Can Zhone make some recommendations on the maximum loop length that can be supported by the FXS card using 0.4mm and 0.5mm wire?
The maximum loop length is normally given in ohms. For the FXS card the value is 1800 ohms. So, the conversion process is .4 mm = 26 gauge wire. 26-gauge wire has a resistance of 428 ohms per mile. Therefore, the distance is 1800/428 = 4.2 miles, or 6.76 kilometers. For the .5 mm wire = 24 gauge wire, 24-gauge wire has a resistance of 265.5 ohms per mile. The distance is 1800/265.5 = 6.7 miles or 10.9 kilometers.