What about Modular DOCSIS 3.0 deployments with external timing sources between the QAM and CMTS
A CMTS DS payload is formatted as an MPEG TS (it even has PIDs; however, no PCR). This in turn establishes cadence for associated downstream devices (eg. they sync to whatever is within allowable tolerances). Any digital 'baseband' output from a so-called 'modular' CMTS (one with external QAM modulation) should likewise serve as a recoverable timing source for the modulator. Inter-DS cadence is allowably async; the only important coupling here are these associated US channels available for a given DS. Unless I've missed something horribly obvious, I can't see where strict timing is important, nor can I see where inter or intra-system synchronization would be critical either, save for corner-cases (i.e. US or DS dynamic channel change, DCC/UDC, etc). Even in these cases, the modem is more than able to rapidly resynchronize with a new/differing cadence in both frequency and phase domains. I guess, in short, don't worry (about this). -Tk