ARA-1 Operations Manual
INTEROPERABILITY NOW
5-3
5.6
Explanation: Trunked Channel Acquisition Delay
Trunked systems including 800 MHz Trunked Radio Systems are a very common public safety
communications format. When trunked system users begin a transmission, their radios must first
communicate with the Trunking Controller. The Trunking Controller has ultimate control of each
radio’s TX function. When a trunked system radio’s PTT input is activated, the Trunking
Controller first ensures that the user’s radio is on an open channel, and then provides a tone to
the user. This tone signals that it is now OK to begin speaking. This is an incomplete overview
of Trunked Radio operation, but the concept essential to interoperability is the time gap between
when a user activates a radio’s PTT switch and when that user may begin speaking.
This gap poses a problem to any Interoperability System. When the trunked radio system is cross-
connected to another radio, the operator of the other radio does not hear the “Channel Ready”
acknowledgement tone (also called the “go ahead” tone), and may not even be aware that he is
cross-connected to a trunked system.
If this radio operator simply begins talking, the first
syllables or words will be lost while the trunked radio is silent and waiting to acquire a free
channel.
This is simply not acceptable in the circumstance when interoperability is most
frequently needed: during a disaster or other unusual event when clear communication is crucial.
The solution is to add delay to the audio that is being patched from other radios into the trunked
system by increasing the
TX Audio Delay
setting of the associated DSP-2 module. This TX audio
delay should match or exceed the channel acquisition time. This holds up the RX audio from
cross-connected radios until the trunked radio is ready to begin transmitting.
Be sure to take into account the fact that channel acquisition times may be increased when the
trunked system is exceptionally busy. Since any type of incident that requires interoperability is
likely to be very busy for all communications, the interoperability system must have the ability
to add sufficient audio delay to compensate.
Also keep in mind that the ARA-1 allows “on-the-fly” adjustment of the delay time through the
web browser interface.
Refer to Figure 5-1 and Figure 5-2 for an illustration of the problem and how it can be resolved.