The downstream and upstream data channels are synchronized to the 4 kHz ADSL DMT (Discrete Multi Tone) symbol rate, and multiplexed into two separate data buffers (fast and interleaved).
ADSL uses the superframe structure shown in figure . Each superframe is composed of 68 ADSL data frames, which are encoded and modulated into DMT symbols. From the bit-level and user data perspective, the DMT symbol rate is 4000 baud (period = 250 s). Because of the sync symbol inserted to the end of each superframe, the transmitted DMT symbol rate is 69/68 * 4000 baud.
Figure: ADSL superframe structure
Eight bits per ADSL superframe are reserved for the crc, and 24 indicator bits (ib0-ib23) are assigned for OAM functions. The "fast" byte of the fast data buffer carries either crc, eoc or synchronization bits.
Each user data stream is assigned to either the fast or the interleaved buffer during initialization.