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.
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.
My calculation worksheet and a PDF is available here.
All the background material needed to understand this post is covered in my earlier post on Australian off-hook phone impedance.
Figure 2 shows an excerpt from a web reference on the BT3 impedance model used by New Zealand.
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
The plot shows that the New Zealand impedance characteristic is fairly similar to Australia's and that of a 26 AWG wire pair.
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.