Troubleshooting Non-fiber Related Problems in Your Link

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Fiber optic communication systems consist of more than just a physical fiber optic link. These links begin and end at electro-optic systems that transmit and receive the signals, and may connect through segments of coax and twisted pair cables, microwave, satellite, or radio.

While each situation is different, troubleshooting must always begin with an examination of the entire network. A technician must look at the big picture to rule out other potential trouble spots before suspecting the fiber or other passive optical components.

Here we look at three common networks:

  • Enterprise networks in campus and building applications

  • CATV networks utilizing analog formats for video signals

  • Telephone networks implementing multiplexed digital systems for voice signal transmission

Enterprise Networks

Premises networks on campuses, at data centers, or at private organizations must serve a large number of users that can change on a daily basis. The first line of defense against network problems and outages is system monitoring software that is overseen by the network manager. The software dynamically monitors all connections across the network looking for those that display excessive bandwidth use or an error rate that is greater than normal.

Network monitoring software is designed to run without operator intervention. Typically, the management workstation actively polls all of the devices on the network every five minutes. If a device misses a poll cycle, an audible alarm is sounded on the network manager’s console. The alarm also triggers automated email and voice mail alerts to be sent to service personnel for immediate response. Network managers can then perform an analysis at the management console to determine the magnitude of the problem.

A number of tools are routinely used to assist in network troubleshooting. The simplest and most convenient of these is the ICMP ping. If one of the interfaces does not respond to the ping, then generally it is a hardware or physical problem. Telnet is used to log into devices for configuring and viewing their status.

Community Antenna Video (CATV) Systems

Most hybrid fiber coax CATV systems support video, voice, and high-speed Internet services. Technicians use a variety of test and measurement techniques to verify network performance. RF sweep generators and receivers are utilized to align the two-way broadband amplifiers that carry signals over the coaxial portion of the network. Spectrum analyzers measure the performance of individual carriers and scan the RF spectrum for unwanted signals — such as ingress or harmonic distortions — that can degrade the quality of the signal.

Digital signal performance is measured with a bit error rate (BER) tester that should include both pre and post error correction measurements. In addition, video signals that use quadrature amplitude modulation schemes are also measured for modulation error rate to determine the quality of signal performance.

Many cable operations have incorporated network operation centers (NOC) that enable full time monitoring of network performance. If a network device fails, the NOC can identify the affected device and direct the repair technician to the precise location of the failure to quickly resolve the problem. Identifying problems early can avoid or significantly reduce costly network downtime later.

Telephone Networks

In telephone applications, many types of communications circuits make up the network. Some of these circuits link the telephone company’s central office to their subscribers while others may traverse the country. Because of the high speeds, vast distances, and high densities involved, some method of testing and analyzing a circuit from end to end is required.
BER testing verifies the integrity and quality of not only the media but the hardware and option settings throughout the circuit, and should be performed as part of the turn up procedure. An end-to-end BER test will travel through channel banks, cross-connects, and various fiber terminals of different companies.

At one location a digital test set can insert a test pattern of ones and zeros to a loopback and receive its transmitted signal. During testing, all of the low speed interfaces are essentially stressed by the test signal passing through. The particular pattern that results from the bit errors will indicate the type of trouble. It is common on a new turn-up for a test pattern to be inserted and left on the system for days with the expected result to be zero bit errors.

In response to alarms or errors, a regional test center will access the circuit via software, and perform tests. The owner of the affected network is then contacted to resolve the problem. The local technician may use a loopback and BER tester to further isolate the trouble. A test set may also be inserted in a circuit to monitor live traffic to detect framing errors.

In Conclusion

The first step for proper troubleshooting is to rule out all possible system related causes, then continue to troubleshoot the optical components of the network. Among the most common problems with optical links are the transmitters and receivers. Transmitters, especially those using laser sources, are subject to higher electrical and thermal stresses than receivers. Lasers also require a source of clean and stable electrical power, so in some cases the problems can be traced to an inadequate or failed AC or DC power source.

Optical receivers require a signal with a power level between certain maximum and minimum values. Too much light will overdrive the photodetector while too little light prevents the detector from accurately reconstructing the original signals without errors.

Regardless of the type of network, the importance of good documentation, processes, procedures, and training cannot be overstated. Good planning, engineering, installation practices, and maintenance will return enormous benefits over the life of the network.

Our training DVD  Troubleshooting a Fiber Optic Link is a great resource of additional information on testing and troubleshooting techniques.