On tiny TRs and fiber prices

Dec. 1, 2001
You did not ask for feedback, but I guess it's the nature of the beast that you get it anyway. Following are some thoughts regarding your colleague's list of concerns, which you addressed in your October column

Q: You did not ask for feedback, but I guess it's the nature of the beast that you get it anyway. Following are some thoughts regarding your colleague's list of concerns, which you addressed in your October column. Power outlets also have limitations, although some people act differently; we have to find the breaker. And you can't carry electrical power on a fiber cable.

Now, about your reply.

  1. BICSI's Telecommunications Distribution Methods Manual, 9th Edition Volume 1, Chapter 7 likes to see telecommunications rooms (TRs) as rooms, not as "tiny TRs." To be exact, Table 7.3 says 10 ft. x 8 ft. ANSI/TIA/EIA-569A recommends 10 ft. x 7 ft. And page 7-4 of the TDMM says, "The minimum ceiling height is 8.5 ft." Not too tiny, is it?
  2. Yes, you can have many TRs. But you have to uplink switches from TRs to equipment rooms. If you want ample bandwidth, the links more likely will be fiber, which will require many high-cost fiber switches.
  3. Even though no minimum distance is stated, if the unshielded twisted-pair (UTP) length is less than 20 or 30 meters, the reflections from connectors will create unacceptable near-end crosstalk, which will show up on a certification test.

Let's hope that the costs of optical-fiber electronics come down quickly, so we can buy reasonably priced fiber switches. The price of fiber cable is not really that expensive; the biggest part of the installed cost is the labor.

Sentekin Can, PE
Ralston Purina Co.
St. Louis, MO

A: Reader feedback is always encouraged. You are correct as to BICSI's description of a telecommunications room-exact match with the TIA-569A recommendation. If you read 569A very carefully, the only requirement is on the size of the door. All other dimensions are recommendations. BICSI documents tend to recommend what is described/required within industry standards and codes, and then provide useful "how-to" information. The concept of the "tiny TR" is new to the TIA's TR-42 Committee, hence not yet addressed within BICSI's TDMM.

You are also correct that optical fiber is the logical choice from the equipment room to the "tiny TR" for beyond 1000Base-T. But I have not been seeing a lot of 1000Base-T to the desk-mostly switched 10/100Base-T. So, until the fiber switch prices meet your budget, use a UTP cable for 1000Base-T to the "tiny TR" and switched 10/100Base-T to the work area.

As for the reflection problems, contact your connector manufacturer. Many connectors available today do not, and in fact never did, experience the "short-link phenomenon" of the 1990s.

Q: My company is preparing to change locations. We are interested in getting a cable-television feed split through our office network on Category 5 cable. Do you have any suggestions on how this would best be achieved?

Alecia Steiner
Katten Muchin Zavis
New York City

A: Call me old fashioned, but I still use RG-6 for cable TV delivery, keeping a few baluns around for short-notice temporary feeds using Category 5. TV over IP means that your network data waits while the "soaps" are expressed through to the television in the break room. Is this what you had in mind? Or, do you just want to use the Category 5 cable as the medium to deliver the service to your television sets? If the latter is true, check out www.magneta-research.com. Magneta Research offers TV-Link, a cable-television distribution system for distributing 125 cable or satellite channels over Category 5 structured cabling. Audio (mono only), video, and channel selection are all provided over this single Category 5 cable for a distance of up to 2,000 feet.

Q: I am trying to locate information about conduit sizing for horizontal pathways. Recently, I have been involved with a telecommunications design project for several call centers, including Spiegel and Canadian Pacific/ Fairmont Hotels and Resorts, located in New Brunswick, Canada.

The electrical consultants doing the pathways design presented the client an almost-embarrassing position that the conduit-fill tables that BICSI follows and promotes (ANSI/TIA/EIA-569A and the CAS T-530) are underrated and at best only cause more-expensive design and construction. The 9th edition of BICSI's TDMM, Chapter 4, pages 49 and 50 make reference to a table and a formula for de-rating the capacity of conduit. An erratum has since removed it.

The benefit of the formula was that, although not perfect, it informed anyone doing the math that de-rating the capacity by 15% for each 90° bend should be considered. I have not been able to find another reference to this formula.

As a result, in my case, the electrical consultant did the calculations for a section of conduit and adamantly claimed that there was no reason why at least 10 Category 5e cables per 1-inch conduit, with two or three 90° bends, would have any negative impact on the installation or system performance.

I know the conduit-fill issue is under review. Is there anything in writing to advise consultants about the status of the review, or information to help give some credibility back to the tables in the pathways standard?
Richard S. Smith, RCDD

Aliant Telecom
Moncton, NB, Canada

A: Yes, the conduit-fill issue is still under review in TIA. And no, there is nothing "official" in writing, that I know of.

Did the electrical consultant consider that annealed copper begins to permanently elongate (stretch) under a stress of approximately 15,000 pounds force/square inch? That means that the absolute maximum recommended pulling tension for a 24-AWG conductor is 4 pounds/force. For 24-AWG UTP cables, multiply 4 pounds/force by the 8 conductors, and you would get 32 pounds/force total for the cable-being mindful that total pulling tension must be equally distributed among all conductors during installation. De-rate a little for a "safety factor" and this is where the 25 pounds/force maximum for pulling tension comes from in the current TIA/EIA-568B.1.

Translation: Depending on the length of the conduit and the number of offsets and 90° conduit bends, the 24-AWG UTP that they started feeding into one end of the conduit may well be 28 to 30 AWG where it comes out the other end. If this were to happen to an electrical power conductor, the wire would get hot. But in our case, the client does.

I contacted Bob Jensen, who said that the Conduit Fill Task Group would present a proposal to TR-42.3 at the group's February 2002 meeting. Jensen believed that the proposal would contain both a formula and table for inclusion in TIA/EIA-569B.

But until TR-42.3 comes up with something better, I have been using this approach:

  • Assume 40% maximum fill-makes sense, as that is what they did on most of the other pathway systems in TIA/EIA-569A.
  • Determine the conduit length, then calculate each offset and 90° conduit bend as an additional 30 feet of conduit length.
  • If the calculated length between pulling points is between 50 and 100 feet, de-rate the conduit fill by 15%.
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Donna Ballast is a communications analyst at The University of Texas at Austin and a BICSI registered communications distribution designer (RCDD). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, The University of Texas, Austin, TX 78713; e-mail: [email protected].

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