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.
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.
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.