The TX submodules translate each received user frame through the TX interface to an Aurora 8B/10B frame. The start of frame (SOF) is indicated by the addition of a 2-byte SCP code group at the beginning of the frame. The end of frame (EOF) is indicated by the addition of a 2-byte End of Channel Protocol (ECP) code group at the end of the frame. Idle code groups are inserted whenever data is not available. Code groups are 8B/10B encoded byte pairs and all data are sent as code groups, so user frames with an odd number of bytes have a control character called PAD appended to the end of the frame to fill out the final code group. Table: Typical Channel Frame shows a typical Aurora 8B/10B frame with an even number of data bytes.