Over the years, as we have been stepping through Categories 3, 4, 5 and 5e, the girth of the cables has been quietly increasing toward the maximum of 6.35 mm (0.25 in.) as stated in TIA/EIA-568-B.2 Commercial Building Telecommunications Cabling Standard Part 2: Balanced Twisted-Pair Cabling Components, Section 4.3.3.4.
TIA/EIA-568-B.2 Addendum 1—Transmission Performance Specifications for 4-Pair 100-Ohm Category 6 Cabling, was released in June 2002, and the diameter of the cables increased again. TIA/EIA-568-B.2-1 Section 6.1.1 states, "In addition to the applicable requirements of ANSI/ICEA S-90-661-1994, the physical design of the horizontal cables shall meet the requirements of clauses 4.3.3.1 to 4.3.3.6 of ANSI/TIA/EIA-568-B.2."
This, of course, includes section 4.3.3.4. Only this time, TIA/EIA-568-B.2, Section 4.3.3.4 and the "laws of physics" are not in agreement.
This predicament remained virtually unnoticed until a designer specified installation of an ANSI/TIA/EIA-568-B-compliant, Category 6 ScTP (screened twisted-pair) cabling system, which would have overfilled the ANSI/TIA/EIA-569-A-compliant pathways. And the telephones started ringing.
Research has shown that manufacture of a cable that meets all the ScTP Category 6 transmission and mechanical requirements is not possible. It seems that what is possible is production of a cable that meets all the requirements except cable diameter. And there is currently a request before TR-42.7, authors of TIA/EIA-568-B.2, to change the mechanical requirements to allow larger-diameter Category 6 cables.
Since these fatter cables would have to fit into the building pathways and spaces, a study of the effects of this proposed change is underway within TR42.3, authors of TIA/EIA-569-A.
Regardless of the outcome of this study, designers who are going to specify these new fatter cables—like ScTP Category 6, augmented ("augie") Category 6, and Category 7—in new-building cabling infrastructure systems need to do the calculations based on 40% fill, and not just refer to a typical fill requirements table.
When sizing a pathway, you must consider the number of cables being placed and the cross-sectional area of the cables being placed in the pathway:
- Determine the number and size of the cables that will be placed in the pathway.
- Determine the cross-sectional area of cables being placed. To find the cross-sectional area of any cable, use 0.79D2 where D is the outside diameter of the cable.
- Total the cable cross-sectional area results, divide by 40 and multiply by 100 to obtain the minimum cross-section area of the required pathway.
To determine how many cables would fit into an existing pathway:
- Determine the cross-sectional area of the pathway.
- Determine the cross-sectional area of each cable type being placed in the pathway.
- Divide the cross-sectional area of the pathway by the cross-sectional area of the cables.
While ScTP Category 6, "augie" Category 6, and Category 7 will likely support 10GBase-T in the future, use of these cables will not be a cost-effective solution if the end user is expected to replace their open-office furniture or perimeter raceways in order to accommodate the size and limited bend radius of the cabling.
The current bending-radius requirements for horizontal cable are in TIA/EIA-568-B.2.
Section 4.3.3.6 states, "Twisted-pair cables shall withstand a bend radius of 25.4 mm (1 in.) at a temperature of -20 deg. +/-1 deg. C, without jacket or insulation cracking, when tested in accordance with ASTM D4565, Wire and Cable Bending Test."
Section K.4.2.3 states, "The cable, when tested in accordance with ASTM D4565, shall withstand a bend radius of 50 mm (2 in.) at a temperature of -20 deg. C +/- 1 deg. C without jacket, shield, or insulation cracking."
It will be interesting to see if the cabling manufacturers attempt to force the construction industry to accommodate an increased bending radius or propose innovative solutions of their own—like surface-mounted boxes or extension rings for mounting faceplates.
Of course, at some point, designers may just consider a currently available, thinner media with an acceptable bending radius—optical fiber. Or, no media at all—wireless.
Fiber-testing TSB completed
After more than two years in the making, TSB-140—Additional Guidelines for Field-Testing Length, Loss, and Polarity of Optical Fiber Cabling Systems, has been approved for publication.
It took fiber experts more than two years of wordsmithing to turn something relatively simple into a document so complex that it was confusing even to them. But having realized the error of their ways, they pared the text back to where it was about a year ago and voted to publish it. Hats off to TR-42.8.
TSB-140 describes field-testing for length, optical loss, and polarity in optical-fiber cabling using an optical-loss test set (OLTS), optical time-domain reflectometer (OTDR), and a visual fault locator (VFL). TSB-140 describes two tiers of field testing:
- Tier 1 includes attenuation testing with an OLTS, and verifying the cabling length and polarity.
- Tier 2 includes the Tier 1 tests plus an OTDR trace.
An optical-fiber cabling link is basically a fiber or several lengths of fiber that have been spliced, cross-connected, or interconnected to create a continuous length of fiber with a connector on each end. The length, fiber type, wavelength, number of splices, and connectors all affect the attenuation (loss) of the link. Severe cable bends, poorly installed connectors, or even the presence of dirt on the endface of connectors will increase link attenuation.
The system designer calculates the attenuation (loss) budget based on the fiber type, wavelength, link length, and number of splices and connectors. For the application to work as designed, the measured attenuation must always be less than the calculated attenuation (loss) budget.
The only way to know if the link that is installed is the link that was designed is to test. And yes, you really do have to use the mandrels as specified. And no, a toilet paper roll is not an acceptable alternative—no matter what the news group says.
Donna Ballast is BICSI's standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: [email protected]