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Homemade cat5 cable using existing phone line fails.

J

James Thompson

Jan 1, 1970
0
Jeff Liebermann said:
The cazapitance per foot is about the same for each pair (I just
measured it on a 500ft roll) at about 30pf per ft. For CAT6, each
pair is solvent welded together at a constant distance making it
constant capacitance. The relatively loose twist rate does not
contribute any change in cazapitance.

The purpose of the different twist rates is to reduce coupling between
pairs. The basic specification is about 11dB NEXT or Near End Cross
Talk. The twisting drastically reduces the external coupling from
each pair. If the pairs were all wound at the same rate, they would
have a much larger number of points of contact between adjacent pairs.
It's these points of contact that cause the most coupling between
pairs due to simple capacitive coupling at the point of contact.
Reduce the number of points of contact and the coupling goes down.

By using non-twisted pair telco wire instead of CAT5, the worst case
cable is created. It has the maximum points of contact (the entire
length), the worst case NEXT as it makes a great distributed
transformer, and the worst loss because the non-twisted pairs will
radiate somewhat more.

I've actually seen commercial cables with such wiring. It's
legitimately CAT3 cable (which usually has a silver colored jacket)
and has 4 unpaired wires in the jacket. Some vendors (Asante) used to
bundle those with their MacIntosh ethernet (AAUI) adapters. I found
an entire skool computer lab connected with the junk. It works so-so
with 10baseT-HDX, but fails miserably with FDX (full duplex) or
100baseTX. I tossed something like 50 cables in the trash and
replaced them with home made CAT5 cables. Unfortunately, someone
fished them out of the trash and they found their way back all over
the skool. Sigh.

I've also used 25 pair telco wire for ethernet. That works just fine
for 10baseT-HDX, but also fails on the better or faster protocols.
Because it's twisted pairs, it doesn't leak too badly. Like the real
CAT5, the various pairs are twisted at different rates to reduce
crosstalk. I've never bothered too measure the impedance or NEXT as
I'm sure it varies and isn't even close to the CAT5 100 ohms. However,
it works well enough for 50ft or less runs.
Ok, I always figured it was a capacitive coupling problem, and is why they
twist them at different rates. Kinda like having tuned l/c tanks so the
signals would not crosstalk so bad. But what you said about contact points,
make a lot of sense too. Always open to new info. JTT
 
S

SMS

Jan 1, 1970
0
Jamie said:
yes, i have looked at it, i have maintain the machines and worked with
the management that are involved in getting the twinning machines
set up to run this product.
they are all twisted the same rate. the only difference is when the
pairs get bunched together, we have a process to make sure they are
coupled together at offset points.

That works too, but using different twist rates eliminates the need for
offsets, which reduce save manufacturing cost.
 
J

Jamie

Jan 1, 1970
0
Doug said:
Did you ever look at it? The different TPI is pretty visable on any
cat5 I've installed/used over the years.

Lets just cut some open I have here and count?

Lets see, I cut off a 10" long chunk and in my sample I have..

Blue Pair = 1.0 TPI
Orange Pair = 1.3 TPI
Green Pair = 1.6 TPI
Brown Pair = 0.7 TPI

7 twists in 10" vs. 16 twists in 10" is pretty visable to me.
yes, i have looked at it, i have maintain the machines and worked with
the management that are involved in getting the twinning machines
set up to run this product.
they are all twisted the same rate. the only difference is when the
pairs get bunched together, we have a process to make sure they are
coupled together at offset points.
it must work because it passes all test our engineering department
put on it along with QC.
 
D

DecaturTxCowboy

Jan 1, 1970
0
Jeff said:
Unfortunately, someone
fished them out of the trash and they found their way back all over
the skool. Sigh.

That why I *always* snip the cable in half - ethernet, phone cord,
handset cord....
 
J

Jeff Liebermann

Jan 1, 1970
0
Jamie said:
i have seen our competitors wire and they do have their pairs
slightly staged in twist. we don't do that , we group the pairs
in a special order when bundling them. this grouping apparently
gives the same results.
in fact while i am composing this, i have open up a cat5 teflon
cable made by us and i can say the twist are no more than maybe an 1/8
of inch
difference between each other and this is most likely due to the
consistency of our twinning machines.

I didn't know that anyone made teflon CAT5. PTFE has a higher
dielectric constant than polyethylene. That means the outside
diameter of each pair is smaller for PTFE than for polyethylene. With
a smaller diameter, it would be possible to twist all 4 pairs in the
opposite direction as the twisted pairs, and have a fairly small
number of points of contact. Also, the smaller diameter insures that
it will not bloat beyond the 0.250" O.D. limit. However, I see a
potential problem making CAT6, where the wires in each pair have to
bonded or solvent welded together. PTFE doesn't that too well.
 
J

Jamie

Jan 1, 1970
0
Jeff said:
Jamie <[email protected]> hath wroth:

Summary of EIA-568 at (because the EIA wants $750 for the printed or
CDROM version of the standard):
http://www.commserv.ucsb.edu/infrastructure/standards/history/EIA-TIA_568.asp
Pair Assembly
The pair twists of any pair shall not be exactly the same as
any other pair. The pair twist lengths shall be selected by the
manufacturer to assure compliance with the crosstalk requirements
of this standard.

Every CAT5 cable that I've worked with has had different twist rates
on each of the 4 pairs. ANSI/TIA/EIA-568 does NOT specify the twist
rate, but simply leaves it to the manufactory to optimize. If you can
meet it with identical twist rates, I'll be suitably impressed.
well then you should notify our engineering staff because they
obviously have something wrong.
i have seen our competitors wire and they do have their pairs
slightly staged in twist. we don't do that , we group the pairs
in a special order when bundling them. this grouping apparently
gives the same results.
in fact while i am composing this, i have open up a cat5 teflon
cable made by us and i can say the twist are no more than maybe an 1/8
of inch
difference between each other and this is most likely due to the
consistency of our twinning machines.
i remember years ago when we were optimizing the process it was a
big deal on how the pairs were bundled together. the offset position
of how these pairs laid together were very important for cross talk
issues for tightly bunched cables.
we did the same thing when we developed the IBM 590 super cable that
had the 12 conductors, that had twisted pairs on the outside of a
shielded twin pairs of foam. cross talk was a big issue on that also.
actually we did the 590 project first then the CAT5 process was after
that..
we were also the first ones in the business to develop the cross
linking polyrads for wire! , i wasn't there with them at that time how ever.
oh well.
 
J

Jamie

Jan 1, 1970
0
SMS said:
That works too, but using different twist rates eliminates the need for
offsets, which reduce save manufacturing cost.
i can concur with you there, i do remember it was a problem getting
things to stay in track, but we end up using a following idler system
that would rack the wheel which control a cam following system to shift
the angle a bit as the pairs came together.. this works perfectly and
allows us to run at a high rate of speed.
at the time we came up with this, we have some of the smartest
engineers in the business.
now adays, a lot of that is going by the waste side.
 
A

Alan B

Jan 1, 1970
0
That why I *always* snip the cable in half - ethernet, phone cord,
handset cord....

Yes, I remember that trick, from long ago in my technician days. It's good
practice. We would also do things like smash the legs on bad IC's, things
like that. Make sure the bad component *stays* bad.
 
A

AES

Jan 1, 1970
0
Alan B said:
Yes, I remember that trick, from long ago in my technician days. It's good
practice. We would also do things like smash the legs on bad IC's, things
like that. Make sure the bad component *stays* bad.

This somehow brought back forgotten memories of weird set of my first
wife's relatives, from very old (pre-Gold Rush) San Francisco family:
bunch of strange, reclusive brothers and sisters all living in big old
brown-shingle mansion on Sixth Avenue, up against the Presidio wall.
Only 2 out of 7 siblings ever married or left the house, and one of
those soon returned to the nest.

Took our small children up to visit them one Christmas. Somehow, a
really old electric light bulb was discovered on the top shelf of a
kitchen cabinet, with a tag attached by some ancient string bearing two
scrawled notes:

"October 14, 1937 -- No good"

and then below this, in different ink:

"August 7, 1943 -- still no good"
 
P

Phil Nelson

Jan 1, 1970
0
AES said:
This somehow brought back forgotten memories of weird set of my first
wife's relatives, from very old (pre-Gold Rush) San Francisco family:
bunch of strange, reclusive brothers and sisters all living in big old
brown-shingle mansion on Sixth Avenue, up against the Presidio wall.
Only 2 out of 7 siblings ever married or left the house, and one of
those soon returned to the nest.

Took our small children up to visit them one Christmas. Somehow, a
really old electric light bulb was discovered on the top shelf of a
kitchen cabinet, with a tag attached by some ancient string bearing two
scrawled notes:

"October 14, 1937 -- No good"

and then below this, in different ink:

"August 7, 1943 -- still no good"

So, was it still no good?
 
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