System Feature Overview 39
68007024085 April 2011
2.3.4.3 Emergency Alarm with Voice to Follow
When configured for Emergency Alarm and with Voice to Follow, the emergency consists of
sending a single emergency alarm, and followed by an automatic transmission of an Emergency
Call. This is referred to as hot microphone. The radio only sends one emergency alarm regardless
if there is channel activity, and then without waiting for an acknowledgement the radio immediately
activates the microphone and initiates an Emergency Call without the need of the user pressing
the PTT. The duration of the hot microphone state is configurable through the CPS in the Digital
Emergency Systems settings. This transmission is considered an Emergency Call, and therefore
includes the embedded emergency indication. Once this hot microphone duration expires, the
radio stops transmitting, but remains in the emergency mode. Any follow up PTT initiates an
Emergency Call, and includes the embedded emergency indication. The user remains in the
emergency mode until he manually clears his emergency. The only way to reinitiate the
emergency alarm and the hot microphone is to re-initiate the emergency.
It is important to note that when configured for Emergency Alarm with Voice to Follow, the radio will
continue to transmit voice for the duration of the provisioned hot microphone timer. Since voice
has priority over data, any data is queued while voice is transmitting, including the GPS update
that was triggered by the emergency. The GPS data cannot be delivered until after the radio stops
transmitting voice, and after the repeater hangtime has expired. The GPS data has no additional
priority over other data queued in the radios, or over any traffic on the channel. Therefore, its
delivery may be delayed if the radio in emergency has pending data queued or if the channel is
busy processing other traffic.
It is recommended when utilizing Emergency Alarm with Voice to Follow and GPS, that the hot
microphone timer be at maximum 30 seconds. There are a few reasons for this. First of all, data
messages will not stay in the queue for ever, 30 seconds is short enough so to give the GPS data
a chance to be transmitted without timing out. Second, if the hot microphone timer is longer than
30 seconds, and the GPS update rate is around the same value, then other GPS messages may
start to fill up in the queue while the voice transmission is processing. This not only occurs with the
radio in emergency, but with all other radios since the channel is busy. Therefore when the voice
call ends, all radios will be attempting to access the channel with their GPS data which increases
the likelihood of collisions and lost messages. Finally, it is important to understand that while the
user is transmitting due to its hot microphone timer, there is no way to communicate back to him.
Most users can explain their situation in less than 30 seconds and require some feedback from the
emergency dispatcher much sooner. That is why it is recommended to keep this value low and if
additional monitoring is required, the remote monitor feature can be utilized. Only use a long hot
microphone timer in specialized applications.
Also, since the emergency alarm itself is not acknowledged nor retried, its reliability is less than
that of the standard Emergency Alarm and Emergency Alarm Only and Call. These considerations
should be taken into account when choosing to operate with Emergency Alarm with Voice to
Follow.
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