It’s all going to be going away soon but, for most of us, our landline phones are still connected the way they were 80 years ago......
The analog Public Switched Telephone Network (PSTN) or Plain Old Telephone Service (POTS) local loop is defined as the twisted pair of copper wires many of us have coming into our home or business. This local loop is sometimes referred to as the “final three miles” or simply the “final mile”. The local loop has been “tuned” to our voice frequencies over the last 100 years and has a bandwidth of approximately 4000 Hz. This bandwidth includes two guardbands to prevent adjacent frequency interference. As can be seen in the figure below, bandwidth available to the local loop circuit for actual voice analog transmission is about 3000 Hz.
The local loop wire pair consists of two wires and runs from a home or business to a Local Exchange Carrier (LEC) Central Office (CO) which is also referred to as the Central Exchange (CE). The CO provides voltage (– 48V DC) for the telephone in our homes and businesses. The wires that make up a wire pair are identified as follows: The “tip” (red wire) is attached to the negative side of the CO 48 V battery and the “ring” (green wire) is attached to the positive side of the CO 48 V battery.
Local Loop Telephone Circuit
This diagram shows a basic local loop telephone circuit. Notice the CO provides the voltage for the telephone. This voltage is provided by batteries in the CO – we’ve all experienced power failures at one time or another and most realize telephones still work even when the power is out. Also notice the battery polarity is inverted and a –48 V DC is being provided to the phone. This is done for electrolytic corrosion reasons. In my next post we’ll look at the local loop in the form of a transmission line.