There are two kinds of switching, Space Division (analog), and Time Division (digital). Voice communication is an analog signal. Analog switches do not modify the input signals. Each connection is like having your own road to drive on. Digital switches move numbers from one port to another using common connection just like all the cars sharing the same road. Voice (analog) must be converted to numbers (digital) to go thru a digital switch then converted to voice (analog) again. Just as paint by numbers (digital) images can never match photographs; there is a small distortion in the voice signal thru a digital switch. The ear does not notice this change. 56k modems cannot transmit from one analog port to another analog port of a digital switch. Analog switches can pick up electrical interference as noise. Digital switches are more resistant to noise. Just as you can still read a news paper if it has some dirt (noise) on it; digital switches will send a clear signal as long as the numbers get thru.

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