| 摘要 |
The invention is concerned with digital communication systems and there is disclosed a digital telephone system wherein any digital telephone may be switched, by control from a local exchange, between a power-up state wherein continuous operating power is consumed by the telephone and a power-down state wherein the telephone is effectively turned off except for the consumption of a small amount of power sufficient only to enable communication between the telephone and the exchange. A telephone is powered-up by detecting, at the telephone, a control signal in the digital data transmitted from the exchange to the telephone. Each telephone includes a detection circuit for detecting the control signal and actuating a power rationing switch to switch the telephone to the power-up state. The power rationing switch may also be actuated momentarily when the telephone goes from the idle to busy condition to initiate a call so as to signal the exchange of the requirement for full operating power. The exchange is capable of deciding whether a telephone wishing to power-up can do so without causing exceeding of an allowable sum of power ration for the telephones of the system. A test center for digitally testing the lines between the exchange and the telephones is also disclosed. The test center causes a digital test signal to be transmitted from the exchange to a telephone at the end of a line under test wherein it is modified in a particular way and transmitted back to the exchange. The returned test signal is compared, at the test center, with a similarly modified non-transmitted test signal enabling a count of the errors to be made. The S/N ratio on the line under test is reduced to increase the errors.
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