On Wed, 09 Nov 2011 17:04:05 -0800, John Larkin wrote:

On 08 Nov 2011 17:32:33 GMT, Glen Walpert wrote:

On Mon, 07 Nov 2011 20:08:40 -0800, John Larkin wrote:

On Mon, 07 Nov 2011 19:37:53 -0800, josephkk
wrote:


Here is my first try:

That worked fine.

The page specifies a max prop delay of 555 ns, but doesn't specify the
length of the cable. Sounds like about 100 meters maybe.

The skew spec is awesome: 50 ns, or about 10% of the prop delay.
That's scary.

The method used to control crosstalk in UTP cables is to vary the twist
rate between pairs, so there is a pretty good difference in length of
wire between the pair with the slowest twist and the pair with the
fastest twist. IIRC the construction of Cat cables has both increased
the twist difference and tightened up the twist tolerance with every
increase in speed grade, to I would expect the skew in Cat 6a to be the
worst, but the most predictable.

Apparently you can buy two kinds of CAT6A cable, individually shielded
and not. I'd hope that the individually shielded stuff has the same
twist on all the pairs.

John

There are also two types of shielded CAT6A; fully shielded and semi-
shielded, which consists of unconnected bands of foil shielding not
connected to anything, providing an additional 10 dB reduction in
crosstalk (at 10 GBPS) without requiring a signal common to connect the
shield to. Fully shielded provides even more reduction in crosstalk, but
I am pretty sure all varieties use twist pitch variation also, including
CAT7 which is only available in fully shielded. Shielding is never 100%
effective, so twist variation is still beneficial.

If low skew is more important than low crosstalk then a CAT type cable is
probably not a good choice. You could make your own cable from 4 pieces
of the same single STP cable for very low volumes, or have someone like
Cooner Cable make a custom cable for you.

I might have mentioned this last week but I have been out of internet
range again (Adirondacks). Went out for a short canoe trip yesterday
morning, but had to cut it short when we came to a spot where the choppy
water (20 to 30 MPH wind) suddenly turned smooth as far ahead as we could
see :-).

Glen