Inside a 150- W STP-A cable, the two wires of an individual pair couple more heavily to their shield than they do to each other. The net result is that the two wires act like a pair of weakly coupled transmission lines, with a near-end crosstalk coupling factor of only 3.7%.  If you have studied coupled transmission lines, you will know that such a system can support multiple modes of transmission and that the speed of propagation among the various modes need not be equal. In particular, the signal on one wire might, due to slight differences in the dielectric composition of the wires, arrive ahead of the signal on the other wire.  In extreme cases, the resultant skew could diminish the size of the eye opening of the far-end received signal. The skew problem in 150- W STP-A is particularly acute because of the short baud intervals involved in typical 150- W STP-A applications. For example, the Gigabit Ethernet 1000BASE-CX specification uses an 150- W STP-A-like cable with a baud period of only 800 ps. Compared to such a tiny baud period, even a few tens of picoseconds of skew can have a measurable effect on the quality of the received signal.
 Working with the two wires of a single pair, drive the (+) wire with respect to the shield and connect the (-) wire through 75 ohms to the shield. The near-end crosstalk induced on the (-) wire is 3.7%. The two wires are well balanced in that they bear equal impedances to the common shield, but they are hardly coupled to each other.
 Early pre-production versions of this cable used a colored dielectric insulation. The various colors of ink used to tint the wires had different dielectric constants that affected the speed of propagation on the two wires. Production cables available today generally use a thin painted coloration, partly to help control skew.
One hundred-fifty-ohm cables with STP-A construction are available with a skew specification as tight as 150 ps in 25 m (this meets the requirements of Gigabit Ethernet 1000BASE-CX).
POINT TO REMEMBER
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
Time-Domain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. Two-Port Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )