| Author |
Message |
Brendon Caligari
Guest
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 Posted:
Tue Jan 04, 2005 12:40 am Post subject:
Cat 5e twists |
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Is there a difference between the pairs of Cat 5e?
I've been doing cabling (not exclusively) for a few years but today
while drawing a temp makeshift patch cord from my study to my bedroom i
noticed that some pairs are more tightly twisted than others. Is it
just cos this happens to be a dirt cheap cable?
B. |
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Doug McIntyre
Guest
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| Posted:
Tue Jan 04, 2005 1:04 am Post subject:
Re: Cat 5e twists |
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Brendon Caligari <bcaligari@nospam.fireforged.com> writes:
| Quote: | Is there a difference between the pairs of Cat 5e?
|
Yes.
| Quote: | I've been doing cabling (not exclusively) for a few years but today
while drawing a temp makeshift patch cord from my study to my bedroom i
noticed that some pairs are more tightly twisted than others. Is it
just cos this happens to be a dirt cheap cable?
|
Nope, thats exactly the way it should be.
Different twists mean that that radiation pattern for that pair is
slightly different than the next pair, reducing cross-talk along the
whole length of the cable between the different pairs. |
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Brendon Caligari
Guest
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| Posted:
Tue Jan 04, 2005 1:19 am Post subject:
Re: Cat 5e twists |
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Doug McIntyre wrote:
| Quote: | Brendon Caligari <bcaligari@nospam.fireforged.com> writes:
Is there a difference between the pairs of Cat 5e?
Yes.
I've been doing cabling (not exclusively) for a few years but today
while drawing a temp makeshift patch cord from my study to my bedroom i
noticed that some pairs are more tightly twisted than others. Is it
just cos this happens to be a dirt cheap cable?
Nope, thats exactly the way it should be.
Different twists mean that that radiation pattern for that pair is
slightly different than the next pair, reducing cross-talk along the
whole length of the cable between the different pairs.
Thanks :D |
Hmmmm......should the 'rate of twist' change over distance to eventually
even out? The twists of this particular cable were quite pronounced and
guess that if it was the same all along one pair drawn out would be
substantially longer than on another pair.
B. |
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James Knott
Guest
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| Posted:
Tue Jan 04, 2005 2:14 am Post subject:
Re: Cat 5e twists |
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Brendon Caligari wrote:
| Quote: |
Is there a difference between the pairs of Cat 5e?
I've been doing cabling (not exclusively) for a few years but today
while drawing a temp makeshift patch cord from my study to my bedroom i
noticed that some pairs are more tightly twisted than others. Is it
just cos this happens to be a dirt cheap cable?
|
Cables are built with different twists in the pairs, to minimize crosstalk.
All pairs are good enough to run ethernet. |
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Dmitri(Cabling-Design.com
Guest
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| Posted:
Tue Jan 04, 2005 3:29 am Post subject:
Re: Cat 5e twists |
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|
Brendon Caligari wrote:
| Quote: | Thanks :D
Hmmmm......should the 'rate of twist' change over distance to
eventually
even out? The twists of this particular cable were quite pronounced
and
guess that if it was the same all along one pair drawn out would be
substantially longer than on another pair.
B.
|
This concern is taken care of by specifying a parameter that's called
"delay skew". Up to the maximum distance the signal can travel (100
meters), the length of the "stretched out" pairs (or simply copper length)
cannot be different more than in takes light to travel 45 ns in copper
(approx. 0.67 speed in vacuum).
So, multiplying it all up we get 4.5E-08 sec x 3E08 m/sec = 13.5 meters or
44 feet 3 inches if you like. That's roughly how much copper length
difference a pair of devices talking over that cable can tolerate. This,
in turn, means that the designers of those devices should allow enough
memory buffer to hold the parts of the data stream for at least 45 ns.
--
Dmitri Abaimov, RCDD
http://www.cabling-design.com
Cabling Forum, color codes, pinouts and other useful resources for
premises cabling users and pros
http://www.cabling-design.com/homecabling
Residential Cabling Guide
-------------------------------------
##-----------------------------------------------##
Article posted with Cabling-Design.com Newsgroup Archive
http://www.cabling-design.com/forums
no-spam read and post WWW interface to your favorite newsgroup -
comp.dcom.cabling - 1021 messages and counting!
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James Knott
Guest
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| Posted:
Tue Jan 04, 2005 4:27 am Post subject:
Re: Cat 5e twists |
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Dmitri(Cabling-Design.com wrote:
| Quote: | This concern is taken care of by specifying a parameter that's called
"delay skew". Up to the maximum distance the signal can travel (100
meters), the length of the "stretched out" pairs (or simply copper length)
cannot be different more than in takes light to travel 45 ns in copper
(approx. 0.67 speed in vacuum).
So, multiplying it all up we get 4.5E-08 sec x 3E08 m/sec = 13.5 meters or
44 feet 3 inches if you like. That's roughly how much copper length
difference a pair of devices talking over that cable can tolerate. This,
in turn, means that the designers of those devices should allow enough
memory buffer to hold the parts of the data stream for at least 45 ns.
|
Other than with Gigabit, how is delay skew a concern? At 10 & 100 Mb, only
one pair is used in each direction. |
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Dmitri(Cabling-Design.com
Guest
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| Posted:
Tue Jan 04, 2005 4:56 am Post subject:
Re: Cat 5e twists |
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James Knott wrote:
| Quote: | Other than with Gigabit, how is delay skew a concern? At 10 & 100
Mb, only
one pair is used in each direction.
|
James, you are absolutely correct. There is no arguing about that.
10BASE-T and 100BASE-TX devices don't care about delay skew, FEXT, PSNEXT
etc.
Gigabit Ethernet, however, is not some obscure application that only
exists in a lab. A Dell PC you'd buy today will come with 10/100/1000 NIC
by default.
10G Ethernet is only couple years away from general acceptance. So, we
better start worrying about delay skew today as the cables we lay today
will definitely see at the very least Gigabit Ethernet (and most likely
10G) before the end of their life cycle.
With best regards,
--
Dmitri Abaimov, RCDD
http://www.cabling-design.com
Cabling Forum, color codes, pinouts and other useful resources for
premises cabling users and pros
http://www.cabling-design.com/homecabling
Residential Cabling Guide
-------------------------------------
##-----------------------------------------------##
Article posted with Cabling-Design.com Newsgroup Archive
http://www.cabling-design.com/forums
no-spam read and post WWW interface to your favorite newsgroup -
comp.dcom.cabling - 1023 messages and counting!
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Robert Redelmeier
Guest
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| Posted:
Tue Jan 04, 2005 8:01 am Post subject:
Re: Cat 5e twists |
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James Knott <james.knott@rogers.com> wrote:
| Quote: | Other than with Gigabit, how is delay skew a concern?
At 10 & 100 Mb, only one pair is used in each direction.
|
Interpair slew (different pair lengths) is a big problem for
multi-pair video (YC or RGB).
I don't think any of the big cable mfrs are anywhere near 45
ns out on 100m, but could reach that proportion over shorter
1-5m lengths.
-- Robert |
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Justin Time
Guest
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| Posted:
Tue Jan 04, 2005 7:13 pm Post subject:
Re: Cat 5e twists |
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Not quite true - 10Base-T and 100Base-T do care about delay skew. The
problem actually surfaced about 10 years ago now. There was a
world-wide shortage of teflon used in Plenum cables so some
manufacturers started making cables with different insulation on some
pairs. These were known as 2+2 and 3+1 cables. The Green / Orange
pairs were teflon, and sometimes the Blue pair as well. I think the
other pair(s) were polyolefin or similar.
As different insulations were used, the insulation thickness was not
the same on all the pairs resulting in all kinds of cabke geometry and
interconductor capacitance problems. This was about the time 110VG
AnyLan was coming out and getting established, so the amount of delay
between one pair and another became a real issue.
Perhaps some of the biggest problems were when using what many people
considered a standard as far as cable quality was concerned, AT&T.
AT&T changed to a 3+1 cable construction and didn't change the model
number of the cable. Their 1062 cable had been pretty solid, although
pricey, for several years and now it was made as a 3+1 without the
model changing. When people tried using the cable for 100VG, it bombed
- big time. AT&T, later Lucent, admitted they changed the material and
started providing date codes on when the cable changed. I don't think
they ever recovered from that fiasco and became one of the "avoid
unless speced" suppliers within the cabling industry.
There is still some of the 3+1 and 2+2 cables out there in walls being
used for 100Base-T and working fine, but if they ever attempt to try to
use all four pair, then they will have to replace the run. |
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Robert Redelmeier
Guest
|
| Posted:
Tue Jan 04, 2005 8:14 pm Post subject:
Re: Cat 5e twists |
|
|
Justin Time <a_user2000@yahoo.com> wrote:
| Quote: | Not quite true - 10Base-T and 100Base-T do care about delay
skew. The problem actually surfaced about 10 years ago now.
|
I think 10baseT & 100baseTX are immune to delay skew because
only one pair is use for each of Rx & Tx. 100VG and 100baseT4
could suffer since they use more than one pair for Rx & Tx
| Quote: | Perhaps some of the biggest problems were when using what
many people considered a standard as far as cable quality was
concerned, AT&T. AT&T changed to a 3+1 cable construction and
didn't change the model number of the cable. Their 1062 cable
had been pretty solid, although pricey, for several years and now
it was made as a 3+1 without the model changing. When people
tried using the cable for 100VG, it bombed - big time. AT&T,
later Lucent, admitted they changed the material and started
providing date codes on when the cable changed. I don't think
they ever recovered from that fiasco and became one of the
"avoid unless speced" suppliers within the cabling industry.
There is still some of the 3+1 and 2+2 cables out there in
walls being used for 100Base-T and working fine, but if
they ever attempt to try to use all four pair, then they
will have to replace the run.
|
Do you have any data that 3+1 or 2+2 won't run 1000baseT
(Gigabit)? Don't the controllers have sufficient buffers
to compensate for slew?
-- Robert |
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Dmitri(Cabling-Design.com
Guest
|
| Posted:
Tue Jan 04, 2005 10:21 pm Post subject:
Re: Cat 5e twists |
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|
Justin Time wrote:
| Quote: | Not quite true - 10Base-T and 100Base-T do care about delay skew.
|
Not quite true - 100BASE-T4 and 100VG-AnyLan do, but 10BASE-T and
100BASE-TX still don't care about delay skew.
| Quote: | The
problem actually surfaced about 10 years ago now. There was a
world-wide shortage of teflon used in Plenum cables so some
manufacturers started making cables with different insulation on some
pairs. These were known as 2+2 and 3+1 cables. The Green / Orange
pairs were teflon, and sometimes the Blue pair as well. I think the
other pair(s) were polyolefin or similar.
|
Polypropylene that is (in most cases). The cable manufacturer's are well
aware of the difference in dielectric constant between Teflon (2.1) and
solid polypropylene (2.25). If you take it directly, the skew due to
insulation material difference can be calculated at 10 ns. However, the
"slower" polypropylene can be foamed to achieve the constant reduction to
any level desired down to 1.55 range to match that of Teflon's. Besides,
the electromagnetic wave's path is not exclusively going through the
insulation (partly through the 'skin" of the copper conductor), so the
difference is going to be even smaller.
| Quote: | As different insulations were used, the insulation thickness was not
the same on all the pairs resulting in all kinds of cabke geometry and
interconductor capacitance problems.
|
The insulation diameter has noting to do with delay skew but impedance of
the cable and, consecutively, return loss.
| Quote: | This was about the time 110VG
AnyLan was coming out and getting established, so the amount of delay
between one pair and another became a real issue.
|
Delay skew requirements for category cables were not yet
established/standardized back in 1995. Having based their equipment on the
missing specification, HP practically gambled with the life of the
application. By the time delay skew was standardized in 1999, 100VG-AnyLan
was dead.
| Quote: | Perhaps some of the biggest problems were when using what many people
considered a standard as far as cable quality was concerned, AT&T.
AT&T changed to a 3+1 cable construction and didn't change the
model
number of the cable. Their 1062 cable had been pretty solid, although
pricey, for several years and now it was made as a 3+1 without the
model changing.
|
The correct part number was 2061 for AT&T SYSTIMAX plenum cable. Has been
2061C to indicate "true" 4+4 plenum insulation and overall performance
improvements since the smokes of the FEP shortage have disappeared in 1996
if memory serves me right.
| Quote: | When people tried using the cable for 100VG, it bombed
- big time.
|
Considering (lack of) acceptance of 100VG-AnyLan I'd call it "puffed" as
oppose to "bombed". Most adverse effects of the 2+2 and 3+1 plenum cable
designs were associated with fire code compliance, and not performance.
Again, cable manufacturers were not stupid justifying the design change:
from the point of view of the cabling standard there was NO performance
difference.
| Quote: | AT&T, later Lucent, admitted they changed the material
|
There has never been any conspiracy as the way you put it implies. If one
was interested, one could have asked and gotten the straight answer. True,
the info did not make it into sales brochures as the shortage was
(appropriately) considered temporary.
| Quote: | and
started providing date codes on when the cable changed. I don't think
they ever recovered from that fiasco and became one of the "avoid
unless speced" suppliers within the cabling industry.
|
One can only talk about personal perception of a product this way. I'm not
sure why you single SYSTIMAX out of all the rest of the cabling world, but
you are talking about nine lo-o-o-ng years here. They have recovered (if
ever suffered), had few great years, then fell prey to stock market plunge
and BAD management, then been pushed around and eventually got bought by
Commscope, but mid-1990s FEP shortage has very little to do with it. BTW,
the way SYSTIMAX has generally been specified is rather "use unless cannot
afford" before the shortage and after.
| Quote: | There is still some of the 3+1 and 2+2 cables out there in walls being
used for 100Base-T and working fine, but if they ever attempt to try to
use all four pair, then they will have to replace the run.
|
The best approach to deal with installed cable plant of unknown
origin/quality is to re-certify it on CAT5E standard. That usually
uncovers any potential problem, including possible cable design issues.
Didn't mean to come down hard on you, Justin (forgot your real name,
sorry). Just feel cranky this morning ;-)
--
Dmitri Abaimov, RCDD
http://www.cabling-design.com
Cabling Forum, color codes, pinouts and other useful resources for
premises cabling users and pros
http://www.cabling-design.com/homecabling
Residential Cabling Guide
-------------------------------------
##-----------------------------------------------##
Article posted with Cabling-Design.com Newsgroup Archive
http://www.cabling-design.com/forums
no-spam read and post WWW interface to your favorite newsgroup -
comp.dcom.cabling - 1029 messages and counting!
##-----------------------------------------------## |
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|
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Brendon Caligari
Guest
|
| Posted:
Wed Jan 05, 2005 3:00 am Post subject:
Re: Cat 5e twists |
|
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| Quote: |
The best approach to deal with installed cable plant of unknown
origin/quality is to re-certify it on CAT5E standard. That usually
uncovers any potential problem, including possible cable design issues.
Didn't mean to come down hard on you, Justin (forgot your real name,
sorry). Just feel cranky this morning ;-)
|
Thanks to all. I just posted a curiosity more than anything and got an
avalanche of relevant meaningful knowledge back.
B. |
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Justin Time
Guest
|
| Posted:
Mon Jan 10, 2005 6:59 pm Post subject:
Re: Cat 5e twists |
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|
Dimitri,
Thank you for the corrections and illuminations. It has been several
years since I was directly involved in the pulling and ordering of
cable. Just one or two other points though. The problem with delay
skew on 10Base and 100Base-T usually cropped up when you had people
splitting a run and putting a second Ethernet on the "unused" pair.
Looking back over the history, this was more frequent than a lot of us
would like to admit, what with the spot price of teflon cable being up
to $300 a box.
While it is true the makeup of the poly can be changed to correct for
the differences in delay skew, it was something that wasn't measured
between '94 and '96, so a lot of people didn't know it existed as a
problem. All I remember about it was that because of the job we were
working, we specifically called out all four pair had to be insulated
with teflon because of the problem.
If people want to do a google search on the topic of delay skew as it
began to unfold in the newsgroup, they will see there was a lot of
discussion on the topic. But as far as AT&T not divulging the fact
they changed their formulation of the 2062 (I stand corrected) going to
a 3+1 and then a 2+2, it all depended on who you were talking to. I
was never able to get an answer from AT&T directly, I finally got the
answer by going to one of the component manufacturers about their jacks
and panels.
Rodgers Platt |
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Dmitri(Cabling-Design.com
Guest
|
| Posted:
Tue Jan 11, 2005 4:00 am Post subject:
Re: Cat 5e twists |
|
|
Justin Time wrote:
| Quote: | Dimitri,
Thank you for the corrections and illuminations. It has been several
years since I was directly involved in the pulling and ordering of
cable. Just one or two other points though. The problem with delay
skew on 10Base and 100Base-T usually cropped up when you had people
splitting a run and putting a second Ethernet on the "unused"
pair.
Looking back over the history, this was more frequent than a lot of us
would like to admit, what with the spot price of teflon cable being up
to $300 a box.
While it is true the makeup of the poly can be changed to correct for
the differences in delay skew, it was something that wasn't measured
between '94 and '96, so a lot of people didn't know it existed as a
problem. All I remember about it was that because of the job we were
working, we specifically called out all four pair had to be insulated
with teflon because of the problem.
If people want to do a google search on the topic of delay skew as it
began to unfold in the newsgroup, they will see there was a lot of
discussion on the topic. But as far as AT&T not divulging the fact
they changed their formulation of the 2062 (I stand corrected) going to
a 3+1 and then a 2+2, it all depended on who you were talking to. I
was never able to get an answer from AT&T directly, I finally got
the
answer by going to one of the component manufacturers about their jacks
and panels.
Rodgers Platt
|
Hi Rodgers!
One day someone will put together a memoir about the Internet boom era,
and we both can contribute a paragraph or to into it ;-) Sort of looking
at the same problem from the different sides of the fence (plus some very
interesting history prospective, looking back). At the time I was working
for the company you just happen to mention as involved into the issue,
hence some personal emotions showing in my previous post. Besides, I have
to admit I would not know exactly how difficult it would have been to get
info on the insulation material from the outside as I had access to this
data from inside. I guess, I assumed it is quite open, but I don't know
exactly how much sales folks told the customers.
Even these days I still get to read it in some cabling project specs that
plenum cables has to be 4+4 and not 2+2 or 3+1, which today reads as a
kind of technical curiosity. Someone needs to update their spec templates!
--
Dmitri Abaimov, RCDD
http://www.cabling-design.com
Cabling Forum, color codes, pinouts and other useful resources for
premises cabling users and pros
http://www.cabling-design.com/homecabling
Residential Cabling Guide
-------------------------------------
##-----------------------------------------------#
Article posted with Cabling-Design.com Newsgroup Archiv
http://www.cabling-design.com/forum
no-spam read and post WWW interface to your favorite newsgroup -
comp.dcom.cabling - 1048 messages and counting
##-----------------------------------------------## |
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James Knott
Guest
|
| Posted:
Wed Jan 12, 2005 7:10 am Post subject:
Re: Cat 5e twists |
|
|
Justin Time wrote:
| Quote: | The problem with delay
skew on 10Base and 100Base-T usually cropped up when you had people
splitting a run and putting a second Ethernet on the "unused" pair.
|
Why would running two circuits over the same cable, cause a problem with
skew? You're still running on only one pair in each direction, for each
circuit. |
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