1. Huawei Enterprise. (2020). GPON Technical White Paper. Section 1.2, "GPON System Composition." This section defines the roles: "An OLT is an aggregation device located at the central office (CO) for terminating the PON protocol." and "An ONU is user-side equipment... It communicates with the OLT." This establishes the OLT as central and the ONU as peripheral, defining the downstream (OLT to ONU) and upstream (ONU to OLT) directions.
2. Huawei Enterprise. (2020). GPON Technical White Paper. Section 2.2, "GPON Downstream Transmission." This section explicitly states: "In the downstream direction, data is broadcast from the OLT to all ONUs." This confirms the OLT's role in downstream transmission.