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Chapter 16: Network Interface Units (NIU) NIUs are sometimes referred to as "Smart Jacks" in that they have intelligent functions that are necessary for the demarcation point between the telco and the customer premises. NIUs bear a strong resemblance to terminating office repeaters that do not feed span power to the facility. They also bear a resemblance to T1 Channel Service Units (CSUs). Different NIUs have different feature sets, but diagnostic loopbacks are most commonly found. The interesting situation is when there are no line repeaters. If the total length of the facility is less than 3000 to 4000 feet, then probably no mid-span T1 line repeaters are necessary. In that case, the DC polarity might accidentally be applied backwards at the CO and there is nothing on the copper line to fail until it gets to the NIU. So, if the NIU does not work on Day 1, first establish that DC is going correctly (this takes a DC voltmeter across the span side, and test for a 6 to 8 VDC voltage drop). Then move to tracing the high frequency signal. A DS-1 signal at 1.544 MHz is pretty unique and can be traced from point to point with the right kind of full-featured DS-1 test set, not a "receive only" DS-1 monitor. Remember that simplex current is only seen from the office repeater or NIU to the outside. Once DS-1 signals are inside the CO, inside from the office repeater, then simplex is no longer present. Inside, the DS-1 signals are +3 volt and -3 volt pulses with no simplex.
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