Fiber automated monitoring systems improve maintenance

March 1, 1999

Fiber automated monitoring systems improve maintenance

As fiber traffic escalates, proactive maintenance of cabling infrastructure is facilitated by remote monitoring systems.

Kurt Galantha

Peter Lancier GmbH & Co. KG

The transmission of light through fibers is influenced by several factors. But only the characteristics that alter the attenuation during the fibers` operational life are of interest to maintenance staff. Impurities or manufacturing-related variations of the transmission capacity are considered part of the basic route conditions. Higher attenuation is caused by bends (when the cable is forced, squeezed, or otherwise maltreated), aging splices, ingress of water (hydrogen diffusion, modification of micro-cracks, deterioration of the coating, ice cracks), loose connectors, and more. In addition, transmission interruptions can result from a variety of reasons ranging from earth moving for construction projects to bird shooting (a missed target spares the bird but spoils the fiber) to a pull on the wrong connector.

Economic reasons for monitoring

Whenever the performance of a fiber-optic cable`s transmission path is negatively affected or interrupted, fault-localization activities are initiated despite the considerable costs for staff, instruments, transport, and keeping the required infrastructure available. If the cable routes are monitored continuously, however, increases in attenuation are detected early and corrective measures can be taken in due time. Data about faults and their location also enables the network provider to avoid expensive traffic interruptions. Even in the case of a break, the monitoring system allows immediate detection of the location. It`s more efficient and cost-effective than sending a technician with an optical time-domain reflectometer (OTDR) to one end of the cable.

A trace of the faulty section can be requested from the monitoring system at any time. Together with a database that includes route maps and geographic information, the traces and provided data streamline fault localization and speed up maintenance. The economic benefits of a monitored infrastructure include minimized downtime, fewer performance problems, and less maintenance staff, resulting in higher quality standards at lower costs.

FAMOS design

To adapt to specific network layouts and monitoring requirements, fiber automated monitoring systems (FAMOSs) should feature a modular design (see Fig. 1). A power supply, central processing unit, and control module for the optoelectronic components are common equipment. Additionally, data storage elements and operational software are necessary for operation of the FAMOS. The type of communications module (modem, Ethernet adapter, and others) depends on the network used for the interconnection of the FAMOS with the supervision centers located at the maintenance offices. A hardwired interface connector allows a maintenance technician to log in locally with a notebook computer.

The OTDR module is chosen according to the required dynamic range and specified monitoring wavelength. Generally, 1625 nm is used for fiber surveillance because it monitors a lit fiber without interfering with the signals in the standard transmission wavelengths (1310 and 1550 nm). It also ensures a high sensitivity to alterations of the monitored fiber path. By means of an optical switch, the OTDR signal is distributed to all the fibers monitored by the FAMOS. The size of the switch is, therefore, determined by the number of fibers.

Finally, the connection panel provides the interface to the monitored fiber-optic cable. Dark fibers are connected directly to an output of the optical switch via a connector, while active fibers require a wavelength-division multiplexer (WDM) to separate the transmission wavelengths from the monitoring wavelength.

Won`t play in traffic

Injecting and extracting the OTDR signal of the FAMOS via a WDM ensures that the transmission equipment will not be influenced by the monitoring system. If the transmission unit at the far end of the monitored fiber includes a receiver, an optical filter stops the OTDR signals from entering and avoids interference. If only a transmitter is located there, no filter is required. A WDM and a filter can be part of the FAMOS connection module, or if additional attenuation should be minimized, they can be spliced into the fiber path (at the cable termination rack, for example). In that case, the WDM`s add/drop output of the monitoring wavelength is connected to the panel in the same way as a dark fiber.

Cascades monitor distribution networks

A FAMOS cascade is often used when the optical signal at the remote end is distributed (see Fig. 2), for example, when a high-bit-rate signal of one fiber is transferred to several fibers with low-bit-rate signals. To include the secondary fibers departing from the distribution point in the monitoring process, the output of these fibers is added to that of the primary fiber, one after the other, by an optical switch at the distribution point. An optoelectronic remote control steps the switch into the desired position to compose the OTDR measurement. The step command is transmitted either as an electric signal via an interstice pair or as an optical signal sent by the OTDR module through the primary fiber. The optical data "telegram" transmitted using the monitoring wavelength is redirected by the WDM at the entry of the distribution device from the signal path to the bypass. There, it passes through a splitter, where a small portion of the signal is routed to the receiver of the optoelectronic remote control. The telegram is decoded and translated into a step command for the optical switch where the OTDR signal is directed to the desired output.

On the secondary side of the distribution device, the OTDR signal is re-injected into the fiber by means of another WDM, thus forming a fiber route that includes the primary and secondary fiber. Due to the considerable investment in the WDMs, splitter, optoelectronic control, and optical switch, the use of a fiber monitoring cascade needs to be economically justified by elevated costs in the case of faults in the secondary fiber paths.

Sometimes the attenuation of fiber-optic cable routes is too high even for OTDRs with the highest dynamic range. The backscattered signal from the remote part of the route is below the noise level. In this case, two FAMOSs, which pick up overlapping attenuation traces, can be placed at the ends of the route to enable the monitoring of the whole length. To avoid damaging the OTDRs` sensitive optical receivers with powerful signals from the adjacent FAMOS, a synchronization logic (such as optosync) has to ensure that only one system works at a time.

Supervision centers

At the supervision centers, all information associated with alarms is needed by the maintenance staff to allow detection of alarm conditions, analysis of fault situations, evaluation of severity, and planning of remedy actions. Therefore, a graphical user interface and easy handling of the data are musts.

Cable routes and other maps of monitored outside-plant areas should integrate incoming data and any alarms referring to the map elements. Wherever possible, existing maps from other computer programs should be transferred or used as background for the creation of special-purpose maps. Effortless drawing of system maps and creation of user-specific macros is an important system requirement. Relevant data such as distances need to correlate with the route maps; the program uses this information for evaluation processes initiated by the user. Based on the route-map data resident in the supervision center equipment and correlated data received from the monitoring stations, the graphic presentation of traces and profiles supports fault analysis and localization.

Automatic reports from the monitoring stations to the supervision centers update the database. All alarm messages are distributed according to the alarm assignment programmed in the monitoring equipment. In some cases, it may be useful to include pagers in the list of terminals to be addressed by the monitoring equipment, so staff on duty is made aware of upcoming alarms.

Events recorded at the supervision station can also offer relevant fiber statistics. A rather complex program will allow the user to define the parameters for selective data collection, to create statistical data indicating the number and type of faults per cable, per exchange, per region, per day, per week, per month, per year, etc. Compressed information from regional supervision centers may be forwarded to a central supervision center, depending on the user`s organizational structure. u

Kurt Galantha is the sales manager for network surveillance systems at Peter Lancier GmbH & Co. KG (Münster, Germany).

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