关于选择非屏蔽布线系统
与屏蔽布线系统的讨论

  这个话题虽然在布线界讨论了很多年,但仍然是争论不休的问题。我们在这里加入在论坛中有价值的发言,公布给大家,并推荐 Henri 博士简明而精彩的发言。

  希望用户能发表自己的观点。读者Sunny.Y已将部分的英文发言翻译成中文,我们在此感谢

  Sunny.Y 发表在论坛中的译文

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安恒公司
网络维护论坛

I have a big dilemma about choosing between UTP or FTP wiring.
Considering the site is a business premises with about 15 km total length
the dilemma is even bigger.
I am aware of the disadvantages of FTP (more expensive, needs more space,
difficult to connect) but anyway I need more arguments for comparing FTP to
UTP.

Can someone help me?

Thank you,

Igor Hostnik
SMART-COM

Reply to Igor Hostnik from Ernst Lopes Cardozo
--------------------------------------------------

Well, you ask a broad question, you get a spectrum of answers! This is of
course one of today's though questions. Some elements to consider:

1. How long will the cabling be used? If you plan to leave the building in 3
years, your decision may be different than when you plan to stay long term.

2. What will the cabling be used for? For data, I reckon that Ethernet at
all speeds (10/100/1000 Mbps) must be supported, hence UTP Cat5E is the
minimum. If you use the same type of cable for telephony (gives you more
flexibility) any extra cost for shielding weighs in more heavily than when
it is data-only.

3. While fiber cabling by itself may have comparable price, in practice I
don't think that fiber-ONLY is an option. Hence your choice is either copper
or copper + fiber. The reason of course is that there are very few
fiber-based telephones, fax machines, etc. on the market. Also, you will
find that fiber-only limits your possibilities and raises cost in other
devices, e.g. PC's with built-in Ethernet, ditto printers, etc.

4. Shielding (STP or FTP) can help, but has its own risks and expense. My
rule is: use shielding only if there is a demonstrated necessity, e.g. high
level of EM noise. The 'if it doesn't help, it can't hurt' approach is not
applicable. Any fault in your shield brings your cable down from
better-than-UTP to worse-than-UTP. Hence I don't think that paying the extra
x% for FTP buys you real peace of mind. Unless, of course, UTP will not work
in your particular environment. Given that it does in the vast majority of
office buildings, your FTP proponent should come up with concrete evidence,
naming the sources of radiation or showing the measurements.

Whish you success!

Ernst Lopes Cardozo"

FTP cables have better immunity to EMI...if you have any EMI source nearby,
it could corrupt your data transfer if you use UTP instead of FTP cables.
In addition, if you need extra data security, if you use FTP cables it is
hard to be bugged from the outside.
FTP cables are a very good balance between UTP price and STP performance.
You cannot fail if you chose FTP instead of UTP!

Aleksandar Marinovic

Hi Igor,

The best thing to do in this case is using fiber to the desk.

Regards

Allan Smith

This topic can become quite political and emotional.

1) No doubt, assuming you have installed all ScTP cabling correctly and made
sure that all shield connections are thorough, you will have less EMI
opportunity with shielded cabling solutions.

2) There is not a really good field test to certify the (RF) quality of the
shield connection. Just refer to the methods for evaluating RF connections
in draft Cenelec or international standards. Connection with a drain wire (a
DC test) is not good enough. There needs to be a nice ground clamp grabbing
onto the shield to get close to satisfactory EMI requirements. If those
connections are not properly implemented, or you use somewhere a UTP patch
cord in a shielded cabling link, you are MUCH worse off than a UTP
implementation.

3) You really have to worry about power line ground loops with shielded
cabling solutions. If your power line distribution system causes significant
voltages between ground connections (in the order of 1 V RMS), you do have a
problem that needs to be corrected. I expect that ground loop problems are
found when deployment of machinery and equipment takes place and not at the
time of testing a new installation. Ground loop problems can be hard to
correct. Most European countries have very good power line grounding
systems.

4) Assuming quality shields, the potential for alien crosstalk is reduced
with shielded cabling. But, if you do have sufficient margin (and that is
the case for most cat 5, and cat 5e installations), this is not an issue.

5) Good UTP cable installations can and will generally meet EMI requirements
(assuming equipment attached to make a "live" disturber). Often computers
radiate more noise than the network cable attached to it.

6) If you really worry about EMI and crosstalk between cables, fiber is of
course the ultimate.

In summary: you will achieve better EMI performance with shielded cabling.
You have to make sure it is properly installed. And most of the true
verification in the field is visual inspection, and must be supported by
supplier supported installation guidelines. No doubt that costs extra money.

But UTP cabling does work as well and does not have to radiate, and avoids
the potential for power line loops and is likely least expensive.

If you are really worried about EMI, fiber is the answer.

Of course there are cost considerations and economics and you have to look
at the system solution, including equipment. I suggest that you make a
careful evaluation of your needs for the first 3 (?) years and do an
economic analysis based on that. If you do your analysis on the basis of
Internal Rate of Return (IRR), any capability that is used after that period
is really showing up as uneconomincal. Maybe it is like buying a PC: you buy
the PC for todays needs, and plan to buy a new one in 2 years.

So, the true answer is "it depends" on your situation, and what you want to do!

Hope this is helpful.

Henriecus ("Henri" or "Riekus") Koeman
Fluke Corporation, MS 279F

Henri,

You make some excellent points. One that is not mentioned is the length of
the channel cable run. At 10/100 mbits one can fudge the 100 meter limit.
However, at 1000mbits, 100 meters is critical. And, in the world of today's
technology, if one doesn't install 5e/6 cable so that it will support
1000mbits, one is wasting time and money (in my opinion). Since the
distance factor is unknown, I personally recommend going with fiber,
particularly with pre-terminated fiber in-a-box, which can support a
variety of combinations (1-12 cable strands, or more). Fiber-in-a-box saves
time and money and allows anyone to successfully "install" fiber to the
desktop.

Gene Dunaway

Gene,

I can understand people's feeling on going for the "ultimate" in bandwidth
and immunity to noise. But that carries a price tag (in equipment), and does
not necessarily get rid of copper wiring for other services (your
telephone).

Considering the rapid change of technology, I am strongly inclined to put
emphasis on my immediate needs, and be prepared for updates in the not
extremely distant (earlier than 10 years) future. That is likely better from
an economics point of view (return on investment).

I am not as concerned as you are on the 100 m limitation. From a
signal-to-noise ratio consideration, of course attenuation requirements need
to be met. Then it is really the ACR, return loss-to-attenuation ratio and
ELFEXT that affect the SNR. Of course propagation delay (for non-switched
Ethernet) and delay skew must be within limits.
In fact, for 1000BASE-T the return loss is a major contributor to the SNR
and that would worry me the most. Now it remains to be seen whether vintage
cat 5 cabling meets the requirements or not. We will see some results of
this as 1000BASE-T is deployed. I personally not too pessimistic about that
(like Y2K).

To say, you have to worry about 100 m is potentially misunderstood (by me).
I do not believe that accurate length measurements based on propagation
delay can be made due to the uncertainty with Nominal Velocity of
Propagation. At best you can expect 10 %, and you can fudge a length
measurement by changing the value of NVP.

What really is important are propagation delay, delay skew and attenuation.
As long as you meet these requirements, the length measurement likely does
not matter too much. I wish that length would go away as a formal test
requirement for the reason that its measurement is relatively unreliable
(and that is NOT the fault of my tester!).

Henriecus ("Henri" or "Riekus") Koeman
Fluke Corpora
tion, MS 279F

I'm glad to see that Henriecus is still with us.

I've made an assumption about FTP that may not be valid. What is FTP,
according to this group?


Robert L. Pritchett
Computer Telephony Engineer

Henri is with us - as strong as ever!

As in regard to the term FTP: This is the most widely accepted name for a
cable made of unshielded twisted pairs cabled and overall shielded with an
aluminum foil.

Amir Yaari
TELDOR

Amir:

Thanks for the clarification.

So the F-word is "Foiled" and not "Fiber", as in Foiled Twisted Pair, while
UTP is for Unshielded Twisted Pair. So how different is ScTP (Shielded
Twisted Pair) from FTP? What is the cost delta between the two (assuming
there is a real difference)?

Have we dropped the name "ScTP" in favor of "FTP" now? If so, this tracks
along with the change made for the name settling activity of the cumbersome
keystone-cutout-RJ45-sized fiber connectors to "SFF" or Small Form Factor.
FTP seems to roll off the tongue better than ScTP. My confusion also is that
FTP is universal netspeak for "File Transfer Protocol".

Of course, there is no such thing as "fiber twisted pair", even though I
just saw in a trade journal yesterday such nonsense as "Fiber Wire" instead
of "Fiber Strand" in comparisons between copper and fiber network designs.
Believe it or not, I think it was in the latest issue of "Communications"
magazine (CATV Trade Journal).

Try as I might, these twist-on wirenuts just don't seem to help with
connectivity when used on fiber (we've heard of such antics done in manhole
systems when cables were cut). ;^)


Robert L. Pritchett
Computer Telephony Engineer
SCM Consultants, Inc.

Hi,
I love this debate on the acronyms.
The IEC 46C community, from the begining, would like to clarify the commonly
used wording dealing with its scope (Symmetrical cables).
Unfortunately the market use coming from various countries lead to more and
more confusion.
Let's say that in IEC SC46C at least it is clear that:

UTP are cables that include several unshielded pairs or quads.
FTP are cables that include several unshielded pairs or quands and that have
an overall screen (aluminium foil)
STP are cables that include shielded pairs and that may have an overall screen.

Obviously every body is free to invent new acronyms to better defined a new
designed such than a STP with a overall screen including a foil plus a braid
and why not a second foil over the braid.

The danger of acronyms is that they are only understood bt those who invented
them. For this reason some of our document/specifications/standards are
quite esoteric.

To illustrate my feeling, I submit you this requirement:

"Where the LPS of the LPZ related to ITE includes a CBN with SPD, FTP ,
ScTP or SFTP shall not be connected to the EB through the PNE nor the PE. "

Of course you can find better examples already used by Ciceron during the
second World War.