Phone Line Impedance Levels: 600 Ohms and 900 Ohms

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Introduction

Figure 1: Old Phone Central Office. (Wikipedia)

Engineering is a pretty conservative profession -- I have been accused of "abhorring change". Once something gets standardized it stays in place even when it does not make sense. This morning provided me a good example of this. Phone lines in the United States are usually characterized as having a characteristic impedance of 600 Ω or 900 Ω. These impedance levels go back to the early days of telephony (Figure 1). However, all the phone cables we work with are Category 3 and therefore have identical characteristic impedance (~725 Ω). So why the different impedance levels? I had a discussion with one of our telephony engineers about it this morning and all we could do is speculate. I thought I would document this speculation here.

Analysis

Assumptions

Our speculations are based on certain assumptions.

• In the old days, 26 AWG wire was used for central office wiring.
• In the old days, 22 AWG wire was used for connecting central offices to homes.
• Telephony circuits were characterized at 1 kHz.
• I normally hear test engineers assuming characteristic impedances of 600 Ω for home wiring and 900 Ω for central office wiring.

When I work with customers, I do see 22 AWG and 26 AWG cables used in older homes and central offices, respectively. We can compute the characteristic impedance of these cables at 1 kHz to see if that explains the 600 Ω and 900 Ω terminations.

Characteristic Impedance Calculation

Figure 2 shows my calculation of the characteristic impedance of 22 AWG and 26 AWG wire at 1 kHz.

Figure 2: Characteristic Impedance Calculations for 22 AWG and 26 AWG cables.

So we see that the old wiring standards did have 600 Ω and 900 Ω values. Note that the impedances are complex, but we normally speak of them as being real in the US. Some countries, like Australia, recognize that the impedances are complex (see this blog post for further discussion).

Conclusion

I have no idea if this is why the 600 Ω and 900 Ω values are used, but it seems reasonable. The interesting thing to me is how we keep designing systems assuming 600 Ω and 900 Ω values even when we know those numbers are not correct. It reminds me of the old story I have included here (Source).

The U.S. Standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. That's an exceedingly odd number. Why was that gauge used? Because that's the way they built them in England, and the U.S. railroads were built by English expatriates. Why did the English people build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used.
Why did ''they'' use that gauge then? Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.
Okay! Why did the wagons use that odd wheel spacing? Well, if they tried to use any other spacing the wagons would break on some of the old, long distance roads, because that's the spacing of the old wheel ruts.
So who built these old rutted roads? The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The roads have been used ever since. And the ruts? The initial ruts, which everyone else had to match for fear of destroying their wagons, were first made by Roman war chariots. Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.
Thus, we have the answer to the original questions. The United States standard railroad gauge of 4 feet, 8.5 inches derives from the original specification (Military Spec) for an Imperial Roman army war chariot. Military specs and bureaucracies live forever. So, the next time you are handed a specification and wonder what horse's ass came up with it, you may be exactly right. Because the Imperial Roman chariots were made to be just wide enough to accommodate the back-ends of two war horses.

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