David J Taylor wrote: > Garrett Wollman wrote: > >> In article <k9wdns4y5piuymfunz2dnuvz_t2dn...@giganews.com>, >> Richard B. Gilbert <rgilber...@comcast.net> wrote: >> >>> USB is nearly useless for NTP! USB has latencies sufficiently large >>> and variable to render it unsuitable for use with NTP. > > [] > > Garrett, Richard, > > You've both commented that USB has drawbacks, but in reality what > performance might be obtained? Not everyone needs microsecond > precision, and USB might allow millisecond precision - i.e. possibly > better than what might be obtained by Internet access alone, or by using > a radio source. > > Has anyone made any actual tests or measurements? Are there any results > available from an actual installation of USB on either Windows or UNIX?
USB is a polled system. i.e. every action is initiated by the host controller and peripheral "interrupt packets" are just proper answers to a host enquiry. The basic rythm of USB is 1ms. HighSpeed introduces 8 subframes of 125us in this 1ms frame. But bus signaling is at FullSpeed . So peripheral responses are rasterized into this 1ms rythm. Depending on OS you get a plethora of "funnies" added into latency behaviour. The potentially best behaviour can be expected from isochronous transfers. uwe _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
