GPON was designed to offer ability to use any third-party or compatible ONT. Full interoperability means that OLT supplier shared OMCI stack, OAM and PLOAM control packets with ONT supplier and both party verify compatibility. Reality is that such a situation is very rare as most of the OLT suppliers protect they market share and they don’t cooperate with ONT suppliers unless they have to. Therefor Broadband World Forum define two standard as like OLT-ONU interoperability test (BBF WT-255) and ONT Certification BBF.247 program to encourage vendors too interoperability.
Second and most common approach is that ONT manufacturer try to make IoP by his own as generally OMCI has been standardized via ITU-T G.988 but also in other standards as like G.984.4. Unfortunately not every vendors is complaint to it as sometimes they use own Management Entities. These create some sort of the gap that complicated interoperability and makes customer scary. Because who is going to support us when somethink when wrong?
These Second approach is no that scary if operator define its service scenario. In most cases Telco don’t need fully features that are implemented in OMCI of it's OLT vendor and part of the ONT management can be moved to TR-069 or SNMP.
On the market there are several (expensive) PON control packet sniffers that can help in troubleshooting and interpretation of GPON Interoperability.
Market show that they are couple examples and Fibrain already has experience in similar project were interoperability was mandatory. Please contact us for reference letter from our customers.