New Zealand Complex Phone Line Impedance

Quote of the Day

I found one day in school a boy of medium size ill-treating a smaller boy. I expostulated, but he replied: 'The bigs hit me, so I hit the babies; that's fair.' In these words he epitomized the history of the human race.

— Bertrand Russell

Introduction

Figure 1: A Map of New Zealand (Source).

I received an email today asking me about the phone line impedance  differences between New Zealand (Figure 1) and Australia.  This is an easy question to answer, and I wrote up a quick Mathcad worksheet to perform the calculation.

New Zealand is a country that I dream about visiting – I want to see Hobbiton. I loved watching The Lord of the Rings, and this put New Zealand on my bucket list.

My calculation worksheet and a PDF is available here.

Background

All the background material needed to understand this post is covered in my earlier post on Australian off-hook phone impedance.

Analysis

Reference Circuit

Figure 2 shows an excerpt from a web reference on the BT3 impedance model used by New Zealand.

Figure 2: New Zealand BT3 Reference Circuit (Source).

Analysis Setup

Figure 3 shows how I modeled the characteristic impedance for 26 AWG wire, which is commonly used in North America. I also show the impedance models for Australia and New Zealand.

Figure 3: Formulae for 26 AWG Characteristic Impedance, Plus Australia and New Zealand Reference Impedances.

Comparison with Australia and US Impedance Levels

Figure 4 shows my plot of the impedances for:

• 26 AWG wire pair
• North American 600 Ω and 900 Ω line impedance model
• Australian line impedance model
• New Zealand (BT3) line impedance model

Figure 4: Plot of Telephony Impedances.

The plot shows that the New Zealand impedance characteristic is fairly similar to Australia's and that of a 26 AWG wire pair.

Conclusion

I have always wondered why so many countries use custom line impedances given that phone wiring is so similar around the world. To ensure that we pass the acceptance tests for each country, we must adapt our equipment to match the impedance requirements for each country. Today, this adaptation can be done using software to configure impedance settings on the Subscriber Line Interface Circuits (SLICs). Back in the old days, it required hardware modifications.

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