Re: Equalization and benefits of Parallel Optics (skew correction)
At 12:19 PM 0/7/28, Roger Ronald wrote:
>It is quite possible to get rid of skew between fibers as a factor that limits
>distance. I speak with some authority here, since our standards committee
>(NCITS T11.1) has a 12-channel parallel 6.4 Gbit networking standard
>(HIPPI-6400) that can compensate for up to 10 nanoseconds of skew.
>This is done dynamically, with timing adjusted every 10 usecs.
>Works great. We don't care much about skew any more. Jitter is our big
>distance limiter since we have a discreet clock and our jitter budget must be
>divided between two signals (clock and data).
Hmm. Ten nanoseconds maximum skew corresponds to (10*10^-9
sec)(2/3)(3*10^8 m/s)= 2.0 meters of fiber, so in a 300-meter
parallel-fiber cable, the optical lengths of all fibers in the cable would
need to agree to within 2/300= 0.7%, if fiber mismatch is allowed to take
the entire skew budget. If the link is limited to 100 meters, then the
fiber optical lengths must agree to within 2/100= 2%. These accuracies
seem a bit tight for field installations. One would probably have to use
an OTDR, and trim each fiber to fit.
How many fibers are in parallel? What's the maximum length of a HIPPI-6400
cable? What does such a HIPPI cable cost, as a ratio to say a typical
duplex fiber cable of the same length?