AU1995992A - Fault detection and isolation of fiber to the curb systems - Google Patents

Description

Fault Detection and Isolation of Fiber to the Curb Systems

Fault detection and isolation are key considerations in the design of fiber to the curb (FTTC) and similar systems. Deployment of fiber in the telephone local loop requires network availability comparable to that of today's copper networks. To achieve this, redundancy needs to be provided in high capacity elements of the network, but automatic fault detection and isolation capabilities required to take advantage of the redundancy are just as important. Automatic isolation of failures significantly improves network availability and reduces maintenance costs through accurate and timely dispatch of repair personnel.

Ellersick et al., U.S. patent application serial no. 07/697,855, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference, discloses a method of translating multiple signaling nibbles contained within a signaling byte into multiple signaling bytes for use in a FTTC system. This patent application teaches using available bits in the translated FTTC signaling bytes for data coding checking so as to achieve a degree of fault isolation and detection capability. The present invention discloses improved means and methods for further detecting and isolating faults which preferably are usable in conjunction with the aforesaid cited patent application teachings to achieve very significant detection and isolation fault capabilities.

An object of the invention is to provide a technique (method and apparatus) for detecting and isolating faults, preferably in a fiber to the curb (FTTC) system. This technique uses overhead or unused time slots in conjunction with pattern generation and verification hardware to perform bit error rate tests on digital time division multiplexed data paths. This allows fault diagnosis without interruption of service, especially telephone service.

Two types of time slots are preferably used to diagnose faults in each half of head end multiplexer electronics, e.g. office interface unit (OIU). The first type is an unused time slot made available by running an OIU multiplexer clock slightly faster than is necessary to provide time slots for the full data capacity of the system(s) serviced by the OIU. The second type is an overhead time slot normally used for establishing framing information on a standard multiplexed digital trunk, such as the European 2.048 Mbps CCITT standard interface or the US 1.544 Mbps Tl standard interface, connected to the OIU from a central office, or remote extension thereof.

An OIU that interfaces to multiple digital trunks from a central office (CO) or remote extension thereof and provides single line interfaces at a subscriber interface unit (SIU), located near customer or subscriber premises, must synchronize all of the trunks to one OIU-SIU frame clock to allow synchronous interconnection of single line channels. This frame clock is passed through the OIU-SIU system using separate hardware signals within the OIU and by transmitting an OIU- SIU framing time slot over the fiber to the SIU fiber interface, the fiber framing time slot preferably being identified by a code violation of a specific byte. Thus, framing information for each digital trunk need not be carried past an OIU point where the digital trunks are synchronized to the OIU-SIU system frame clock, and time slots associated with each digital trunk can then be made available by the OIU as overhead fault diagnosis channels.

In the receive direction from the CO to the OIU, synchronization to the system frame clock is usually accomplished in a digital framer chip that uses a two frame deep receive first-in-first-out (FIFO) memory that uses frame clock information to clock data in from each digital trunk, and system frame clock information to clock data out to the rest of the FTTC system toward the SIUs. In the opposite transmit direction, the FIFO is not needed as data is transmitted synchronous to the system frame clock, with the framing channel for each digital trunk being regenerated from the system frame clock in the OIU digital framer. Digital framers such as ones made by Base2 Systems, Inc. allow individual 64 kbps channels to be looped back internally. In the transmit direction, this internal loop back capability is implemented before the framing channel is generated in the transmit direction. In the receive direction, this internal loop back is implemented after framing information from the framing channel in the digital trunk is used to clock data into the receive FIFO. According to a preferred embodiment of the invention, the framing channel is looped back toward the SIUs and used for testing without affecting the use of the framing channel on the digital trunk. By providing pattern generation and verification circuitry near cross connect circuitry of the OIU, the time division multiplex (TDM) data path from the cross connect circuitry all the way through most of the digital framer can be checked, without requiring service to be interrupted as would otherwise be the case.

A preferred object of the invention is to provide a fault diagnosis apparatus for a telecommunication system that transmits and cross- connects telephone signals between multiple telephone exchange office feeder lines and telephone subscriber equipment, comprising:

means for retransmitting the telephone signals in a digital time division multiplex format from the multiple feeder lines to the subscriber equipment and vice versa at a transmission rate in excess of that required by a quantity of telephone voice channels and telephone signaling channels received from the multiple feeder lines so as to create at least one spare time slot channel in a retransmission frame which is not necessary for transmitting voice or signaling information;

pattern generation means for transmitting a predetermined bit pattern in the at least one spare time slot channel;

pattern verification means for receiving the bit pattern in the spare time slot channel and determining an absence or presence of a bit error therein. These and other objects of the invention will be further explained by reference to the following drawings and detailed description.

FIG 1 is a block schematic diagram of a fiber to the curb system;

FIG 2 is a block diagram of an office interface unit illustrated in FIG 1;

FIG 3 is a block diagram schematically illustrating system loop backs for an office interface unit and a subscriber interface unit illustrated in FIG 1; and

FIG 4 is a further block diagram of the subscriber interface unit illustrated in FIG 1.

FIG 1 shows a typical digital FTTC system 1, consisting of exchange feeders 2, an Office Interface Unit (OIU) 3, an Operations and Maintenance system (O&M) system 4, distribution fibers 6, Subscriber Interface Units (SIUs) 7 and subscriber loops 8. Fault detection and isolation strategies for digital services and plain old telephone services (POTS) are first discussed for such a system 1, followed by fault detection and isolation algorithms for failures inside the FTTC system.

ISDN and other digital services have built-in cyclic redundancy check (CRC) checksums which provide excellent fault detection. The exchange or subscriber equipment will detect failures of FTTC system elements for ISDN and other digital services, using the CRC checksums passed through the entire system. Using CCi^'lT recommended maintenance channels, the exchange is relayed failure information from CRC checkers in the Line Interface Units (LIUs) in the SIUs as well as in the subscriber equipment. With this information, and the use of loop backs in each LIU, failures can be isolated with good confidence to the exchange, exchange feeders, the FTTC electronics, subscriber loop or subscriber equipment. The FTTC system is also relayed failure -6 - information from CRC checkers in the exchange and subscriber equipment, and can thus distinguish between FTTC system and external failures. Isolation of internal FTTC system failures is discussed in more detail below.

POTS presents unique and difficult fault detection problems, as there is no capability for fault detection at the subscriber equipment 10 (telephone), and CRC checksums on digital feeders are generally terminated at the OIU's line interface units 11 (LIUs), as they cannot be maintained through cross-connect circuitry in the OIU. Thus, internal fault detection on POTS is needed throughout an FTTC system, and the present invention is directed to fulfilling this need.

The invention combines three fault detection techniques: CRC checksums and coding propagated through the system, data comparisons between redundant modules, and non-service affecting loop back tests. After detection, emphasis is placed on fault isolation techniques that clearly identify the failed module. These techniques rely on loop backs at module and subsystem interfaces, comprehensive self tests, and on utilization of shared Passive Optical Network (PON) topologies to isolate failures to head end or remote equipment.

Digital feeders and LIUs are tested by CRC checksums and line coding generated and verified in the LIUs at each end of the feeder. By carrying intemal data coding or checksums up to the analog POTS LIUs in the SIUs, an FTTC system can achieve nearly instantaneous fault detection on all data paths carrying more than one phone call.

Faults are detected on POTS LIUs and subscriber loops by running tests at service turn up or on demand. LIU tests include analog reflection and idle channel noise tests. Subscriber loop tests include foreign voltage, impedance and leakage current tests. Failures on POTS service channels can be isolated to the LIUs or subscribers loops by running digital BER and analog reflection tests. Loop failures can be isolated to customer premise wiring by detecting demarcation circuitry at the end of the subscriber loop.

To allow comprehensive firmware to be developed with finite resources, the fault isolation approach of the invention focuses on tests which clearly isolate faults to field replaceable units (FRUs), such as bit error rate (BER) tests with loop backs at FRU interfaces or comprehensive FRU self tests.

In a preferred FTTC system, service is carried digitally, mostly in time division multiplexed (TDM) data paths. According to the invention, spare time slots are generated within the OIU by running an OIU multiplexer frame clock faster than is necessary to provide time slots for a full data capacity of systems serviced by the OIU. More specifically, according to a preferred embodiment, an OIU intemal bus is run at a speed in excess of 5 megahertz, giving a capacity of 640 time slots, or nominally 512 DSOs plus 120 ABCD signaling byte time slots with 8 spare time slots, otherwise referred to as available overhead, as also more fully described in the copending application cited above. BER tests on these spare time slots provide an inexpensive, effective solution for detecting faults on TDM data paths. With loop backs implemented near FRU interfaces, these BER tests can also be used to isolate faults.

According to the invention, each ABCD signaling byte is cross- connected in the OIU to its own dedicated time slot or channel so that, in the example indicated, for transmission feeder lines which include 120 ABCD signaling nibbles, generally stacked in 60 time slots, the OIU connects these 120 signaling nibbles into 120 signaling time slots. Accordingly, since each signaling time slot generated within the OIU contains 4 bits corresponding to the ABCD signaling information, 4 additional bits remain unused which can be utilized for fault detection, for example by utilizing a binary 1 or 0 checksum algorithm. --r-

While BER tests using "robbed" in service time slots are a standard technique for fault diagnosis, the allocation and use of overhead or spare time slots for BER testing according to the invention is new and provides excellent fault detection and isolation capabilities without service interruption, as occurs when time slots are robbed.

BER tests on unused or overhead time slots are an inexpensive, effective solution for detecting faults on TDM data paths. With loop backs implemented near FRU interfaces, these BER tests can also be used to isolate faults. Space division multiplexed data paths (e.g. cross- connect memories) can be effectively checked by comparing the outputs of redundant hardware.

Failures of clock generation hardware can take down service to all customers served by the system, and can be difficult to isolate with BER tests. To ensure rapid fault isolation and recovery, clock generation circuitry is perhaps most effectively checked by comparing the outputs of redundant modules. Disagreements between redundant hardware modules that compare their outputs can be resolved with comprehensive self tests, or by checking the results of nonintrusive BER tests with each of the disagreeing modules in service.

Faults on digital feeders or distribution fibers can be isolated by looping back at FRU interfaces. PON-based systems can utilize knowledge of their topology to isolate failures between SIUs and shared distribution fibers, as an SIU failure will leave other SIUs error free.

Where possible, a processor's functions should be limited to maintenance and provisioning of service, and associated service carrying hardware should be designed to continue operating under processor failures. Thus, processor or firmware failures do not add to system downtime. In addition, redundancy is not required on non- service affecting processors, reducing hardware costs and firmware complexity. Fault detection on processors is provided by watchdog timers and periodic polling of distributed processing elements. - β- Refeπing to FIG 2, the OIU comprises a shelf with plug-in

FRUs. The FRUs comprise El Modules 13 (ElMs) which transfer data between 2048 Kbps lines 2, a Global Cross-connect Module 14 (GCM), and a timing generation module 17 (TGM). The GCM programmably connects data channels between the El Ms and the Distribution Fiber

Modules 15 (DFMs), and also contains pattern generation and verification circuitry for automated or on demand testing of the system.

Central Processing Module 16 coordinates fault isolation, provisioning of service, and interfaces with the O&M system 4. An OIU backplane which interconnects the FRUs has redundant timing and data busses to prevent loss of service due to a driver or receiver failure and contains no active components to minimize the chance of field backplane failures.

The TGM and GCM each compare their outputs with redundant hardware, e.g. a redundant TGM and GCM, providing excellent fault detection coverage, and use comprehensive self tests and redundancy switching to isolate faults to the failed FRU. If the two GCMs' or TGMs' outputs do not match, the offline board is self tested. If the self test fails, the offline board is labelled "faulty" and the fault has been isolated. Otherwise, the offline board is switched in and the previously online board is self tested. If it fails self test, it is labelled "faulty". If both boards pass self test, and there are no secondary failure indications that point to one of the boards, they are both labelled "suspect", the previously online board is switched back online, and a manual fault isolation procedure is necessary.

For digital services, E1M and DFM faults are detected through CRC checks in the LIUs at the exchange, El Ms, SIUs and customer equipment. Once a failure has been isolated to the FTTC system as described above, BER tests from the known good GCM (due to redundant GCM output comparison) on the failed time slots isolate the failure using loop backs at FRU interfaces (see FIG 3). For POTS, the El Ms loop back unused time slots toward the GCM, in their LIUs just before CRC checksums and coding are added. The GCM performs BER tests in the background on the unused time slots through each LIU on the El Ms to detect ElM errors up to the LIUs. With a known good GCM, a BER test failure indicates an ElM failure, and no further isolation is needed. The CRC checksums and line coding on the 2048 Kbps lines are checked in the LIUs in the exchange and the El Ms, detecting failures of the LIUs or exchange feeders, which are isolated using loop backs close to the line interfaces of the LIUs.

Again for POTS, coding is added and checked on data between the GCM and SIUs. Failures in the DFMs will result in coding errors, which are isolated by taking advantage of the shared optical architecture of the FTTC system. Coding errors on a single SIU's data indicate an SIU failure, whereas coding errors on multiple SIUs indicate a DFM or fiber failure, which is further isolated to the DFM or fiber with good confidence using loop backs on the DFM.

All processors are non-service affecting, eliminating the need for redundant processors and associated hardware cost and firmware complexity. Failures of the CPM or embedded controllers in other boards or their firmware are detected (and isolated) by watchdog timers and periodic polling by the CPM.

The SIU comprises a Fiber Interface Unit 21 (FIU) and Line Cards 22 for various services, as shown in FIG 4. The SIU backplane again has no active components to minimize the chance of field failures.

For digital services, FIU and Line Card faults are detected through CRC checks in the LIUs at the exchange, El Ms, SIUs and customer equipment. Once a failure has been isolated to the FTTC system as described above, BER tests from the OIU on the failed time slots isolate the failure using loop backs at FRU interfaces. For POTS, coding is checked on data between the GCM and SIUs.

Failures in an SIU FIU or POTS Line Card's digital path will result in coding, errors, which can be isolated to the SIU as described above. SIU errors can be further isolated to the FIU or Line Card using BER tests from the OIU and loop backs at the Line Card/FTU interface.

Sophisticated self tests of POTS Line Card analog circuitry, and analog loop tests preferably are implemented in the SIU, and are performed on demand through the O&M system.

Though the invention has been described by reference to a fiber- to-the-curb system whereby subscribers are connected to SIUs, the invention is not dependent on any particular distribution architecture or topology, e.g. bus, star, PON, etc. Also, the invention, described by reference to an El transmission format, is not to be limited to only this format and applies to any format, i.e., Tl. Accordingly, the invention is not to be limited by reference to any particular preferred embodiment described, rather it should only be limited by the appended claims.

Claims (4)

What is claimed is:

1. A fault diagnosis apparatus for a telecommunication system that transmits and cross-connects telephone signals between multiple telephone exchange office feeder lines and telephone subscriber equipment, comprising:

means for retransmitting the telephone signals in a digital time division multiplex format from the multiple feeder lines to the subscriber equipment and vice versa at a transmission rate in excess of that required by a quantity of telephone voice channels and telephone signaling channels received from the multiple feeder lines so as to create at least one spare time slot channel in a retransmission frame which is not necessary for transmitting voice or signaling information;

pattern generation means for transmitting a predetermined bit pattern in the at least one spare time slot channel;

pattern verification means for receiving the bit pattern in the spare time slot channel and determining an absence or presence of a bit error therein.

2. The apparatus of claim 1, the pattem generation means and pattem verification means being located in a common field replaceable unit module.

3. The apparatus of claim 1, the pattem generation means and pattem verification means being located in different field replaceable unit modules.

4. The apparatus of claim 2 or 3, the field replaceable unit module being selected from a group of modules consisting of an El feeder module, a global cross-connect module, a timing generation module, a - II- distribution fiber module, a line interface unit module, a fiber interface

5 unit module, and a line card module.

6 i 5. The apparatus of claim 1, the telephone voice and signaling

2 channels comprising a plurality of DSO channels, each signaling channel

3 received for retransmission including first and second ABCD signaling

4 nibbles, each retransmission signaling channel generated by the

5 retransmitting means containing only one ABCD signaling nibble

6 therein with four additional bits being useable for fault diagnosis. 7 i 6. The apparatus of claim 1, the retransmitting means creating a

2 plurality of the spare time slot channels.

3 i 7. A fiber to the curb telephone system including an office interface

2 unit for receiving telephone signals from multiple telephone central

3 office feeder lines and retransmitting these signals to telephone

4 subscriber equipment, the system comprising:

5 β means for retransmitting the telephone signals in a digital time

7 division multiplex format from the multiple feeder lines to the

8 subscriber equipment and vice versa at a transmission rate in

9 excess of that required by a quantity of telephone voice channels 0 and telephone signaling channels received from the multiple feeder lines so as to create at least one spare time slot channel in a retransmission frame which is not necessary for transmitting voice or signaling information;

pattern generation means for transmitting a predetermined bit pattem in the at least one spare time slot channel;

pattern verification means for receiving the bit pattem in the spare time slot channel and determining an absence or presence of a bit error therein.