Telephones: Digitizing and Delivering
The concentrator digitizes your voice at a sample rate of 8,000 samples per second and 8-bit resolution (see How Analog and Digital Recording Works for information on digitizing sounds). It then combines your voice with dozens of others and sends them all down a single wire (usually a coax cable or a fiber-optic cable) to the phone company office. Either way, your line connects into a line card at the switch so you can hear the dial tone when you pick up your phone.
If you are calling someone connected to the same office, then the switch simply creates a loop between your phone and the phone of the person you called. If it's a long-distance call, then your voice is digitized and combined with millions of other voices on the long-distance network. Your voice normally travels over a fiber-optic line to the office of the receiving party, but it may also be transmitted by satellite or by microwave towers. (See How does a long-distance call work? for a more detailed description.)
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