Beware of Rogue ONTs!

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What is a rogue Optical Network Terminal? A rogue ONT used in FTTx passive optical networks (PONs) is one that doesn’t behave within its expected operation, and therefore creates problems for one or more subscribers and the service provider responsible for the quality of signal received.

To best describe rogue problems is understanding how a PON works, and how to identify and resolve locating rogue ONTs. To begin, the PON works by using a star topology to broadcast optical signals downstream from an optical line terminal (OLT) via optical splitters to a number of ONTs located at subscriber’s buildings. The complexity in all PONs is from the upstream time division multiple access (TDMA) transmission where multiple ONTs transmit within time slots assigned by the OLT.

In developing the various PON standards, the ITU, FSAN, and IEEE working groups addressed proper operation in their standards.

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This is required to provide upstream transmissions while preventing collisions from multiple ONTs. Bandwidth maps (BWmaps) are used to define these upstream time slots.

A rogue ONT is one that transmits outside of its assigned allocated BWmap. Several examples are when upstream transmission occurs outside the specified time slots where the OLT perceives the transmission as invalid data, or collisions occur which impact multiple ONTs. This not only fails to deliver proper service, but also degrades or totally disables the service of other customers on the same fiber network.

What causes an ONT to operate rogue? One is a change in the timing of the transmissions that causes this in which the OLT can identify through its operation, administration, maintenance, and provisioning (OAM&P) functions as defined in the standards. Another can occur within the ONT’s temperature specifications and is often caused by temperature variations caused by the ONT’s uncooled Fabry-Perot lasers. As rogue events may be random, this can be quite frustrating to subscribers and even more so to service providers as they try to resolve it. Generally speaking, it is a hardware failure.

Traditional techniques to identify rogue ONTs involved first troubleshooting the network with using optical power meters to check power levels and making sure all connections were clean. This was then followed by replacing suspect ONTs and seeing if the problem was resolved. This was time-consuming, involved taking subscribers off-line, and didn’t always find the culprit ONT.

Recently, I learned about the use of a type of test equipment which may assist in the identification of rogue ONTs. The equipment performs optical power levels, data rate measurements, and transmission errors along with service disruptions on the systems OLTs and ONTs in the network.

PON networks can be divided into several functional segments: the backbone, the GPON network, and the customer premises. The challenge is to identify the cause and location of the culprit. One tool called the GPON Tracer can be placed in-line either after the OLT or prior to an ONT for troubleshooting.
(See Figures 1 and 2.)

Test showing a good upstream/downstream transmission.

Figure 1. Test showing a good upstream/downstream transmission (Courtesy TraceSpan Communications Ltd.)

Test showing a rogue ONT disrupting two other ONTs.

Figure 2. Test showing a rogue ONT disrupting two other ONTs. (Courtesy TraceSpan Communications Ltd.)

Manufacturers have told me the quantity of rogue ONTs are decreasing, but with increased migrations to next-generation 10G and WDM-PON systems, the proof will be in the pudding.

I’d love to hear more from you as service providers about your experiences. To share ideas, questions, or comments, please email Fiberstory@gmail.com or visit https://www.fiber-story.com. Follow FiberStory on Twitter @Fiber_Story.

 

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About Author

Larry Johnson, President of FiberStory, started his career in fiber optics in 1977, and has written over 20 courses and developed 10 certifications on fiber optics through The Light Brigade which he founded in 1986. Besides his work on various standards groups, he is a member of multiple industry technology committees including the Utility Telecom Council and the Fiber Broadband Association. FiberStory is involved with the history of fiber optics, provides technology assessments to organizations including the outside plant, and represents industry organizations in fiber optic technologies. To share ideas, questions, or comments, please email Fiberstory@gmail.com or visit www.Fiber-Story.com.

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