The diagram below shows the deployment of the NCM with dual tandems. At PSAP A is an NCM Alarm Unit equipped with an NCM card and a transfer switch. At Tandem Switch A is a 19-Slot NCM Rack with two NCM cards. And at Tandem Switch B is a 19-Slot rack with one NCM card.

The NCM located in the Alarm Unit at PSAP A will send a heartbeat to the NCM card at tandem switch A. If the heartbeat is not received by the NCM card at tandem A then the NCM card at the tandem and in the PSAP Alarm Unit will give a visual and audible alarm indicating the alternate answer circuit is not functioning. The second NCM card at Tandem A will send a heartbeat to the NCM card at Tandem B. If the heartbeat is not received by the NCM card at Tandem B then the NCM Cards at Tandem A and B and the Alarm Unit in PSAP A will give a visual and audible alarm indicating the alternate answer circuit is not functioning.

The second purpose of the NCM is to send a signal that will saturate a scan point at the tandem switch and re-direct the 9-1-1 calls to an alternate PSAP. By activating a switch on the NCM Alarm Unit at PSAP A, a signal will be sent via the alternate answer circuit to the NCM cards at the tandems to activate the remote make busy feature at both Tandem Switch A and B and re-direct 9-1-1 calls to PSAP B. The NCM Alarm Unit will give a visual indication (a green LED will light) that the signal to saturate the scan points at the switches has been received at the tandem.

Each NCM card comes with four communication channels, so separating wire-line trunks and wireless trunks can be easily handled with separate transfer switches on the Alarm Unit.

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