Question by Peter McArthur posted 02 Mar 2006
| twisted pair questions|
My question is a little unusual, apart from standard networking I use both cat5 & cat6 for media extenders ie. DVI,USB,A/V etc. mostly I have issues with DVI extenders running 1920 x 1080 res or 1600 x 1200. With the pair twist ratio differing particularly on Cat6 cable does anyone know the different frequency response or impedance that each pair gives? the reason I am asking is that some equipment specifies T-568-A and some T-568-B wiring configuration. So apart from the obvious colour code differences what exactly is it that the differentiates the 2 specifications? I mainly use Belden 700A for cat5 and Belden data and media twist for Cat6.
Answer by Dmitri Abaimov posted 03 Mar 2006
The delay skew parameter in the cabling standards is actually set for the worst case between all four pairs in a cable and not between any two particular pairs. Beyond the standard, the manufacturers are free to use whatever twisting schemes they see fit. As a matter of fact some of them even patent their twisting schemes. So, with that said, you cannot really expect a particular standard delay from pair #1, then another standard delay from pair #2 and so on. I guess, you can reasonably expect that pairs #1 and #4 are the least twisted (which may not be true with newer cables anyways) and therefore is the shortest and either pair #2 or #3 are the longest due to tighter twist. However, invariably some cables will have #3 shorter, and some will have #3 longer of the two, and these two are the only ones different between T568A and T568B.
It all boils down to the requirement to have enough memory buffer at the receiving end to hold the signals for at least 45 nanoseconds for an extenders designed specifically for CAT5E and 25 nanoseconds for a CAT6 one (which I haven’t really seen yet – all I know of can work on both media) so the slowest traveling signal can arrive in time.
Additionally, there is a lot of circuitry in those media extenders – there are operational amps, line drivers, buffers etc., and each component adds probably more delay than the cable itself. This is what explains sometimes drastic difference in price between different media extenders.
All of these problems, of course are made worse when you go for a higher screen resolution because you require more bandwidth from all the pieces of the system – not only the cable.
So, to (finally! ;-) directly answer your question: I think that the requirement to use a T568A vs. T568B is only caused by the way the PCB of the extender was laid out, and that only made sense with the cable the manufacturer used in their lab. As soon as it gets out into the real world, the device should work with all the different cables out there and therefore it has to be made for worst case scenario anyways, not matter the pin layout.
Here is an interesting read from the manufacturer of the ICs used in some of those video extenders
Dmitri Abaimov, RCDD
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