How to Identify and Isolate a Rogue ONU in a GPON Network

The term 'Rogue ONU' is used in GPON FTTH technology for an ONU or an ONT which not only fails to deliver proper service – but also degrades or totally disables the service of other customers on the same fiber network.

To understand how this happens, let's recall the principles of GPON transmission. For the sake of simplicity we will use the term 'ONU' to represent an ONU or an ONT. The GPON network topology is a tree, where the fiber going from the OLT to the first optical splitter is shared among all the ONUs on the PON. To allow every ONU to transmit its data on one hand and avoid collisions on the other hand, the OLT assigns timeslots for the upstream transmission of each ONU. These timeslot allocations are called bandwidth maps or BWmaps in short. Each ONU must obey these BWmap allocations and transmit in, and only in, these timeslots.

Rogue ONU blog post - picture 1

A rogue ONU is one that transmits outside of its allocated BWmaps, potentially causing two main issues:

  • This ONU's transmission isn't received by the OLT in the expected timeslots, and thus the OLT may regard it as invalid data.
  • Its transmission may collide with valid transmissions of one or more "good" ONUs, making all these transmissions corrupted.

Rogue ONU blog post - picture 2

The GPON shared-network architecture makes the task of analyzing and pinpointing a rogue ONU challenging and extremely consuming in time and resources. The most common way to handle it is by turning off all the ONUs, and then turning them back on one-by-one until the faulty one is found. On top of being time-consuming, this process requires disconnection of traffic to every one of the customers on the PON. Needless to say, this is something that any service provider would like to avoid.

Some OLTs have built-in mechanisms for identifying and disabling a rogue ONU, but we've heard mixed opinions about this kind of solution. One of the major European service providers told us that this solution has created more damage than benefit in its network because it sometimes misidentified good ONUs as rogue ones, resulting in a series of frustrating and costly events for both the service provider and its customers. First the OLT disabled the suspected ONU, thus disconnecting the customer's service; then the service provider had to send someone to the customer's home to replace the ONU; and finally the service provider had to send the ONU back to the vendor for re-activation, and pay for the shipment and the service, which wasn't necessary in the first place!

A more productive approach can be introduced by using an independent testing device, which can passively monitor and display the transmissions of the different ONUs in comparison with the OLT's timeslot allocation. That's the direction we took with the GPON Tracer. It's a handheld testing device, and one of its capabilities is detecting and identifying a rogue ONU, usually without having to disconnect or disable any other ONU in the PON.

5 Responses

  1. Very informative and concise. Would like more information of connection in the GPON network (i.e. connection losses -in series or parallel operation)
    • Hi Nevio Grinzi, Are you asking about the connection of GPON Tracer to the network? Well, it's connected on the PON, either in series or in parallel. Serial connection means that a fiber goes into the equipment and a fiber comes out, so that GPON Tracer closes the optical path and the traffic goes through it. Parallel connection makes use of optical splitters or couplers that are pre-installed on the fiber and are used as connection points for GPON Tracer. The main advantage of the parallel mode is that it doesn't require disconnection of the fiber for hooking up the equipment, meaning no disconnection of the user traffic, but it can only be used when the splitters or couplers are available on the PON. Plus, in this mode GPON Tracer doesn't introduce any loss on the fiber. In the serial mode it does introduce some loss, but it's minimal and usually not more than the spare power budget that operators have on their networks anyway. Oded
  2. How is it possible to detect rogue ONT with the help of GPON tracer(how does the whole mechanism works),and how does Gpon tracer knows about the timeslots allocated to each ONT (in case it uses the timeslots to see which ONT is not transmitting in it's allocated timeslot)
    • Hi Imran Raja, Here is a general description of how it works: 1. GPON Tracer identifies the BWmap allocations in the frames that the OLT sends - this is what tells each of the ONUs when it should transmit. 2. For every BWmap allocation that it identifies, GPON Tracer expects an upstream transmission from the ONUs in the same timeslots. When it identifies an upstream transmission it compares its timeslots with the expected ones and compares the ONU-ID field in the frame with the expected ONU-ID. Based on these comparisons it determines which transmissions are as expected and which - if any - are outside their expected BWmap allocations, indicating a rogue ONU. If you would like more details or are interested to see a demo of GPON Tracer please contact us by sending an e-mail to info@tracespan.com. Oded
  3. Hi, In my scenario it is occurring almost the same problem, as ONU stop working, but the problem goes to another OLT.

Leave a comment