| 摘要 |
A network termination device which will operate correctly even if its connection to a central office has reverse polarity. A network termination device at customer premises is connected to a line termination device located at a central office via a two-wire subscriber loop. If this connection was made with reverse polarity, the supply voltage provided by the line termination device would oscillate at specific intervals, alternating between normal polarity and reverse polarity. In such a case, a polarity switch controller in the network termination device directs its local polarity reversing switch to alternate the polarity at predetermined intervals longer than the cycle time of the above oscillation. This produces such a situation where a reverse supply voltage is present and the polarity reversing switch is in reverse mode, resulting in correct polarity as a whole. Under this condition, frame synchronization can be achieved between the peer devices, and a circuit termination unit thus asserts a synchronization indication signal. Upon receipt of this signal, the polarity switch controller commands the polarity reversing switch to stop changing and hold the current polarity. Eventually, a normal connection path is restored between the two devices, despite of the reversed subscriber loop wiring.
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