FAQ: Older Cable Types

FAQ Older Cable Types

Question: What are TIA/EIA T568B connectors?

Answer: This question refers to two authorized ways of connecting twisted pairs to an RJ-45 style jack or plug. These methods are designated T568A (standard) and T568B (optional). These designations are not to be confused with the generic cabling standards TIA/EIA 568-A (older) and TIA/EIA 568-B (new and improved). The two connection methods swap the positions of the orange and green wires (pairs 2 and 3) on the connector.

According to the standard, pin assignments should be made according to the T568A style, or optionally the T568B style "if necessary to accommodate certain 8-pin cabling systems" (and if you want to permanently confuse everyone who works on your wiring). The U.S. Federal Government publication "NCS, TRP 109-1977" recognizes the only style T568A.

The technical issue here is that the worst crosstalk on a category 3 style T568A uncompensated connector occurs between pairs 1 and 2. Style T568B moves the worst-case crosstalk to pairs 1 and 3. Depending on what services you plan for which pair, one or the other style could be best.

Upgrading to cabling of category 5e or better renders the issue moot.

Question: My European customers have unusual old cables installed in their buildings . How do I support them?

Answer: If you can't work on their cable, then do them a big favor by insisting that they install ISO/IEC 11801-compliant cabling of at least class D (or whatever you need) to every outlet. The new cables will likely support all their older applications.

Question: Didn't a lot of buildings used to have 25-pair cables near the work area?

Answer: Yes. Such cables are relics left over from the era of mechanical key-system telephones. TIA/EIA 568-B.1 , Annex C, states regarding the use of 25-pair horizontal cabling, "Although such an arrangement may provide installation efficiencies, it should not be used for the general case."

The sanctioned mode of installation runs individual 4-pair cables from each work area faceplate all the way to a switch room located within roughly 100 m. The use of individual cables from each faceplate provides terrific crosstalk isolation between devices connected to different work areas.

Question: My system runs on only two twisted pairs. Can I let my customers operate two systems through the same four-pair cable?

Answer: You will need to carefully check your crosstalk budget before supporting this feature. Even if you can support two systems in a single 4-pair cable, it likely won't increase your market. That's because modern buildings are cabled with a separate cable to each data faceplate. At the RJ-45 data faceplate it is not always easy to split out two of the pairs to run to a separate jack. Also, users don't expect to be able to put two systems on one cable, because the feature is not guaranteed by most LAN specifications.

Question: What is the difference between two-pair UTP and quad cable?

Answer: The difference is in the twist. Two-pair UTP incorporates two pairs of wire, each of which twists round and round its mate, but not around the other pair. The low-crosstalk properties of this cable derive from the way alternating positive and negative crosstalk effects cancel as the wires twist.

High-quality quad cable, or star quad, is found mostly in Europe. It holds all four wires in a square configuration and then twists the whole bunch. The wires of each pair stay on opposite corners of the square at all times (Figure 7.5). The low-crosstalk properties of this cable derive from its exact geometrical symmetry. Quad cables are generally available only in two-pair configurations.

Figure 7.5. Types of twisted-pair cable (EIA definitions).

Persons familiar with telephone cable might remember the old telephone-grade quad cable, which also had four wires but no controlled symmetry. In the old telephone-grade quad cable, the wires of each pair were not twisted, and could flop into any position. With no controlled symmetry, such cabling has terrible crosstalk. Do not use it for data.

For further study see: www.sigcon.com