Abstract:
In a communication network, a method and apparatus is provided to detect the presence of loopback errors over facilities interconnecting switches of the network. According to the test protocol, an originating switch generates a tone on a forward trunk associated with the facility and monitors the return trunk for the presence of that tone. If the return trunk contains the tone, the originating switch fails the facility for a loopback condition. The loopback test protocol complements conventional continuity tests and results in greater reliability of communications networks.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of checking the integrity of a communications network and, more particularly, to a method of detecting a loopback condition in the communications network. 
     A communication network may be populated by a number of switches, as shown in FIG.  1 . The switches are typically interconnected by optical or electrical cables (“facilities”) that carry communication traffic. In a time division multiplexed (TDM) scheme, facilities carry communication data for one call in a time slot (“trunk”) multiplexed with data for other calls in other time slots. For two way communication, a call is assigned two trunks. A forward trunk on a first facility  10  carries data in a forward direction, such as from an originating switch  100  to a terminating switch  200 ; a return trunk on a second facility  20  carries data in a reverse direction, such as from the terminating switch  200  to the originating switch  100 . By convention, the same trunks of the two facilities are assigned to the same call. For example, if trunk no. 2 of facility  10  is assigned as the forward trunk, trunk no. 2 of an associated facility  20  is assigned for the return trunk. 
     Control signals typically are used for call setup and tear down in communication networks (i.e., to establish call paths through the network during call setup and termination). Although communication traffic from an originating switch  100  is transmitted to a terminating switch  200  over trunks, the trunks do not carry control signals. To setup and terminate calls, an originating switch sends call setup information over an A LINK  30  to a signal transfer point (“STP”)  300 . The STP  300  communicates the call setup information to the terminating switch over a second A LINK  40 . Through this out-of-band signaling, the originating switch  100  and terminating switch  200  confirm that each is operating and establish trunk assignments for the new call. 
     Out-of-band signaling does not permit the originating switch  100  or the terminating switch  200  to confirm the integrity of the facilities that interconnect them. Although the originating switch  100  and terminating switch  200  communicate over the A LINK, they would not be able to determine whether one or more of the facilities that interconnect them are damaged. If facility  50  is severed, for example, communications data placed on the facility  50  by the originating switch  100  would be lost; it would never reach the terminating switch  200 . Although the terminating switch  200  would expect to receive communication traffic over the facility  50 , the absence of communication traffic on the facility would not normally generate an alarm condition and/or a report. 
     To detect facility failures, it is known to conduct a continuity test in which, before carrying communication traffic, the originating switch  100  instructs the terminating switch  200  via the A LINK to switch all the data that it receives on the forward trunk to the return trunk. The originating switch  100  generates a test tone on the forward trunk and monitors the return trunk to detect the tone. The originating switch  100  determines that the facilities are operable when it detects the test tone on the return trunk. When it confirms that the facilities are operating properly, the originating switch  100  completes the call setup process. 
     The continuity test, while it is useful to detect certain facility failures such as open conditions, is vulnerable to other network failures, such as loopback conditions. A loopback condition occurs when a facility originates and terminates at the same switch, shown as  60  and  70  in FIG.  1 . Most often, loopback conditions are caused by human error during facility installation or maintenance. The facilities are configured incorrectly. When a continuity test is run on a facility  60  that loops from a switch back to the same switch, the continuity test generates a “false positive;” the originating switch  100  detects the test tone on the “return trunk” even though the forward trunk failed to reach the terminating switch. 
     Although a loopback facility  60  cannot carry data between the originating switch  100  and the terminating switch  200 , the continuity test cannot identify this type of equipment failure. The limitations of the continuity test result in unsuccessful calls and customer dissatisfaction. 
     Accordingly, there is a need in the art for a test protocol in communications networks that detect the presence of loopback conditions. Further, there is a need in the art for a continuity test protocol that tests for loopback conditions and maintains the viability of the continuity test protocol. 
     SUMMARY OF THE INVENTION 
     The disadvantages of the prior art are alleviated to a great extent by a test protocol that tests for loopback conditions according to the following method: An originating switch engages a normal call setup operation to communicate with and confirm the operation of the terminating switch. The originating switch generates a test tone on the forward trunk. Unlike the continuity test, the originating switch does not instruct the terminating switch to switch the forward trunk to the return trunk. If the originating switch monitors the return trunk and detects the test tone, the originating switch fails the facility due to the presence of a loopback condition. 
     The loopback test protocol complements and works with the continuity test. The tests may be run in succession to detect facility integrity and the absence of loopback errors. For example, while setting up on a first call, the continuity test may confirm the viability of the facility through the known continuity test. Then, on a subsequent call, the loopback test procedure may be run. By alternating the continuity and loopback tests, the present invention detects both error conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 discloses a communications network for use with the present invention. 
     FIG. 2 discloses a first test protocol incorporating an embodiment of the present invention. 
     FIG. 3 discloses a second communication network for use with the present invention. 
     FIG. 4 discloses a second test protocol incorporating an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIG. 2, there is shown a test protocol employed by the originating office  100  according to the present invention. Call setup procedures begin according to customs known in the art (Step  1000 ). At some point during that procedure, forward and return trunks are assigned to the new call on facilities between the originating switch  100  and the terminating switch  200 . After the trunk assignments are made, the originating switch  100  generates a test tone on the forward trunk (Step  1010 ). The originating switch  100  monitors the return trunk (Step  1020 ) and determines whether it contains the test tone (Step  1030 ). If the return trunk does not contain a test tone, the originating switch  100  allows call setup to complete normally (Step  1040 ). 
     However, if the originating switch  100  determines at Step  1030  that the return trunk does contain the test tone, the originating switch  100  fails the facility as having the loopback condition (Step  1050 ). Preferably, the originating switch  100  generates a report and/or alarm indicating a failure of the facility. The originating switch  100  then determines whether other facilities are available to complete the call (Step  1060 ). If other facilities are available, the originating switch  100  selects another facility and trunk pair (Step  1070 ) and repeats the test protocol starting with Step  1010 . If the originating switch  100  determines at Step  1060  that other facilities are not available, the originating switch  100  fails the call setup procedure according to conventional procedures (Step  1080 ). 
     The test protocol of FIG. 2 finds application between any two switches of a network that are connected directly by one or more facilities. For example, in the embodiment shown in FIG. 3, a loopback condition may exist between an end office  400  and a first network switch  500 , between the first network switch  500  and a second network switch  600 , and between the second network switch  600  and a second end office  700 . In this example, the end office  400  could be considered the originating switch to test the facilities between it and the first network switch  500 . Similarly, the first network switch  500  could act as the originating office to test facilities between it and the second network switch  600 . The test protocol of the present invention may be extended for as many switches as are found in the network. Note that in a situation where the end office  400  acts as the originating switch, if the end office  400  proceeds to Step  1040  to complete call setup, the end office  400  may have to wait for other switches in the network also to complete the loopback test protocol. 
     The test protocol of FIG. 2 consumes network resources and, therefore, is not run each and every time an originating switch  100  completes call setup. In the preferred embodiment, the originating switch  100  performs the test protocol at some predetermined rate, for example, a few times per day or as a percentage of the number of calls that the originating switch  100  sets up on a specific facility. Also, because lopback conditions frequently are caused by human error that occurs during facility installation or maintenance, the loopback test protocol preferably is run immediately after such installation or maintenance. 
     In a preferred embodiment, the loopback test protocol operates in conjunction with the continuity test protocol, shown in FIG.  4 . Call setup procedures begin according to known customs. Referring to FIG. 1, after the forward and return trunks are assigned, the originating switch  100  determines whether a continuity test or a loopback test is to be performed (Step  2000 ). If a loopback test is to be performed, the originating switch  100  generates a tone on the forward trunk in accordance with Step  1010  of FIG.  4  and proceeds through the loopback test already described with respect to FIG.  2 . If the originating switch  100  determines at Step  2000  that a continuity test should be performed, the originating switch  100  enters a continuity test protocol. The originating switch  100  also may determine that no test should be run, in which case the originating switch  100  completes call setup normally. 
     To perform the continuity test, the originating switch  100  instructs the terminating switch  200  over the A LINK to connect the forward trunk to the return trunk (Step  2010 ). In effect, the terminating switch  200  generates a temporary loopback condition through the terminating switch  200 . The originating switch  100  generates a test tone on the forward trunk (Step  2020 ). The originating switch  100  monitors the return trunk (Step  2030 ) and determines whether the return trunk contains the test tone (Step  2040 ). If so, the originating switch  100  completes call setup normally (Step  1040 ). 
     If the originating switch  100  does not detect the tone on the return trunk, the originating switch  100  fails the facility for a continuity error (Step  2050 ). Preferably, the originating switch  100  generates a report and/or an alarm signal representing an error with respect to the facility. The originating switch  100  then determines whether other facilities are available to connect the new call (Step  2060 ). If another facility is available for the new call, the originating switch  100  selects a trunk on the other facility for the new call (Step  2070 ) and repeats the continuity test protocol at Step  2010 . If additional facilities are not available, or if the facility failure is repeated a predetermined number of time such as twice, the originating switch  100  drops the call and discontinues call setup (Step  1080 ). 
     According to the present invention, a method and apparatus is provided which detects the presence of loopback failures in facilities that interconnect switches in a communication network. Also, the method and apparatus provide a means to detect loopback errors and yet also detect continuity errors between such switches. The present invention achieves benefits in that continuity and loopback errors in the communications network are detected before subscribers are connected through such faulty hardware.