Proper way to install shielded ethernet

We're having our office rewired; old Cat3 and Cat5 wiring is being replaced with a shielded Cat6. (The reason for shielded is another subject completely and is a done deal, so please, no "Why Shielded? You don't need it" posts. I will say that having a shielded solution is of lesser importance today, but will take on greater importance in the future.)

My concern has to do with the proper way to implement a shielded ethernet solution. We got two quotes. One was very specific about materials, from someone whose expertise I trust. We sent his itemized list of materials to another company and asked them to match the materials and give us a price. Their quote was less detailed, but since it was supposed to be based on the first quote I assumed they quoted the same or equivalent stuff. The cost was similar. For reasons of business politics too complicated to go into, we selected the second company's "equivalent" quote.

Now stuff is starting to arrive. Where the first quote explicitly provided a shielded solution from end-to-end, some of the materials arriving are indeed not shielded products. Instead of shielded patch cables we're getting unshielded. Instead of an empty multimedia panel into which shielded jacks would be installed, we're simply getting a cat 6 patch panel, which I haven't been able to verify supports a shielded solution (I'll check it out when it arrives).

This leads to my questions:

- I am assuming that a shielded patch panel or jack provides the means of connecting all the shields from the horizontal cabling and the patch cables, effectively creating a single ground point for all cable segments from end to end. Is this correct?

- If I'm only concerned about EMI in my horizontal cabling locations, and think EMI will not be an issue in my rack or office locations at present, is it appropriate to use unshielded patch cables, as long as the shielded horizontal cabling is properly grounded to the patch panel (which is itself grounded thru a properly-grounded rack)?

- If the cable shield is ungrounded, does the shield simply provide no benefit, or does it create the potential to cause harm? I've read speculation that an ungrounded shield could act as an antenna that actually increases EMI exposure to the pairs inside.

Please understand, I'm not asking for an opinion about whether EMI is likely to pose a problem in my environment; I'm asking for sound ethernet grounding theory.

Thanks in advance!

Bryan

Reply to
BJ
Loading thread data ...

(big snip)

I first learned about the problems of grounding looking at a big rack full of electronic equipment, all with the ground connection on the power cords cut off...

Are you asking about EMI from ethernet to other equipment, or EMI from outside into ethernet?

Shielding and grounding is a complicated subject once you have more than one piece of equipment that is line powered. Electrically, the best way is to have only one point connected to earth ground, with everything else grounded through the shield. This often doesn't agree with electrical codes requiring equipment to be grounded through the power line.

Ethernet is transformer coupled, and is pretty immune to outside EMI. The possible exceptions are factories full of arc welders, or inside a building with a high powered radio transmitter. There are possible resonant situations where the shield length might be the appropriate multiple of a half wavelength for a nearby EMI source, or for the ethernet signal itself. In some cases a resistor between the shield and the rest of the system might be needed.

Without knowing the possible EMI sources, it is hard to say. Sound grounding theory says ground the cable to earth at only one point, everything else grounds through the cable shield. There can only be one grounding path between any two pieces of equipment, and that path is through the shield. RJ45 connectors are not very good at making ground contact, so you should connect from the shield to a screw connection on each end of the cable.

-- glen

Reply to
glen herrmannsfeldt

Glen replied:

I was asking about EMI from outside into ethernet, although the reverse might also be a concern. Many of my runs will be supplying PoE to end devices, so I suppose the potential exists for EMI between bundles of wires running parallel for some distance.

So, do I understand you to say that if my shield remains ungrounded for the time being, the possibility *does* exist for the shield to "pick up" on EMI -- but that the chance of just the "right" source of EMI being around is unlikely?

So, if my EMI concerns are, at present, related to bundles of wires running PoE, and to in-ceiling and in-wall locations, and NOT related to my rack and office environments, should I still worry about having shielded, grounded patch cables in my rack and in my offices?

--Bryan

Reply to
BJ

Ethernet is transformer coupled. There is a transformer at each end of each twisted pair, which guarantees no common mode signal. That is, the sum of the voltage and current on the two wires of a pair is always zero. The transformer also keeps common mode signals from outside, such as a cable might pick up being near a power line, away from the ethernet receiver. It has been suggested here, though I never got around to testing it, that ethernet will work with a 240 volt AC common mode signal. (I will be careful when testing it!)

If you want more detail, we need to know the source of the EMI that you are trying to keep out. It makes a big difference. Note that low frequency magnetic fields will go right through the shield, for example. High frequencies can get in through small unshielded gaps at the end.

Also, it is fairly easy to get a signal going down the outside of the shield. If you want to keep that away, you need the ferrite rings that you see on many cables these days. (A bulge in the cable near the connector.)

(snip)

I think that is true, but it depends on what sources you expect.

PoE usually means that the device won't have a power line ground, which is good. In that case, ground to the rack and don't worry about the patch cables.

One I haven't heard about at all is the effect of 802.11 devices at the end of an ethernet cable. You could easily couple that into your shielded cable, even from inside the box, beside the fact that most have a plastic box, anyway.

-- glen

Reply to
glen herrmannsfeldt

Well, that action is specifically prohibited by the National Association of Purchasing Managers' Code of Ethics. For a number of very good reasons.

sPh

Reply to
sPh

What a slime ball you are. You let someone you trust do all the work of spec'ing things out, then give that info to someone else, probably with the amount of the first guy's bid. I hope you get screwed!

It is my most sincere wish that your project fails miserably or, at the very least, ends up costing you more than the quote from the vendor who would have done it right the first time.

Who did you say you worked for?

-QR

Reply to
News 2 Me

Point well taken.

I had been trying to communicate materials specs, my reasons for them, and quantities to the engineer, but it was slow and difficult going because the sales guy was acting as a go-between, and he wasn't particularly technical. I finally gave up and pasted the materials and quantities to him so he could forward them to the engineer. After some time and thought, I started to feel a bit shady about what I'd done. Not that it changes anything, but the first company was the one I preferred to work with - I'd used them in the past. But there was a business/personal relationship between one of our owners and the other company, and he (and consequently much of management) felt strongly that they'd like to go with them if at all possible. In the end the business relationship won out, even though I preferred the other company.

I hadn't heard about the National Association of Purchasing Managers so I looked them up. I think I've found some articles and resources about ethics in supply management and will read them, and perhaps I can use them in future dealings, both with other companies and with management. Thanks for the resource.

As for the guy who posted below you, I understand his anger and won't argue with it. He might get his wish, as we may well end up paying more than we would have if we'd gone with the best guy for the job in the first place. I've now run the risk of ticking off the first company and not being able to do business with him again, and I frankly don't like the position I find myself in now; and I put myself here. I'll consider this a hard lesson learned. Doing the right thing is important to me, but in the push to get the job nailed down I didn't notice I was in questionable territory until things had gone a bit too far. If there's a set of ethics guidelines specifically set forth for working with vendors and suppliers, I'll learn and refer to them so I don't mess this up again.

BJ

Reply to
BJ

You're right. I did wrong, and didn't realize the full extent of it until it was too late. I'll be careful not to do it again. I'm somewhat of a newbie when it comes to working with vendors to define the scope of a project and get bids. Any resources you could point me to that will help me do it right and do it ethically are welcome.

BJ

Reply to
BJ

I'm concerned mainly about EMI from power runs in the ceiling, and *lots* of fluorescent lighting. Also, this cable *should* be capable of running

10gig, when it finally comes to that. Since (as I understand) alien crosstalk in large bundles of cables may be one of the major obstacles to 10gig, we hoped a high-grade, shielded cable might increase our chances of getting thru the 10gig generation of products before another rewire would be needed. We briefly considered fiber, but realized early on that we wanted to do PoE, not to mention the other challenges of fiber. So.

So that's what those are. Thanks, I'll remember that.

It sounds like having shielded patch cables isn't worth making a fuss about, as long as the cable is grounded to the rack. If that's so, that'll make things easier.

BJ

Reply to
BJ

As far as I know, fluorescent lighting isn't a problem, even if you wrap the cable around the fixture or lamps a few times. Arc welders I might believe. (Someone asked about that here some time ago.)

Alien crosstalk I might believe is worth considering. To be a problem, I believe two cables have to run right next to each other over a fair distance. In most cases, they don't stay close enough for a long enough distance, unless the installer is really trying to pack them in.

My favorite cable, as far as electrical characteristics go, is the Belden datatwist 350. It has the two wires of a pair bonded together (sort of like miniature lamp cord). It is supposed to be good up to 350 MHz. Many of the problems with ordinary UTP cable come because the spacing between the wires can vary, causing impedance variations. That is probably what I would go for, but I haven't been following the 10G over copper discussion much at all.

(I don't have any connection to Belden, but the physics makes sense to me. I hear it is harder to terminate, though, requiring a special tool to separate the wires.)

-- glen

Reply to
glen herrmannsfeldt

I wanted to follow up here with more info on the ethical stew.

I lost sleep last night over this issue, so first thing this morning I met with management and explained to them I felt we hadn't done right by the first company. He consulted with us quite a bit, and in the end we used his information and expertise in striking the final deal with the other company. I asked for permission to offer to pay a consulting fee to the first guy for his time and work with us, and they agreed it was the right thing to do. I called and extended our offer to him, apologizing that we hadn't thought about it sooner. He was quite friendly and I didn't sense any anger or ill will at all.

We also just started exploring the possibility of expanding our office into some adjacent space that's come up. He knows about that, and I told him frankly that if/when that comes to pass, I won't shop cabling at all; he's my vendor of choice.

Much thanks to those here who piqued my mind and concience on this issue.

BJ

Reply to
BJ

BJ wrote in part:

Well, I'm glad you realized it and didn't get defensive.

IMHO, purchasing customs and ethics are one of the unacknowledged drivers of anglo-saxon (particularly US) commercial dominance. Many other countries have heavy relationship involvement or bazaar systems that are inflexible or create doubt.

There probably are some on the NAPM website. The specific one you violated was around supplier confidentiality.

-- Robert

Reply to
Robert Redelmeier

"Robert Redelmeier" wrote in message news:ximCg.4537$ snipped-for-privacy@newssvr21.news.prodigy.com...

Yes, I found it here, under section 5:

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I also found Section 7, about reciprocity, to be helpful. This wiring project actually stems from another project, and it's the other project where reciprocity was the real issue. The wiring decision was strongly influenced by the final decision on the other project. No wonder I felt uncomfortable with the pressure that was being brought to bear to select the supplier who had personal ties to one of our bigger customers. As the guidelines suggest, I perhaps should have sought legal counsel. I was at least frank with management that in choosing the supplier we did, we were not necessarily getting the best bang for buck today. However, we were still getting a quality product that I think will prove to be a good investment for the future; perhaps even better in the long run than our other option. So under reciprocity guidelines, although I was under improper pressure to select our customer as the supplier, the supplier could arguably be the best source even though they were not the cheapest. (I'll say this: the selected supplier offered a Cisco solution, where the "value" supplier offered some great features at a lower cost, but it wasn't Cisco. It was a very tough call, and if it had gone the other way, I wouldn't necessarily feel any better; both suppliers had many compelling points on their side, and neither could match everything offered by the other.)

But enough; I've enjoyed all I can stand of this project. Now that we're in the implementation phase, I just want to get thru it as smoothly as possible and with a minimum of turbulence.

Thanks Robert for the kind words. It's been a long while since I lost sleep over a feeling that I'd done wrong.

BJ

Reply to
BJ

I had that happen to me. Did walkover for a school district for 200 drops,

20 RS-232 drops, and building 2 new telco closets. I made up my bid with all the parts written. Next thing I know they brang in another person to be the boss and he brang in his own guys after I gave them all the documentation for the project which includes parts lists and drop locations. Scumbags....

It sucks but then again what are you going to do? If you charge for a estimate they probably wont even call ya...

Perkowski

Reply to
Perkowski

Perkowski wrote in part:

Yes. The worst part is they do this to everyone, and pretty soon only sleasebags will work with them on anything. So they get what is coming to them -- ripped off. The problem is they're tax funded :(

In my estimation, govt agencies have far worse ethics than corporations. They do anything and justify everything as "in the public interest, saving money for the taxpayer." Only it isn't so. Any savings are very short term, and will be paid back many times over in the future.

So don't give breakdowns, particularly not to the part level. Just describe the job in enough detail that they can't later say "but we wanted 200 duplex drops". Even line breakdowns can be trouble since they may "reopen" with "the other guy's cable is cheaper". Even if it isn't!

They _do_ want the estimate, and for free too. So you get to decide what's in it. They need a bottom line. Anything more is gravy.

-- Robert

Reply to
Robert Redelmeier

Fiber will have greater importance tomorrow. Shielded copper will not. Too little advantage costing too much and causing too many problems. Grounding being the most important as you've identified it yourself.

It really does sound like "business politics" were really so complicated that they blinded you to any technical arguments against shielding.

The unshielded patch cables alone pretty much nix all the potential benefits of using shielded components in the rest of your cabling.

All in all, I would be interested to find out why the contractor that's being paid to install the shielded system and who sold is to you is unable to provide you with an answer about how to install it? How did THEY anticipate installing it? Otherwise how did they calculate at the very least the labor factor, not even mentioning the proper bill of materials?

It all sounds like whoever approved the purchase has wasted a lot of your company's money. I hope you were not directly responsible.

Reply to
Dmitri(Cabling-Design.com

Fiber is bright. But it soon looses it brightness if you need PoE :-)

Reply to
Gerard Bok

See my other posts referring to alien crosstalk at 10 gig. IIUC, to reduce alien crosstalk at those frequencies requires either more space between cables or shielded cable. Fiber is great, but as I said in my post, we require PoE. (Later); I checked out your site and was interested to read your shielded vs unshielded info. I posed a couple of questions below.

The "politics" didn't involve the type of solution, just the provider.

If the source(s) of EMI are localized (i.e. just in the ceiling), why does the shield need to extend thru the patch cables? As long as the shield is properly grounded for the EMI-exposed segment, wouldn't that be sufficient, and if not, why not?

I was given bill of materials with only general descriptions and quantities; none of the references to patch panels, patch cables, jacks, etc, included the word shielded. When I questioned the company about this, I was told that the components should all be shielded or support shielding, since I

*had* after all explicitly requested a shielded solution. I decided to wait for stuff to arrive before taking a stronger stand. Patch panels and jacks arrived today, and they are clearly not made to support shielding. I'm going to bring that up with the supplier and see what they have to say.

Management understood that the shielded cable, while costing more, was to hedge our bets against *possible* future needs. Even if it's not working for us now, it may pay off in the future (a fair bit down the road, to be sure). I just want to make sure that in the here and now, if it's not working for us, it's at least not working against us.

Finding a consensus on how to implement shielded ethernet cable has been surprisingly difficult. In spite of a decent amount of research, I still haven't found agreement on whether a single or dual ground is required for proper shielding, or on the effects of leaving the shield ungrounded. Among the arguments I've heard:

- Two grounds can create a ground loop, and the conditions for a possible fire hazard. - Two grounds are necessary - Grounding only one end is best - Grounding only one end causes the shield to act as an antenna - Leaving the shield completely ungrounded is safe and has no detrimental effect - Leaving the shield completely ungrounded causes the shield to act as an antenna (or somehow increases the EMI exposure to the pairs inside)

Finding an authoritative answer that can clearly and unambiguously separate fact from misinformation is obviously important to me, since I haven't found a scenario yet that doesn't have an opposing viewpoint.

I specified a shielded solution. If, however, it turns out that I get shielded cable that remains ungrounded, and unshielded terminations and patch cables, I'm fine with that for now, *provided* there is no hazard or detriment to performance in leaving the shielded cable completely ungrounded. (And as long as I didn't pay shielded prices for the unshielded components). If there IS a detriment to leaving the cable ungrounded, then I would need to know exactly what's required to eliminate the performance hazard.

The truth is out there.

Thanks Dmitri for your thoughts.

Regards,

BJ

Reply to
BJ

it would be interesting to learn a bit about your environment that prompted the STP.... EMI from radio, radar, microwave, etc ?

I can't recall exactly all the wiring logistics when we installed the IBM Token Ring (STP) back in the 1980's and the grounding issues, etc...

All I know from general electronics is that floating grounds and ground loops are really bad...

Reply to
Phil Schuman

Association

don't we do that all the time - cars, frigs, retail products ? I know I each time I meet with a vendor, I learn something new that I will take to the table at any future meetings with another vendor. It's like asking for a RFI or RFP and taking the info supplied and forming a list or query for any other responses... back and forth...

The error here was not making the 2nd company specify exactly the same material list as the 1st company -

BTW - it has been interesting reading the thread... both tech + biz

Reply to
Phil Schuman

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