Cable Modem Theory

by: Rolf V. Østergaard

Upstream Data Formats - Page 11

Upstream data is arranged in short bursts. The DAVIC/DVB standard requires a fixed length burst, whereas the MCNS standard specifies variable length bursts.

Since the upstream data is just a short burst of data, the demodulator needs something to trigger on. That is the unique word, that is prepended to the data. For DVB/DAVIC the unique word is 32 bit of data that triggers the demodulator to demodulate the burst.

Without the unique word, the demodulator could easily start to demodulate various noise signals etc. And then be all busy doing that when the real data arrives. Also the unique word provides resynchronisation at every burst.