DETECTION OF LOOP-BACK CONDITIONS ON FAULTY DIGITAL TRUNKS

Abstract: A method of detecting loopbacks on trunks in digital networks, comprises the steps of detecting an out-of-service trunk, continually placing a predetermined bit pattern on an outgoing channel of a trunk card, continually monitoring a receive channel of the trunk card to detect said bit pattern, and identifying the presence of a loopback when said bit pattern is persistently detected on the receive channel. This method avoids the need to dedicate a trunk channel for signaling purposes. By detecting the presence of loopbacks, intelligent networks can be prevented from unjustifiably clearing an alarm condition and returning traffic to a fault path.

Description: DETECTION OF LOOP-BACK CONDITIONS ON FAULTY DIGITAL TRUNKS This invention relates to the detection of faults on digital trunks, such as Tl or El (DS1) trunks. In digital networks there is an on-going need to perform maintenance and diagnostic operations. One technique employed for this purpose is known as loopback. The transmit pair on a Tl card, for example, is connected directly back to the receive wire pair. If data on the Tl trunk is becoming corrupted, placing loopbacks at various points in the network can help isolate the fault. In order to manage the network, some means must be provided for detecting when loopback occurs . Our co-pending application describes a technique where out- of-band signaling is used to detect the presence of loopbacks. However, services provided by some Carriers require that all twenty-four channels Tl trunk on a Tl carry 64Kbps data. Loopback detection using out-of-band signaling only allows twenty-three Tl channels to carry 64Kps data since one channel must be reserved for the out-of-band signaling. An object of the invention is to remove this restriction. According to the present invention there is provided a method of detecting loopbacks on trunks in digital networks, comprising the steps of continually placing a predetermined bit pattern on at least one outgoing channel on an out-of-service trunk at a network node, continually monitoring at least one receive channel on said trunk at said network node corresponding to said at least one outgoing channel to detect said bit pattern, and identifying the presence of a loopback when said bit pattern is persistently detected on said receive channel. The invention determines when a loopback has been placed on the trunk, which might be a Tl, El or any other primary rate transmission medium that carries channelized information. This information is used by the equipment at the endpoints to determine if it is safe to switch back to the preferred trunk. If there are loopbacks, then the link status on the preferred path is not OK and the traffic remains on the alternate route. Loopback detection occurs only when the link is out of service and none of the 24 channels are being used to carry user data. The invention also provides an apparatus for detecting loopbacks on trunks in digital networks, comprising means for continually placing a predetermined bit pattern on an outgoing channel of a trunk at a network node, means for continually monitoring a receive channel of said trunk at said network node to detect said bit pattern, and means for identifying the presence of a loopback when said bit pattern is persistently detected on said receive channel. In one embodiment, a detector is provided to detect the out-of-service trunk. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 shows a loopback present in an out-of- service Tl trunk; Figure 2 shows a multi-hop network employing loopback detection in accordance with the invention; and Figure 3 shows a Tl card operating in accordance with the invention. Referring to Figure 1, a Tl card 1 is mounted in the backplane of a Newbridge Networks Mainstreet™ 3600 multiplexer. The card 1 has a transmit pair 3 and receive pair 4 that are normally connected to the appropriate terminals of a similar card in a far-end multiplexer. As shown in Figure 1, a loopback 5 is provided between the transmit pair 2 and the receive pair 4 for the purpose of isolating faults in the network. In a conventional network, a Tl card is designed to enter an alarm state when it detects faults on the Tl trunk. The problem with placing a loopback on the trunk is that it can cause the alarm state to clear even though the fault is still present. In an intelligent network, when a Tl failure occurs, the traffic that was carried on the failed Tl link, can be switched onto a back-up or an alternate Tl trunk. Once, the failure has been corrected, a switch-back to the preferred route occurs. An example of a network using preferred and alternate Tl trunks is shown in Figure 2. In Figure 2, nodes A and B in a digital network comprise Mainstreetw 3600 multiplexers shown connected to respective representative terminals 12, 13. The nodes A and B are connected by a preferred Tl path 14 and an alternate Tl path 15. Both paths pass through the network "cloud" 16. When a fault occurs on the preferred path Tl, the Tl card at node A automatically enters an alarm state in a known manner, and redirects the traffic formerly carried over path 14 over alternate path 15. When a loopback 18 is placed in path 14, as shown in Figure 2, this causes the alarm condition in the Tl card at node A to clear this card sees incoming data as if the trunk were operating normally, causing the Tl card to send data out over the faulty path 14. This results in the loss of data. The invention determines the presence of a loopback on the path Tl and ensures that the alarm condition is not cleared in the Tl card at node A while the loopback is present. In accordance with the invention, a known unique data pattern is continually placed onto the link when it is taken out of service. The received data is monitored to determine if it matches the transmitted data pattern. If a match occurs persistently, then a loopback is deemed to have been placed on the link. The unique pattern is a twenty bit pattern consisting of the following information: - major node number (10 bits) of the equipment. For example, each Newbridge Networks 3600 Mainstreet™ multiplexer is assigned a unique node number in a Tl network. The major node number is analogous to a unique address. - slot in shelf. (6 bits ) This is the physical slot in the card rack where the Tl interface card sits. - Tl link number. ( 4 bits ) Each Tl interface card can terminate multiple Tl trunks. This identifies which Tl. The 16 bit pattern has the following format: Tl Channel Pattern Pattern Channel Bits Description Bits TS23 5,4,3 Major Node 9,8,7,6 Number TS22 5,4,3 Major Node 5,4,3,2 Number TS21 5 Major Node 1, 0 Number TS221 3 Slot in Shelf 5,4 TS20 5,4,3 Slot in Shelf 3,2,1,0 TS19 5,4,3 Tl Link 3,2 / 1,0 Number The unique pattern can be carried across networks using RBS or JB7 signaling and not get corrupted. A Tl card in accordance with the invention has an out-of-service detector 20 that detects an out-of-service condition on one of the trunks. This causes bit pattern generator to transmit the above bit pattern on the outgoing pair 3. Bit pattern detector 22 detects the presence of this bit pattern. If it persists, the presence of a loopback on the trunk is identified. The loopback must be detected before the fault clears. To this end the loopback is detected within one second. Once detected the loopback alarm will be cleared 10 seconds after loopback removal. The main advantages of this implementation is that it allows loopback detection on a Tl, El or other primary rate Link that uses all 24 channels to carry user data/voice. On Tl trunks, generally channels 18 to 23 are used for loopback signaling, whereas on El trunks, generally channels 21 to 31 are employed for this purpose. In addition, invention offers an inexpensive means of detecting loopbacks, and does not require proprietary in-band signaling techniques to implement. The invention is thus useful for customers who are using protected connections in their network and do not have a timeslot dedicated to signaling, ϋndesired switch¬ backs to the preferred route are avoided while loopbacks are present.

Claims: Claims : 1. A method of detecting loopbacks on trunks in digital networks, comprising the steps of continually placing a predetermined bit pattern on at least one outgoing channel on an out-of-service trunk at a network node, continually monitoring at least one receive channel on said trunk at said network node corresponding to said at least one outgoing channel to detect said bit pattern, and identifying the presence of a loopback when said bit pattern is persistently detected on said receive channel. 2. A method as claimed in claim 1, wherein the bit pattern identifies the address of the equipment containing a trunk card for the out-of-service trunk. 3. A method as claimed in claim 2, wherein the bit pattern further identifies the physical slot in a card rack where the trunk card is located. 4. A method as claimed in claim 2, wherein the bit pattern further identifies particular outbound faulty trunk emanating from the trunk card. 5. A method as claimed in claim 1, wherein the trunk card is a Tl card. 6. A method as claimed in claim 1, wherein the trunk card is an El card. 7. A method as claimed in claim 1, further comprising the step of identifying said out-of-service trunk prior to applying said predetermined bit pattern. 8. An apparatus for detecting loopbacks on trunks in digital networks, comprising means for continually placing a predetermined bit pattern on an outgoing channel of a trunk at a network node, means for continually monitoring a receive channel of said trunk at said network node to detect said bit pattern, and means for identifying the presence of a loopback when said bit pattern is persistently detected on said receive channel. 9. An apparatus as claimed in claim 8, further comprising means for detecting said out-of-service trunk.

Inventors: KITOWICZ MARK, KUHL TIM, NOLAN BRUCE, KITOWICZ, MARK, KUHL, TIM, NOLAN, BRUCE
Applicants: KITOWICZ MARK, KUHL TIM, NEWBRIDGE NETWORKS CORP, NOLAN BRUCE, KITOWICZ, MARK, KUHL, TIM, NEWBRIDGE NETWORKS CORPORATION, NOLAN, BRUCE
Application year: 1996
Main classification: H04L1
IPC codes: H04J3, H04L1, H04L12, H04M3