Arlyn S. Powell, Jr.
The working group of the Telecommunications Industry Association (TIA, Arlington, VA) that is responsible for the TIA/EIA-568A commercial telecommunications cabling standard has issued a warning about copper cable properties following its quarterly meeting this past winter. According to the TIA press release, "Some high-speed local area network specifications require cabling with requirements not now specified in 568A. These requirements are propagation delay and delay skew."
Delay skew is defined as the difference in propagation delay between any pairs within the same cable sheath. Reports made to the TIA`s engineering committee at its quarterly meeting indicate that the delay-skew specifications included in some transmission standards of the Institute of Electrical and Electronics Engineers (IEEE, New York, NY) are not being met on some cable constructions when measured at 100-meter distances.
The IEEE 100Base-T4 standard, for instance, permits a delay skew of not more than 50 nanoseconds between the fastest and slowest cable pairs for transmission frequencies between 2.0 and 12.5 megahertz. The IEEE 100Base-T2 standard specifies an identical delay skew for frequencies between 2.0 and 25 MHz. Geoff Thompson, chairman of the IEEE committee responsible for the 100Base-T4 and 100Base-TX standards, adds that "there is also a [delay-skew] limit in projects under development."
It is a further requirement that once the cable is installed, the skew between all pair combinations resulting from environmental conditions shall not vary by more than 10 ns within the above requirements.
Paul Kish of Nordx/CDT (St. Laurent, Quebec) has drawn the following conclusion from this information: "This leaves 40 ns for the combined skew of the cable, connecting hardware and patch cords. A reasonable upper limit for cable skew is 35 ns per 100 meters."
The TIA press release points out that horizontal copper-cable links of less than 100 meters may not fall outside the IEEE specification. It adds that "most of the existing 568A-compliant installed cable meets the delay-skew specification."
However, TIA Working Group TR41.8.1 is making the specification of delay skew a priority item at its next scheduled meeting this month. If agreement on delay-skew requirements can be reached at the meeting, a Telecommunications Systems Bulletin will be issued setting parameters for certification of new cable and related components.
Telecommunications industry analyst Donna Ballast, who represents BICSI (Tampa, FL) on various TIA standards committees, points out that the delay-skew problem has arisen, indirectly at least, because of the recent shortage of fluorinated ethylene propylene (FEP), a material used in the conductor insulation of Category 5 cables to help them meet fire ratings for plenum spaces. In addition to looking for FEP substitutes, cable manufacturers have stretched their FEP supplies by using so-called 3-and-1 and 2-and-2 strategies. These strategies depend on insulating one or two of the copper-wire pairs in a 4-pair Category 5 cable with FEP substitutes.
The problem with this, according to Ballast, is that "the dielectric of different jacketing materials is not the same, so the velocity of propagation of the signal, which is affected by the dielectric, is not the same. So, if there`s an application using all four pairs, it`s like sending a synchronized drill team marching down the pairs and having each team member arrive at the other end at a different time."
Ballast adds that most transmission equipment is designed to compensate for a certain amount of delay skew, but above the delay-skew limit a signal may be garbled. Ironically, a signal that is garbled on Category 5 cable may not be affected on lower-rated Category 3 cable because all its pairs are insulated with the same material.
Ballast also hints that the TIA is looking at a problem with electrical imbalance in connectors used in Category 5 links. However, the organization has thus far issued no official warning on this problem.
Outside-plant Standard Is on the Way
Bob Jensen, dbi
Many of the methods and procedures used in the design and construction of outside-plant cabling projects are proprietary company practices and, unfortunately, the specifications they use are not standardized within the telecommunications industry. Recognizing this fact, the Telecommunications Industry Association (TIA, Arlington, VA) approved in the spring of 1994 a proposal to write a standard for customer-owned outside plant (COOP).
Since that time, a TIA working group has been laying the groundwork for a document that will complement related standards, such as the commercial building wiring standard TIA/EIA-568A. One of the first tasks of the working group was to define the boundaries of COOP.
The group has agreed that the definition consists of two parts--cable and pathway. The cabling that extends from the first interior building termination to the exterior of a building in a campus is defined as being part of the outside-plant cabling. The group also decided that the demarcation point between the inside-building and outside-plant pathway is the place where a pathway first emerges from the exterior of a building. Pathways in the COOP standard will include aerial, direct-buried, underground-conduit and tunnel methods of distribution.
Cabling components covered by the standard will mainly be cable and connecting hardware such as splices and terminals. Connecting hardware will provide the interface linking various types of copper and fiber cable to the different buildings on a customer-owned campus. Protection hardware will also be covered.
The standard will specify minimum performance levels for COOP cable and components. In the process of setting these performance levels, the task group is reviewing many existing products. For example, at a meeting this past winter, the group looked at an enhanced 100-ohm outside-plant cable that promises a significant increase in bandwidth over traditional OSP cables.
Although TIA standards are recommended guidelines, other rules applying to outside-plant construction may be mandatory. Local building-code requirements, legal requirements applying to rights-of-way, and the National Electrical Code and National Electrical Safety Code are examples of the binding standards that may apply at a particular job site.
Presently, an editing group is developing a model chapter for the COOP standard, with other chapters to follow, once the model is established. Anyone interested in joining the effort to develop a COOP standard should contact the TIA at (703) 907-7700.
Bob Jensen, Registered Communications Distribution Designer (RCDD), is the senior design consultant for Austin, TX-based dbi, a telecommunications infrastructure design company. He is the secretary for the TR-41.8.4 committee.