8M083X25 R14
APX Series P25 Interface
DVRS Installation & Programming Guide
–
PART II
January 2019
Page 96 of 153
Primary / Secondary - Operation and Programming Overview
The DVRS has a built-in simulcast prevention mechanism, which prevents more than one
DVRS from transmitting on the same frequency, at the same time, in the same location.
IMPORTANT!
The DVRS voting algorithm is designed to handle stationary, same scene
deployment scenarios only. It cannot be applied to moving vehicles with active
DVRS! Moving vehicles must have their DVRS switched to OFF or VR Disabled
mode.
The DVRs are capable of communicating with each other and negotiating which DVR is a
Primary i.e. elected to repeat and which DVRs are Secondary (i.e. on stand-by) for as long
as there is a Primary.
The simulcast prevention algorithm has two phases:
•
Primary / Secondary Phase
•
Busy Lockout Phase
During the ‘Primary / Secondary’ Phase, the DVRs exchange P25 messages on their Rx
frequency. When a DVR is activated (switched to SYSTEM or LOCAL Mode), it sends a
query
– “Is there a Primary?” If it receives a Primary (or Permanent Primary) ‘Heart Beat’,
then the DVR assumes Secondary state. If no Primary Heart Beat is received, the DVR
becomes the Primary and starts sending ‘Primary Heart Beats’ notifying other DVRs of its
status.
The ‘Secondary’ DVRs monitor any DVR-related activity while in a Secondary mode and
execute Primary/Secondary checks periodically.
The ‘Primary’ DVR services any communication requests in the area and periodically
‘communicates’ its status to the other DVRs.
The Busy Lockout phase serves as an added protection by forcing the Primary DVR not
only to periodically communicate with the other DVRs in the area (using its Rx frequency)
but also to check for any activity on its transmit frequency prior to every repeat.
In order to accommodate the dynamically changing “real life” environment i.e. DVRs
arriving or leaving the scene at random time, DVR channel / mode changes, Permanent
Primary activation, remote DVR steering etc., the Primary / Secondary algorithm has
several built-in layers, configurable in the DVR programmed personality.
The transmit power during DVRS-to-DVRS transmission is set by the
‘R-Tx Pwr’
field in
the
Repeater Channel Setup
Menu. Typically this field is selected to be 4-5dB lower than
the
‘Tx Pwr’
setting.
Before a Primary DVR sends out a ‘Heart Beat’, it checks for RF activity on its receive
frequency to ensure the channel is free. If an RF signal is present and it exceeds the
programmed
‘Rx RSSI Th’
, the DVR will wait until the channel is free before it transmits
its ‘Heart Beat’. When a Secondary DVR stops receiving ‘Heart Beats’, the DVR will check
for any activity on the transmit frequency to determine if the Primary DVR is busy
transmitting. If there is a transmission detected on the transmit frequency and it exceeds
the programmed ‘Tx RSSI Th’, the DVR will continue operating as a Secondary and
continue waiting for a ‘Primary Heart Beat’. If no activity is detected above the
‘Tx RSSI
