Method and apparatus for improved upstream frame synchronization in a passive optical network

In a passive optical network, upstream transmission frames from an ONU to an OLT can include additional delimiters that assist in overcoming high signal distortion at the beginning of the frame that can obscure an initial frame delimiter. The second delimiter can be inserted into the frame at various locations within the frame known to the OLT such that the OLT can active a delimiter detector at the beginning of a timing window for delimiter detection. The ONU can assist the OLT in synchronizing the frame by ensuring that a frame header immediately follows a second delimiter or appears at the start of a first FEC code block following the second delimiter. Depending on where the ONU inserts the second delimiter in the frame generation process, the second delimiter may be scrambled and/or may form part of the FEC.