| 摘要 |
A switched access emergency (i.e., 9-1-1) telephone system that uses the trunks of a public telephone network (PTN) with the same reliability as a 9-1-1 telephone system that uses a dedicated line is disclosed. The switched access system includes a trunk dial unit (TDU) connected to the central office receiving 9-1-1 calls and a call access unit (CAU) connected to the 9-1-1 public service answering point (PSAP) that the caller desires to contact. 9-1-1 calls to the caller's central office are directed to the TDU, which calls the PSAP via the PTN and the CAU. The TDU captures the automatic number identification (ANI) information associated with the calling telephone and sends the ANI information to the PSAP when requested to do so. During idle periods, the TDU regularly communicates with the CAU via the PTN to determine the availability of the CAU and, thus, the PSAP. The TDU can be programmed to contact one or more secondary PSAPs if a primary PSAP is unavailable. In such embodiments of the invention, the TDU regularly communicates with the CAUs associated with the alternative PSAPs to determine the availability of the CAUs and, thus, their associated PSAPs, if the CAU associated with the primary PSAP has failed, the primary PSAP has failed or no primary PSAP number has been programmed. Alternatively, or in addition, the TDU can be programmed to contact different primary and secondary PSAPs at different times of the day. Regardless of the complexity of the overall switched access system, failure of the TDU to contact one or more of the CAUs creates an audible and/or visual alarm that alerts maintenance personnel to the loss of communication capability.
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