150-Ohm STP-A Cabling

Ohm STP A Cabling

In 1985 IBM rocked the world of integrated voice and data cabling with the introduction of the IBM wiring plan. So bold, so daring, so robust, so overbuilt, so incredibly heavy, so expensive, and so difficult to install, it landed with a deep blue thud on the doorsteps of MIS managers around the world. Few picked it up. [79]

[79] If you are one of the unfortunate customers who installed this cable, it's probably still in the walls. It's so heavy I don't think you can rip it out once it's installed. I've heard stories about old buildings in Manhattan that depend on IBM cables for structural support.

What remains of the system today is 150- W twinax, also called 150- W STP-A, or IBM Type-I cable (see Figure 9.1). IBM managed at one point to get this twinax cable written into the EIA and ISO building wiring standards, although it is rarely used today and is not expected to be carried forward into future versions of international building-wiring standards. Nevertheless, the performance of the cable is so extraordinary it merits discussion.

Figure 9.1. Construction of two-pair 150- W STP-A.

One hundred-fifty- W STP-A generally contains two independent twisted pairs. [80] The cable incorporates an overall outer braid shield and also a plastic and metal-laminated foil shield. As shown in the figure, the metal-laminated foil shield may wrap individually around both pairs, isolating them from each other. The finished outside jacket diameter of this cable is 0.43 inch. Compared to a four-pair UTP diameter of 0.25 in., the cross-sectional area (and thus the cable reel volume and weight) for two-pair 150- W STP-A is three times larger, yet it contains only half as many pairs.

[80] Some of the initial IBM variants contained as many as two twisted data pairs, plus four additional twisted pairs for telephone usage.

The time you want to think about specifying this cable is when you need a quick and dirty transceiver for a first product release (or beta-trial) and don't have time to design a complete, professional, UTP solution. The 150- W STP-A solution will be easier to design, because the cable bandwidth is so terrific , but it places a terrible installation burden on your customer.

POINT TO REMEMBER

• Think about 150- W STP-A when you need a quick and dirty transceiver for a first product release (or beta-trial).