective back-up circuits, the method comprising the steps of:—
         continuously monitoring both ends of a back-up circuit for the presence of a fault condition, and, while a fault condition is detected at a said end;   preventing, at said end, the switching of traffic from the associated primary circuit to the back-up circuit.       

     Preferably, the step of continuously monitoring both ends of a back-up circuit for the presence of a fault condition comprises continuously monitoring said ends for the correct receipt of a test signal, and, while the correct receipt of a test signal is not detected at a said end;
         preventing, at said end, the switching of traffic from the associated primary circuit to the back-up circuit.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A telecommunications network in accordance with the invention will be described, by way of example only, with reference to the accompanying drawings in which:— 
         FIG. 1  shows a schematic diagram of an example of a telecommunications network; 
         FIG. 2  shows a schematic diagram of two of the exchanges of  FIG. 1  showing interconnection thereof; 
         FIG. 3  is a flow chart showing the sequence of events showing how remote switching to a back-up circuit is inhibited under certain conditions; 
         FIG. 4  is a flow chart showing the sequence of events which causes traffic to be switched to a back-up circuit as a result of a locally detected fault; and 
         FIG. 5  is a flow chart showing the sequence of events which causes traffic to be switched to a back-up circuit as a result of another exchange having previously switched traffic to a back-up circuit. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 1  there is shown a plurality of network terminating equipments (NTE)  2  connected to a fully interconnected high speed digital telecommunications network comprising a plurality of service switching points (SSP)  3 , also known as switching nodes, and arranged to carry traffic in accordance with the transmission protocol known as Plesiochronous Digital Hierarchy (PDH). The network  1  is a global network, and the SSPs  3  are situated in respective countries. The transmission links interconnecting the SSPs  3  are shown as single hop links, but in practice will involve intermediate networks, such as network  36  of FIG.  2 . 
       FIG. 2  shows two SSPs  3  in the form of exchanges  28 ,  28 ′, interconnected by a two-way primary circuit  26  and a two-way secondary, i.e. back-up, circuit  27 , via a transmission network  36 . At the exchange  28 , a primary circuit  19 , forming part of a transmission link between exchange  28  and a preceding exchange (not shown), is terminated by a receiver  21  and a transmitter  23 . The output of the receiver  21  is connected to the input of an associated transmitter  20 , and the input of transmitter  23  is connected to the output of an associated receiver  24 . 
     The transmitter  20  and the receiver  24  are connected via a switch  22  of the exchange  28  to the two-way primary circuit  26 , and, similarly, at the exchange  28 ′, a transmitter  20 ′ and a receiver  24 ′ are connected via a switch  22 ′ of the exchange  28 ′ to the two-way primary circuit  26 . Associated receiver  21 ′ and transmitter  23 ′ are connected to a primary circuit  19 ′ forming part of a transmission link between exchange  28 ′ and a following exchange (not shown). Such primary and secondary circuits are constituted by allocated timeslots in a time division multiplexed digital highway forming part of a higher order transmission scheme, as is known in the art. The respective switches  22 ,  22 ′ enable traffic signals from the transmitters  20 ,  20 ′ to be switched between the primary circuit  26  and the secondary circuit  27 . Respective pattern generators (P)  41 ,  41 ′ are connected to the secondary circuit  27 . When switching takes place to connect the transmitters  20 ,  20 ′ and the receivers  24 ,  24 ′ to the secondary circuit  27  the pattern generators  41  are connected to the primary circuit  26 . Instead of the circuit  19 ,  19 ′ being part of an inter-SSP link, it can be the or part of the link from an NTE  2 . 
     In normal operation, traffic signals from the transmitter  20  are transmitted through the switch  22 , then via the primary circuit  26  though the transmission network  36 , through the switch  22 ′ to the corresponding receiver  24 ′. The operation of the forward path of the primary circuit  26 , with respect to the exchange  28 , is monitored using a non-intrusive tee-piece at monitor point  34  at the exchange  28  and similarly at monitor points  32 ′ and  33 ′ at the exchange  28 ′. Similarly, the operation of the other path of the primary circuit  26 , with respect to the exchange  28 , is monitored at monitor point  34 ′ at the exchange  28 ′ and at monitor points  32  and  33  at the exchange  28 . The pattern generator  41  generates a predetermined data pattern of all zeros, which is transmitted through the switch  22  via the secondary circuit  27  through the transmission network  36  to the exchange  28 ′. The operation of the secondary circuit  27  is monitored at a monitor point  35  at the exchange  28  at the incoming end of the reverse path of the secondary circuit  27  and at a corresponding monitor point  35 ′ at the exchange  28 ′, at the far end of the forward path of the secondary circuit  27 , i.e. with respect to the exchange  28 . 
     Each exchange  28 ,  28 ′ contains a respective test apparatus  30 ,  30 ′ which is connected to its exchange monitor points  32 ,  33 ,  34  and  35 , ( 32 ′,  33 ′,  34 ′ and  35 ′). The test apparatus  30 ,  30 ′ can detect various conditions of a monitored signal. These include Loss of Signal (LOS), Alarm Indication Signal (AIS), Excessive Error Threshold exceeded (EET), Loss of Frame Alignment (LFA) and receipt of a data signal having the predetermined pattern. 
     The test apparatus  30 ,  30 ′ are connected to a Central Management Unit (CMU)  29 . This connection is shown in  FIG. 2  as being direct, but in practice is via the public switched telephone network. Data relating to the performance of the exchange and the status of the monitor points is transmitted to the CMU at intervals, for example every 15 minutes. If the status of any of the monitor points indicates a fault condition then an alarm condition is reported to the CMU. To minimize the amount of data transmitted, only the highest priority alarm is reported for any one event. Not all conditions detected at the monitor points necessarily generate alarms, some are simply recorded for later analysis. 
     The conditions which are used in this invention are now described more fully. Loss of Signal is detected when the signal level becomes very low, for example, when the detected signal has a signal level of less than or equal to a signal level of 35 dB below nominal. Alarm Indication Signal is detected on the receipt of a predetermined alarm signal, for example, all ones, which is generated by NTEs  2  when a local fault is detected. Receipt of such a predetermined alarm signal indicates that there is a fault elsewhere in the telecommunications network, i.e. not at the exchange  28 . In the PDH, traffic signals are formatted into sequences of frames which commonly comprise a predetermined header part prepended to an information data part. Commonly, one or more parity bits are appended to the information data part. An errored frame may be detected, for example, if there are errors in the prepended header data part, or if the parity bit(s) indicate(s) an error. An Errored Second (ES) is defined as any second in which an error is detected, and a Severely Errored Second (SES) is defined as a second in which the average error rate is greater than 1 per 1000 bits. The number of errors detected are monitored over a predefined period of time and if a predefined number of errors is exceeded then an Excessive Error Threshold (EET) condition is detected. The EET may be defined, for example, as more than 15 SES&#39;s or more than 150 ES&#39;s for a 15 minute period. Loss of Frame Alignment is detected when a number (e.g. 3) of consecutive prepended header data signals contain errors. 
     With regard to the predetermined pattern, the test apparatus  30 ,  30 ′ is arranged to provide an “alarm” signal when the signal having that predetermined pattern is not present. In other words, if a “one” were to be detected in the signal received on the back-up circuit  27 , this would indicate an error. In alternative embodiments of the present invention other forms of predetermined pattern are used. 
     The detection at monitor points  35  and  35 ′ of the predetermined pattern generated by the pattern generators  41  and  41 ′ indicates that the back-up circuit can be used for transmission of telecommunications traffic data if so required. 
     The pattern generators  41 ,  41 ′ are controlled by the ISMNE  30  to apply or remove the predetermined pattern which is used to indicate that the back-up circuit is free of faults and thus available for use. When telecommunications traffic is switched to the back-up circuit by the ISMNE  30 , the back-up circuit  27  becomes the primary circuit and the faulty primary circuit  26  becomes the back-up circuit. The pattern applied by the generators  41 ,  41 ′ is restored on the originally faulty circuit  26  if the fault clears. 
     Referring now to  FIG. 3  together with FIG.  2 .  FIG. 3  is a flow chart showing the sequence of events which result in removal or restoration of the expected predetermined pattern from the back-up circuit  27 . Monitor points  34 , and  35  are continually monitored. If prespecified fault conditions are detected at either of these points at step  311  then the pattern generator  41  is caused to remove the expected predetermined pattern from the back-up circuit at step  312 . 
     Removal of the expected predetermined pattern inhibits the remote exchange  28 ′ from switching traffic to the back-up circuit as shown in  FIGS. 4 and 5  and discussed more fully below. 
     If any one of a number of prespecified fault conditions, namely LOS, LFA, AIS and EET, is detected at monitor point  34 ,  34 ′ then this indicates that a fault has occurred which is external to the network  36  or the exchanges  28 ,  28 ′, so switching traffic to the back-up circuit  27  would serve no useful purpose. If loss of the predetermined pattern is detected at monitor point  35 , or any of fault conditions LOS, AIS and EET, then there is a fault on the back-up circuit  27 . Again, switching traffic to the back-up circuit would serve no useful purpose. 
     If the conditions detected at step  311  at monitor points  34  or  35  are no longer any of the prespecified fault conditions, then a determination is made regarding whether or not a predetermined pattern is on the back-up circuit at step  313  and then the pattern generator  41  is caused to restore the predetermined pattern to the back-up circuit at step  314 . 
     Referring now to  FIG. 4  together with FIG.  2 .  FIG. 4  is a flow chart showing the sequence of events which causes traffic to be switched to the back-up circuit as a result of a locally detected fault. If one of a number of prespecified fault conditions is detected at monitor point  33  at step  411 , then, to check whether the fault is likely to have resulted from the incoming traffic signal on the primary circuit  26  or from a faulty monitoring apparatus for the monitor point  34 , it is ascertained whether there is a corresponding condition at monitor point  32  at step  412 . If there is not, then it is assumed that monitor point  35  is in error, and an appropriate alarm is sent to the CMU  29  at step  416 . If there are corresponding prespecified fault conditions at points  32  and  33  then traffic would normally be switched from the primary circuit to the back-up circuit and the pattern generator  41  may be switched to transmit the predetermined pattern on the primary circuit at step  414 . However, if switching has been inhibited by the detection of the absence of the predetermined pattern on the back-up circuit, which is detected at monitor point  35  at step  413 , then such switching will not take place. 
     If such a switch occurs, the original primary circuit  26  becomes the back-up circuit, and the original back-up circuit  27  becomes the primary circuit, with a corresponding change in the function of the monitor points  33  and  35 . Clearly, if the fault is persistent then the prespecified fault condition will cause the pattern to be subsequently removed from the faulty circuit  26  as described above with reference to FIG.  3 . 
     If there is a fault detected at point  32  only, then there is a fault in the switch  22 , or monitor point  32  is in error, and an appropriate alarm is sent to the CMU. The prespecified fault conditions detected at monitor points  32  and  33  which cause traffic to switch to the back-up circuit in this embodiment are LOS, LFA, AIS and EET. 
     It will be appreciated that the monitoring at monitor point  33  must have the same capability as at monitor point  35  so that at any time either circuit  26  or circuit  27  can be the primary circuit. 
       FIG. 5  is a flow chart showing the sequence of events which causes traffic to be switched to a back-up circuit as a result of a remote exchange having previously switched traffic to a back-up circuit. If a traffic frame is detected at monitor point  35  at step  511  then the remote exchange  28 ′ has switched traffic to the back-up circuit due to a fault detected by the ISMNE  30 ′. Normally, the expected predetermined pattern would be detected right up to the detection of the traffic frame, and it would not be possible for the ISMNE  30 ′ to switch traffic from the primary circuit onto a faulty back-up circuit, i.e. at a time when the pattern generator  41 ′ had been inhibited. However, a manual override facility at the exchange  28 ′ might be incorrectly operated by network management personnel resulting in traffic being switched from the primary circuit to a faulty back-up circuit. Thus, step  512  provides a check on whether the pattern was detected immediately before the detection of the traffic on the back-up circuit, and, if not, then an appropriate alarm is sent to the CMU at step  515  and traffic is not switched to the back-up circuit. Otherwise, traffic is switched from the primary circuit to the back-up circuit and the pattern generator  41  is switched to transmit the predetermined pattern on the primary circuit at step  513 . 
     In an alternative embodiment, the pattern generator  41  generates a different pattern from that generated by pattern generator  41 ′, and it will be appreciated that the present invention requires only that the two directions of the back-up circuit are continuously monitored for a fault condition, and that this can be achieved with identical patterns or with different patterns in the two directions. In other words, the pattern generators are providing a pseudo traffic signal of known data content to exercise the back-up circuit continuously. 
     The following is a summary of how the status of the monitor point affects any action taken in this embodiment of the invention where:
     1 indicates that a prespecified fault condition has been detected at that monitor point   0 indicates that a prespecified fault condition has not been detected at that monitor point   T indicates a traffic frame has been detected   P indicates the expected predetermined pattern has been detected   N indicates no signal has been detected   * indicates the status of that particular monitor point does not affect any action taken   

     The last line of this summary is in respect of no signal (predetermined pattern) detected at monitor point  35  simultaneously with the pattern generator  41  generating the predetermined pattern. If no signal is detected at monitor point  35 , this will cause the pattern generator  41  to be switched off, but in practice there will be a very small delay between this detection and the cessation of the pattern, and should a fault be detected and verified on the primary circuit during this brief period then the decision whether to switch from the primary circuit to the back-up circuit cannot be based on the state of the pattern generator alone, but must also include whether or not there is a received signal at monitor point  35 . 
     
       
         
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Action taken 
               
             
          
           
               
                   
                 expected 
                 Switch 
               
               
                 Monitor Points 
                 predeter- 
                 traffic 
               
             
          
           
               
                   
                   
                   
                   
                 signal 
                 mined 
                 to back-up 
                 Send alarm to 
               
               
                 34 
                 35 
                 32 
                 33 
                 at 35 
                 pattern on? 
                 circuit? 
                 CMU? 
               
               
                   
               
               
                 * 
                 1 
                 * 
                 * 
                 * 
                 N 
                 no 
                   
               
               
                 1 
                 * 
                 * 
                 * 
                 * 
                 N 
                 no 
               
               
                 0 
                 0 
                 * 
                 * 
                 T 
                 Y 
                 yes if 
                 if no expected 
               
               
                   
                   
                   
                   
                   
                   
                 expected 
                 predetermined 
               
               
                   
                   
                   
                   
                   
                   
                 predeter- 
                 pattern detected 
               
               
                   
                   
                   
                   
                   
                   
                 mined 
                 before traffic 
               
               
                   
                   
                   
                   
                   
                   
                 pattern 
                 frame, switching 
               
               
                   
                   
                   
                   
                   
                   
                 detected 
                 inhibited at remote 
               
               
                   
                   
                   
                   
                   
                   
                 before 
                 end =&gt; alarm 
               
               
                   
                   
                   
                   
                   
                   
                 traffic 
               
               
                   
                   
                   
                   
                   
                   
                 frame 
               
               
                 0 
                 0 
                 0 
                 0 
                 P or 
                 Y 
                 no 
               
               
                   
                   
                   
                   
                 N 
                   
               
               
                 0 
                 0 
                 1 
                 0 
                 P or 
                 Y 
                 no 
                 conditions at 32 
               
               
                   
                   
                   
                   
                 N 
                   
                   
                 and 33 do not 
               
               
                   
                   
                   
                   
                   
                   
                   
                 correspond 
               
               
                 0 
                 0 
                 0 
                 1 
                 P or 
                 Y 
                 no 
                 conditions at 32 
               
               
                   
                   
                   
                   
                 N 
                   
                   
                 and 33 do not 
               
               
                   
                   
                   
                   
                   
                   
                   
                 correspond 
               
               
                 0 
                 0 
                 1 
                 1 
                 P 
                 Y 
                 yes 
               
               
                 0 
                 0 
                 1 
                 1 
                 N 
                 Y 
                 no 
                 switching to 
               
               
                   
                   
                   
                   
                   
                   
                   
                 protection circuit 
               
               
                   
                   
                   
                   
                   
                   
                   
                 inhibited at remote 
               
               
                   
                   
                   
                   
                   
                   
                   
                 exchange (due to 
               
               
                   
                   
                   
                   
                   
                   
                   
                 prespecified fault 
               
               
                   
                   
                   
                   
                   
                   
                   
                 conditions detected 
               
               
                   
                   
                   
                   
                   
                   
                   
                 at 34′ or 35′) 
               
               
                   
               
             
          
         
       
     
     The embodiment described here uses a test apparatus which monitors a digital binary data signal in a packet switched telecommunications network, although it will be appreciated that the invention could be used in an analogue network, or in a digital network which is not packet switched.