| http://www.w3.org/ns/prov#value | - said filters to said central office, the presence or absence of said frequency f2 ; (7) using an audio frequency square wave having a frequency fn to gate a pulse stream operating at said carrier bit rate, at said central office; and (8) converting said gated stream into unipolar light output for driving an nth optical cable from said nth repeater, and so on, wherein n is an integer greater than
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